BACKGROUND
the oil palm is a perennial crop that belongs to the genus elaeis and to the botanical family palmae.
within the genus elaeis, two species are distinguished, the economically important oil palm  originally native to africa and the economically less important south american relative, elaeis oleifera .
the oil palm produces two distinct types of oil based on the fatty acid composition.
the mesocarp of the fruit produces an oil  which has a predominantly higher palmitic  and oleic acid  profile.
in contrast, the endosperm  produces oil  in which the lauric fatty acids  are predominant.
the main feature of the e. oleifera palm that distinguishes it morphologically from the commercial species e. guineensis is its procumbent trunk, distinctly smaller sized fruits and smaller canopy.
moreover, the angle of insertion of its leaflets is in a single plane as compared to a double plane for e. guineensis  <cit> .
in e. oleifera, up to 65% of the fruits tend to be parthenocarpic  <cit>  and have a much lower oil content  <cit> .
as such, the oil yield of e. oleifera is much lower, with oil to bunch ratio of 5%, as compared to the e. guineensis  with oil to bunch ratio of more than 25%  <cit> .
nevertheless, e. oleifera possess certain important characteristics that are of significant interest to oil palm breeders.
this includes the low annual stem height increment   <cit> .
the fatty acid composition of its cpo is especially of interest since its iodine value  can be as high as  <dig> as compared to the average of  <dig> of e. guineensis  <cit> .
the cpo derived from the e. oleifera oil has high levels of oleic and linoleic acid and lower levels of the palmitic acid and other saturated fatty acids, thus imparting a property quite akin to olive oil in composition.
in south america, interest in the e. oleifera was driven by the fact that it shows resistance to bud rot disease  <cit> .
in view of the apparent lack of variability for traits associated with high oil yield within e. oleifera and because e. guineensis has all the desired attributes for high oil yield, the only viable proposition  is to carry out interspecific hybridization between the two species.
fortunately, the e. guineensis and e. oleifera hybridize readily, producing fertile offspring in spite of their different areas of origin, which implies that they share a common ancestry before the two continents  drifted apart some  <dig> million years ago.
the fact that the two species can still hybridize to produce viable offspring itself suggests that the species isolation barrier is incomplete  <cit>  despite the millions of years of separation.
the interspecific hybridization approach is viewed as a viable method to introgress the traits of interest i.e.
namely higher oil unsaturation   <cit> .
this is a long term breeding strategy, with results obtained thus far showing that oil quality, taken as unsaturated fatty acid content, is better in the hybrids and in their backcrosses than in the commercial e. guineensis  <cit> .
however, the conventional breeding approach is severely hampered by the fact that being a perennial crop, the oil palm has a long selection cycle of between  <dig> and  <dig> years  <cit> .
furthermore, it requires enormous resources in terms of land , labour and field management in breeding trials.
the development of marker-assisted selection  techniques would greatly facilitate hybrid-breeding programmes as well as speed up the development of planting materials with an oil composition high in unsaturated fatty acids .
with mas, selection can be carried out in segregating generations of interspecific hybrids and their backcrosses more discriminately using molecular markers linked to the specific fatty acids.
for the purpose of mas, a number of dna marker systems have been applied to genetic mapping in oil palm.
restriction fragment length polymorphism  markers from genomic libraries have been applied to oil palm linkage mapping  <cit> .
this map harbouring  <dig> rflp markers in  <dig> groups of two or more was generated using a selfed guineensis cross.
moretzsohn et al.
<cit>  reported genetic linkage mapping for a single controlled cross of oil palm using random amplified polymorphic dna  markers and the pseudo-testcross mapping strategy described by grattapaglia et al.
<cit> .
more recently, billotte et al.
<cit>  reported a simple sequence repeat -based high density linkage map for oil palm, involving a cross between a thin shelled e. guineensis  palm and a thick shelled e. guineensis  palm.
the map consisting of  <dig> ssr markers and  <dig> amplified fragment length polymorphism  markers represents the first linkage map for oil palm to have  <dig> independent linkage groups corresponding to the haploid chromosome number of  <dig> in oil palm  <cit> .
mayes et al.
<cit> , moretzsohn et al.
<cit>  and billotte et al.
<cit>  reported the identification of rflp, rapd and aflp markers respectively, linked to the shell thickness locus, an important economic trait which exhibits monofactorial inheritance.
however, most of the traits of economic interest in oil palm exhibit quantitative inheritance.
in this area, rance et al.
<cit> , expanding on the genetic map developed by mayes et al.
<cit> , reported the detection of qtls associated with vegetative and yield components of oil palm.
the work reported above represents important developments in the application of mas in oil palm breeding programmes.
despite the advances being made and the progress achieved in genetic mapping of oil palm, only a limited number of economically important traits have been tagged to date.
furthermore, none has been reported for fatty acid composition.
this is probably because of the lower variability for most fatty acids within the e. guineensis populations.
in this study, we hoped to exploit the use of complementary dna  probes as rflp markers for linkage map construction.
the cdna clones represent gene fragments that occur in the expressed regions of the genome.
their identity can be determined via sequencing and such sequences are known as expressed sequence tags .
the usefulness of ests as markers has been demonstrated in several plant species  <cit> .
ests help to map known genes apart from providing anchor probes for comparative mapping.
furthermore, mapping ests closely linked to or co-segregating with a trait allows the gene for that trait to be identified by the candidate gene approach.
this could eventually expedite the application of mas in oil palm breeding programmes.
the strategy adopted in this research was to capitalize on the differences between the two species of oil palm and use an interspecific hybrid for the analysis of qtls associated with palm oil fatty acid composition.
this study employed both dominant  and co-dominant  markers to generate a linkage map.
the map was subsequently used to locate qtls associated with the fatty acid composition.
RESULTS
marker screening
a total of  <dig> polymorphic aflp loci were scored in the progeny by using the  <dig> aflp primer pairs .
generally, for the majority of the segregating markers scored,  <dig>  were in the pseudo-testcross configuration where either the male parent was heterozygous, and the fragment was absent in the female parent  or vice versa  .
* five rflp markers detected two loci each
** four ssr primers detected two loci each
# three of the ssr markers showing 1:1:1: <dig> segregation ratio  were grouped here for simplicity of presentation
a refers to the number of markers having each segregation pattern among the progeny of the up <dig>  × t <dig>  interspecific cross
a total of  <dig> cdna probes from various cdna libraries were tested for their ability to detect segregation in the progeny using the rflp approach.
of the  <dig> probes screened,  <dig>  showed polymorphisms with at least one restriction enzyme,  <dig>  were monomorphic and  <dig>  gave no clear banding pattern.
the percentage of polymorphic probes identified  was similar to the rate of 25% polymorphic rflp probes  reported previously by mayes et al.
<cit>  for oil palm.
out of the  <dig> rflp probes showing polymorphism,  <dig>  were inherited from the male e. guineensis parent.
five of these  <dig> probes revealed two polymorphic loci each, giving a total of  <dig> polymorphic loci .
the rflp probes used in this study appeared to have mainly scanned the homozygous regions of the e. oleifera parental palm that were not segregating in the mapping progeny, thus reducing the number of polymorphic probes revealed.
among the  <dig> ssr primer pairs developed in the course of this study, nine were informative and segregating in the mapping population.
of the  <dig> single-locus ssr primer pairs reported by billotte et al.
<cit> , seven segregated in the mapping population.
six segregated in the male e. guineensis parental gametes only, while one segregated in the female e. oleifera gametes.
three of the five est-ssrs tested  showed polymorphism in the mapping population.
all three informative primer pairs segregated only in the male parent e. guineensis gametes.
four of the informative ssr primers segregating in the male gametes revealed two loci each .
information on the informative ssr primer pairs is provided in tables  <dig> and  <dig>
* genbank
of the  <dig>  markers identified segregating in the mapping population,  <dig>  were segregating in the gametes of the male parent, nigerian e. guineensis and  <dig>  were segregating in the gametes of the female parent, the colombian e. oleifera .
this indicated that the male e. guineensis parent is more heterozygous than the female parent, e. oleifera.
as such, sufficient markers could only be generated to enable development of a genetic linkage map for the male parent.
it is therefore concluded that it would be more appropriate to analyze this cross as a "one-way pseudo-testcross" in which the male, e. guineensis is considered to be the heterozygous parent and the colombian e. oleifera, the homozygous parent.
linkage analysis
only markers showing "type b, e, f and g" profiles  were used for linkage analysis.
markers showing "type c" profile with a 3: <dig> segregation ratio  were not employed as the recombination frequencies obtained with such markers are typically inaccurate  <cit> .
in the initial attempt,  <dig> markers were shortlisted to generate a linkage map for the male t <dig> parent.
fourteen markers had to be removed from the analysis as they showed very significant distortion .
in addition,  <dig> markers with more than  <dig> missing data points were also removed.
finally,  <dig> markers were used for map construction.
both the independence lod and recombination frequency methods agreed with respect to the grouping of markers in the linkage groups.
however,  <dig> of the markers  remained unlinked.
these unlinked markers could be sampling parts of the genome where there are few other markers, in which case they would be very valuable in the future  <cit> .
in the initial map constructed, markers of two linkage groups  exhibited irregular patterns.
in order to improve the map order, the total number of recombinations for each palm across linkage groups was evaluated.
out of the  <dig> palms used in the analysis, eight palms with relatively high recombination frequencies were identified.
these eight palms were then removed from the analysis and map construction was repeated for all groups as before using the remaining  <dig> palms and the  <dig> markers that were shortlisted.
in the second attempt, similarly,  <dig> markers had to be removed from the analysis as they showed very significant distortion .
in addition,  <dig> markers with more than  <dig> missing data points had to be removed and hence  <dig> markers were finally used for map construction.
the same  <dig> markers  that were unlinked in the previous attempt remained unlinked in this effort.
the new map order was generally similar to the order produced previously and the "plausible position analysis" showed that marker order of all groups showed a regular pattern and all markers were indeed located at their "best estimated position".
a graphical representation of the genetic linkage map obtained is shown in figures  <dig>   <dig> and  <dig>  in total,  <dig> markers  mapped in  <dig> linkage groups.
the average number of markers per linkage group was  <dig>  the total genetic distance covered by the markers was  <dig> cm, with an average interval of  <dig> cm between adjacent markers.
the map distance of the tenera t <dig> parental palm was close to the tenera map distance of  <dig>  cm reported by billotte et al.
<cit> .
excluding the two smallest groups  which had three and four markers respectively, the length of individual linkage groups varied from  <dig>  cm to  <dig> cm, with an average of  <dig> cm.
the average length of the linkage groups is close to the expected size of 100– <dig> cm found in most agricultural crops  <cit> .
the markers were well distributed over all the  <dig> linkage groups.
there was only one interval of  <dig> cm in group  <dig>  there were no gaps larger than  <dig> cm in any of the other groups.
this indicates that the map is relatively homogeneous with regards to marker distribution and will be useful for tagging traits of economic interest for the purpose of marker-assisted selection.
of the  <dig> rflp loci used for linkage analysis,  <dig> were successfully mapped.
the  <dig> rflp loci were generated from  <dig> independent cdna probes .
the rflp markers were generally well distributed throughout the linkage groups.
there were certain instances  where two rflp markers were not interrupted by aflp loci, which in fact tended to flank the rflp markers.
however, there were many regions where both marker systems intermingled and as such, probably do not at this stage represent distinct regions.
twenty-four of the rflp sequences had significant similarity with genbank accessions, particularly to genes from rice and arabidopsis .
however, five of these matched with unknown or hypothetical proteins.
the location of some putative genes  were determined on the present linkage map.
# putative gene identity was inferred from homology search using blastx.
* the rflp markers concerned detected more than one segregating loci
fifteen of the  <dig> ssr loci segregating in the male parent gametes were successfully mapped.
due to the low number of ssr markers employed, only ten of the groups had at least one ssr marker each.
nevertheless, the presence of ssr markers in these groups together with the rflp markers makes it more convenient for genetic map integration or comparison.
development of additional ssrs from the existing ests collection is in progress, and it is anticipated that the est-ssrs will assist with map saturation in the future.
the proportion of markers exhibiting distorted segregation ratio in this study was about 21% .
this was slightly higher than that reported for oil palm previously   <cit>  and other species, such as eucalyptus   <cit>  and apricot   <cit> .
however, the segregation distortion was much lower than those observed for roses   <cit>  and coffee   <cit> .
nevertheless, 79% of the markers  segregated in the expected ratios, indicating that a majority of the markers were inherited in a stable mendelian manner.
groups  <dig>   <dig> and  <dig> in particular had a large percentage of distorted markers.
quantitative traits
a major objective of this study is to map qtls associated with iodine value  and fatty acid composition  in oil palm.
generally, all of the traits showed a pattern of continuous distribution around the mean, although some traits did not follow a perfect normal distribution .
the frequency distribution of iv, c16: <dig>  c18: <dig>  c18: <dig>  c18: <dig> and c18: <dig> did not differ significantly from normality.
this agrees with the co-dominant theory of inheritance for the fatty acids as proposed by ong et al.
<cit> .
however, the frequency distribution of c14: <dig> and c16: <dig> showed deviation from normality.
deviation of a trait from a perfect normal distribution has been observed in qtl analysis experiments  <cit> .
the correlation coefficients between the various traits and their values were computed and provided in table  <dig>  as expected, the iv content is positively correlated with the unsaturated fatty acids c18: <dig> and c18: <dig>  the results also indicate that the saturated fatty acids c14: <dig> and c16: <dig> are negatively correlated with iv, c18: <dig> and c18: <dig>  the results obtained here are as anticipated and similar to those reported previously  <cit> .
however, c18: <dig> showed no significant correlation to c16: <dig> and c18: <dig>  weak correlation between c16: <dig> and c18: <dig> has also been reported previously for rapeseed  <cit> .
nevertheless, perez-vich et al.
<cit>  had reported that the c18: <dig> and c18: <dig> contents were negatively correlated in sunflower.
the lack of correlation of c18: <dig> to c18: <dig> could be due to the low levels of inherent c18: <dig> in oil palm including the interspecific hybrids.
note: correlation carried out using pearson correlation test implemented via the spss software package.
** correlation is significant at the  <dig>  level
* correlation is significant at the  <dig>  level
qtl analysis
at a genomic wide significant threshold of p <  <dig>  and p <  <dig> , significant qtls were detected for iv , c14: <dig> , c16: <dig> , c16: <dig> , c18: <dig> , c18: <dig>  and c18: <dig>  using the interval mapping approach .
significant qtls were not detected for c18: <dig>  the lod score profiles obtained are shown in figure  <dig>
of lod peak
a loci flanking the likelihood peak of a qtl
in the subsequent multiple-qtl model  analysis, the significant qtls for iv, c16: <dig> and c18: <dig> were maintained on group  <dig>  however, additional qtls for c14: <dig>  and c18: <dig> were also revealed on group  <dig> .
all five qtls showed similar shaped lod profiles suggesting that the same qtl is influencing the five traits.
the qtls mapped on group  <dig> for iv, c16: <dig> and c18: <dig> explain a significant proportion of the variation observed for the traits, that is  <dig> % for iv,  <dig> % for c16: <dig> and  <dig> % for c18: <dig>  the variation explained for c14: <dig> and c18: <dig> on group  <dig> was  <dig> % and  <dig> % respectively, indicating that it was a minor qtl influencing these two traits.
the qtl for unsaturation  had an opposite effect to the qtl for saturated fatty acids , suggesting that the alleles at this qtl locus affect the saturated and unsaturated fatty acids differently.
of lod peak
the mean value of the quantitative trait for palms having the locus was higher compared to palms not having the locus
the mean value of the quantitative trait for palms having the locus was lower compared to palms not having the locus
the above analysis was possible as markers linked to the qtls were in the pseudo-testcross configuration
a loci flanking the likelihood peak of a qtl
p: significance level
significant qtls for c14: <dig> and c18: <dig> were located on group  <dig>  which explained  <dig> % and  <dig> % of the variation respectively.
another major qtl located on group  <dig> was that for c16: <dig>  which explained  <dig> % of the variation.
a minor qtl for c18: <dig> was also located around the same region on group  <dig> , explaining about  <dig> % of the variation respectively.
the lod profiles of the qtls were also very similar , indicating that the same qtl is influencing the traits concerned on group  <dig>  in contrast to what was observed in group  <dig>  the minor qtl for c18: <dig> on group  <dig> was in the same direction with c18: <dig>  similar results were also observed by zhao et al.
<cit>  for rapeseed and could be an indication of a pleiotropic effect of a single qtl.
mqm analysis revealed a third minor qtl on group  <dig> for c18: <dig>  the minor qtl detected for c14: <dig> on group  <dig> through interval mapping was found to be not significant in the mqm analysis, and as such, was not considered as a locus influencing c14: <dig> in this study.
with respect to c18: <dig>  only a single qtl was detected on group  <dig> , which explained about  <dig> % of the variation observed.
the rank sum test of kruskal-wallis was subsequently used to confirm whether the individual markers linked to the qtls were actually significant.
the kruskal-wallis test is regarded as the non-parametric equivalent to the one-way analysis of variance  <cit> , and the results are summarized in table  <dig>  for all traits, the markers flanking the qtl were also significant  for the presence of a segregating qtl in the kruskal-wallis test.
the kruskal-wallis test provides further confirmation of the marker-trait association, and indicates that the results of the qtl analysis were not influenced by segregation distortion or non-normal distribution of certain traits .
segregation of markers associated with qtls
this study also correlated the actual segregation of rflp and ssr markers  and the traits of interest in the mapping population.
the rflp and ssr markers were chosen, as they are practical for application in plant breeding and had significant lod scores for the traits concerned.
since the pseudo-testcross strategy was used in map construction, palms in the mapping population were separated as either having the band present  or absent  for a particular marker associated with the qtl.
the trait values were averaged and compared between palms with the "aa" and "ab" genotypes.
the results obtained are summarized in table  <dig>  as is shown for the rflp marker cb75a, there was a significant difference for iv between palms having the "aa" and "ab" genotypes.
the absence of the cb75a band  resulted in high levels of iv, in other words, high levels of unsaturation of the oil.
similar results were observed for c18: <dig>  the rflp probe cb75a was also associated with the qtl for c16: <dig> .
in a similar analysis, there were significant differences in the c16: <dig> content between palms having the "ab" and "aa" genotypes.
in this case, the presence of the cb75a band is correlated with a higher level of the saturated fatty acid c16: <dig>  the results are interesting as the presence of the cb75a band points to a higher level of saturated fatty acid , lower levels of unsaturation  and vice versa, in this particular mapping population.
the sequence of the rflp probe cb75a was however not associated with any of the genes in the fatty acid pathway.
means of the genotypes "aa" and "ab" were compared using the independent t test via the spss statistical package.
the means of the different genotypes for the markers associated with each trait were found to be significantly different
aa: band absent
ab: band present
in a similar way, the traits c16: <dig> and c18: <dig> were negatively correlated and the qtls overlapped in the same position on group  <dig>  the presence of the ssr allele, p4t <dig> , which is located about  <dig> cm from the estimated position of the qtl, resulted in high levels of c18: <dig> but reduced levels of c16: <dig>
discussion
the facs of palm oil and palm kernel oil render these oils applicable to both edible and non-edible uses.
however, to venture into new markets, at least in the malaysian palm oil industry, the focus is to change the oil towards a higher unsaturated fac content particularly oleic acid , at the expense of saturated fatty acids such as palmitic acid   <cit> .
an oil with such properties has the potential to open up wider markets for palm oil in the salad oil sector, especially in the cooler climatic regions of the world where currently rapeseed, sunflower and soyabean are preferred  <cit> .
in addition, it is envisaged that such an oil could be industrially useful for producing chemical derivatives, which could serve as alternatives to petrochemical feedstock  <cit>  as well as be a potentially cheaper feedstock for production of biofuels by virtue of having a lower cloud point.
the ultimate objective is to try to produce breeding lines that can produce oil with iv content of above  <dig>  palmitic acid content of below 25% and oleic acid content of 60%, without sacrificing the palm oil yield per unit area.
this will ensure that maximum benefit could be achieved from diversifying away from the present commercial planting material that has a higher saturated fatty acid profile and into a more liquid oil without sacrificing the inherent high oil yield potential of the crop  <cit> .
two approaches are being taken to achieve this objective: i) genetic engineering of oil palm  <cit>  and ii) using the more conventional breeding approach of interspecific hybrid breeding.
the work carried out in this study was also intended to develop probes to help expedite the latter approach, which is not complicated by issues of bio-safety and bioethics.
the mapping population chosen for this purpose met two important criteria; segregating for the trait of interest and is relevant in the long term breeding scheme or strategy to improve the oil quality trait.
although the female parent  was mostly homozygous for the loci analyzed, the male parent, e. guineensis fortunately was highly heterozygous, hence contributing to a significant level of genetic variability that was exploited for qtl analysis.
in fact, it has been reported that the range of fatty acid composition observed in nigerian based materials such as the male parent palm t <dig> used in this study, extends beyond that of the breeding materials currently in use  <cit> .
this suggests that the nigerian based e. guineensis materials are more suited for breeding oil palms with improved fatty acid composition  <cit> .
the variation captured in the male parent palm t <dig> can also prove useful for selection within e. guineensis, which can directly affect desirable changes in fatty acid composition in hybrids created subsequently.
nevertheless, it is also acknowledged that to fully exploit the value of oil palm interspecific hybrids and to capture the variation between the two parents, backcross populations have to be analyzed in the future.
the genetic map constructed had an excess of linkage groups in relation to the haploid chromosome number despite the relatively high number of markers used.
failure to obtain the basic chromosome number despite applying high numbers of markers has also been reported for other species  <cit> .
the reason for this could perhaps be due to the relatively small sample size of the f <dig> progeny used in this study.
another possible explanation is the lack of polymorphic markers in particular chromosomal regions, which could be due to the marker systems being employed and/or presence of large homozygous regions in the genome of the female e. oleifera parental palm used to create the interspecific hybrid population used in this study.
furthermore, it has to be stressed that very strict criteria were used to carry out map construction in this study.
only markers that fit extremely well in a linkage group were retained.
markers that caused even a slight friction were discarded in order not to compromise the subsequent qtl analysis.
this also explains why only  <dig> markers  were successfully ordered in the genetic map.
the genetic map reported in this study depicts the mapping of expressed genes.
the sequences of the rflp markers mapped in this study have been submitted to genbank.
since the rflp markers were well distributed across the linkage groups, they can be used as potential anchor markers for integration or comparison of maps of different populations.
as the oil palm est database is growing rapidly  <cit> , additional probes either as rflp markers or est-ssr markers will be placed on the genetic map concerned.
more importantly, the growing oil palm est database will allow the selective mapping of genes associated with the fatty acid composition pathway.
the use of allele specific markers linked to genes underlying the synthesis of seed oils has been demonstrated in brassica  <cit> .
in this study,  <dig> qtls were detected for iv and the six components of the fatty acid composition  in four different linkage groups.
for c18: <dig>  two qtls were detected, one major qtl in group  <dig> and a minor qtl in group  <dig>  which collectively explained  <dig> % of the total phenotypic variation.
two qtls were detected for c14: <dig> and three for c18: <dig>  explaining  <dig> % and  <dig> % of the total phenotypic variation observed respectively.
one qtl each was detected for iv, c16: <dig> and c18: <dig>  for the first time, this study has revealed qtls associated with fac in oil palm.
the traits were largely controlled by a limited number of genomic regions with large effects.
qtls for five traits  were located in group  <dig>  all traits showed similar shaped lod profiles suggesting that the same qtl is influencing all five traits.
the fact that four of the traits are significantly correlated further supports this assumption.
furthermore, looking at the pathway for fatty acid biosynthesis where c16: <dig> is in fact elongated to c18: <dig> by the enzyme β-ketoacyl acp synthase ii , and c18: <dig> is subsequently desaturated by Δ9-stearoyl acp desaturase to form c18: <dig>  supports the fact that the same locus could be influencing these traits.
also considering that iv is a measure of unsaturation of oils and fats, c18: <dig> is the most abundant unsaturated fatty acid while c16: <dig> is the most abundant saturated fatty acid in palm oil, it is only logical to assume that the same genomic region is influencing these traits in oil palm.
qtls for c14: <dig>  c16: <dig>  c18: <dig> and c18: <dig> were located on group  <dig>  the stearoyl acp desaturase enzyme, although highly specific to the conversion of c18: <dig> to c18: <dig>  is also known to sometimes act on c16: <dig> as a poor substrate and convert it to c16: <dig>  <cit> .
this probably explains the strong negative correlation  between c18: <dig>  and c16: <dig>  and why the same qtl may be influencing the traits.
as expected, the effect of the qtl for c16: <dig> and c18: <dig> is also in the opposite direction.
the likelihood profile for the qtls affecting the two traits in group  <dig>  is also very similar, adding further strength to the argument that the same locus is influencing both traits.
previously, a single preliminary qtl was reported for iv in oil palm in a similar population consisting of only  <dig> palms  <cit> .
the lod peak of  <dig>  reported is not significant at the threshold level employed in the present study.
since no other similar work especially for fac has been reported for oil palm, it was not possible to carry out a direct comparison with findings from other research groups.
however, a comparison with other crops  is possible.
for example, in maize, alrefai et al.
<cit>  detected  <dig> qtls  associated with c16: <dig> only.
similarly, mangolin et al.
<cit>  detected  <dig> qtls distributed in eight chromosomes for kernel oil content in maize.
the low number of qtls detected in this study, were however in agreement with the work by somers et al.
<cit>  and jourdren et al.
<cit> , who found that a few qtl loci could explain a large proportion of the phenotypic variation associated with one of the fatty acid components, c18: <dig>  in brassica napus.
furthermore, the same genomic region influencing two or more fatty acid components have also been reported for sunflower  <cit>  and brassica napus  <cit> .
however, the experience in soybean was different where panthee et al.
<cit>  reported lack of common markers associated with c16: <dig>  c18: <dig> and c18: <dig>  although the same genomic region appears to be influencing the three 18-carbon unsaturated fatty acids .
nevertheless, it is important to note that the differences in qtls mapped in this research cannot be directly compared to those reported above because of the different crop, type of markers, mapping population structure and the density of the genetic maps used in the analysis.
the population size employed is another major factor that may explain differences in studies on qtl analysis, as the population size can affect the power to detect qtls.
the population size for qtl analysis in this study was  <dig> palms, smaller than that reported for some annual crops  <cit> , which also makes direct comparison with other studies more difficult.
it is also noted that qtls could not be detected for c18: <dig>  the small population size employed limited power to detect qtls of smaller effect.
analysis of further populations, particularly backcrosses derived from the same cross, may yield qtls for this trait.
the qtls identified in this study will provide breeders with a valuable tool to manipulate the fac content in oil palm.
for example, absence of the rflp probe cb75a could be indicative of palms having oil with higher unsaturation level.
the absence of the rflp marker resulted in an increase of about  <dig> % above the family mean for iv , and a decrease of about  <dig> % below the family mean for c16: <dig> content .
if the marker/qtl linkage holds across different pedigrees, this rflp marker could be used to enrich for genotypes with higher levels of unsaturation.
the association reported here was found only in a particular mapping population and as such may not yet be applicable for molecular breeding.
many researchers have pointed out that associations established in one cross may not hold true in other crosses  <cit> .
however, grattapaglia et al.
<cit>  were of the opinion that substantial linkage disequilibrium can be maintained for marker/traits associations established in a single cross.
the linkages established however can only be defined as "confirmed linkages" once they have been confirmed in a further sample, preferably by an independent group of investigators  <cit> .
nevertheless, it is heartening to note that qtls for fatty acid composition have generally been validated across populations, even those associated with minor qtls  <cit> .
although the efforts in malaysia are largely directed towards decreasing levels of saturation, increasing levels of certain saturated fatty acids can also have some economic benefits.
in this respect, there is interest in increasing the stearic acid content , which can give rise to new applications such as cocoa butter substitution and personal care products   <cit> .
this is also partly motivated by the substantial price differential between cocoa butter and commodity oils  <cit> .
like most plant oils, the oil palm has low stearate content of less than 5%  <cit> .
the ssr marker p4t <dig> could play an important role in mas for high stearate palms.
the presence of the ssr alleles resulted in an increase of  <dig> % above the family mean for c18: <dig> content.
an important point to note is that the saturated fatty acids, e.g.
c16: <dig> and c18: <dig> are negatively correlated with total unsaturation  .
furthermore, the qtls for saturated and unsaturated fatty acids are largely in the opposite direction.
as such, it is unlikely that a particular palm for high unsaturation and c18: <dig> can be bred.
it may be more practical to select separately palms for high saturated and unsaturated oils.
rajanaidu et al.
<cit>  reported that repeatability of measurements for fac is high indicating that a single measurement is sufficient to describe the fatty acid content of a bunch.
rajanaidu et al.
<cit>  also predicted high heritability for most of the fatty acid traits in oil palm.
arasu et al.
<cit>  reported that genotype × environment  interaction was not detected for any of the fatty acid traits in the  <dig> e. guineensis nigerian germplasm populations analyzed.
as such, good repeatability, high heritability and minimum g × e interaction suggest that fac content is actually amenable to improvement with simple selection procedures.
fac can be improved rapidly using the strategy pointed out by hospital et al.
<cit> .
after having established the linkages, the genetic gain can be accelerated by scoring for markers associated with the qtls for two generations without phenotypic observation.
if the marker/qtl linkage holds true, rance et al.
<cit>  predicted that such a strategy could reduce the generation time by almost half for oil palm as the crosses can be made right after flowering , without having to wait for the fruits to be formed and analyzed .
CONCLUSIONS
in this study, the qtls were only detected for the male e. guineensis parent, t <dig>  as such, we cannot conclude if the marker/qtl linkages will hold true for e. oleifera.
nevertheless, we believe that the linkages established between the marker and qtls could be followed in backcross populations, which usually involve backcrossing the f <dig> hybrid to the e. guineensis parent.
the qtls identified in this study would also be potentially useful in exploiting the huge e. guineensis germplasm that malaysia  has accumulated.
qtls with favorable alleles can be identified in the germplasm collection for incorporation into the existing breeding programmes.
the high phenotypic variation explained by most of the qtl improves confidence in their application for mas.
nevertheless, certain drawbacks should also be pointed out.
there is always a possibility of linkage drag occurring, especially when involving germplasm collections and in oil palm interspecific hybrids, where unfavorable alleles such as that responsible for low yield are also incorporated together with the favorable alleles for higher unsaturation.
however, as pointed out by rance et al.
<cit> , this can be minimized by selecting for qtls with small confidence interval that defines a very narrow region.

 the oil palm is a perennial crop that belongs to the genus elaeis and to the botanical family palmae. within the genus elaeis, two species are distinguished, the economically important oil palm originally native to africa and the economically less important south american relative, elaeis oleifera. the oil palm produces two distinct types of oil based on the fatty acid composition. the mesocarp of the fruit produces an oil which has a predominantly higher palmitic and oleic acid profile. in contrast, the endosperm produces oil in which the lauric fatty acids are predominant. the main feature of the e. oleifera palm that distinguishes it morphologically from the commercial species e. guineensis is its procumbent trunk, distinctly smaller sized fruits and smaller canopy. moreover, the angle of insertion of its leaflets is in a single plane as compared to a double plane for e. guineensis. in e. oleifera, up to 65% of the fruits tend to be parthenocarpic and have a much lower oil content. as such, the oil yield of e. oleifera is much lower, with oil to bunch ratio of 5%, as compared to the e. guineensis with oil to bunch ratio of more than 25%. nevertheless, e. oleifera possess certain important characteristics that are of significant interest to oil palm breeders. this includes the low annual stem height increment. the fatty acid composition of its cpo is especially of interest since its iodine value can be as high as as compared to the average of of e. guineensis. the cpo derived from the e. oleifera oil has high levels of oleic and linoleic acid and lower levels of the palmitic acid and other saturated fatty acids, thus imparting a property quite akin to olive oil in composition. in south america, interest in the e. oleifera was driven by the fact that it shows resistance to bud rot disease. in view of the apparent lack of variability for traits associated with high oil yield within e. oleifera and because e. guineensis has all the desired attributes for high oil yield, the only viable proposition is to carry out interspecific hybridization between the two species. fortunately, the e. guineensis and e. oleifera hybridize readily, producing fertile offspring in spite of their different areas of origin, which implies that they share a common ancestry before the two continents drifted apart some million years ago. the fact that the two species can still hybridize to produce viable offspring itself suggests that the species isolation barrier is incomplete despite the millions of years of separation. the interspecific hybridization approach is viewed as a viable method to introgress the traits of interest i.e. namely higher oil unsaturation. this is a long term breeding strategy, with results obtained thus far showing that oil quality, taken as unsaturated fatty acid content, is better in the hybrids and in their backcrosses than in the commercial e. guineensis. however, the conventional breeding approach is severely hampered by the fact that being a perennial crop, the oil palm has a long selection cycle of between and years. furthermore, it requires enormous resources in terms of land, labour and field management in breeding trials. the development of marker-assisted selection techniques would greatly facilitate hybrid-breeding programmes as well as speed up the development of planting materials with an oil composition high in unsaturated fatty acids. with mas, selection can be carried out in segregating generations of interspecific hybrids and their backcrosses more discriminately using molecular markers linked to the specific fatty acids. for the purpose of mas, a number of dna marker systems have been applied to genetic mapping in oil palm. restriction fragment length polymorphism markers from genomic libraries have been applied to oil palm linkage mapping. this map harbouring rflp markers in groups of two or more was generated using a selfed guineensis cross. moretzsohn et al. reported genetic linkage mapping for a single controlled cross of oil palm using random amplified polymorphic dna markers and the pseudo-testcross mapping strategy described by grattapaglia et al.. more recently, billotte et al. reported a simple sequence repeat -based high density linkage map for oil palm, involving a cross between a thin shelled e. guineensis palm and a thick shelled e. guineensis palm. the map consisting of ssr markers and amplified fragment length polymorphism markers represents the first linkage map for oil palm to have independent linkage groups corresponding to the haploid chromosome number of in oil palm. mayes et al., moretzsohn et al. and billotte et al. reported the identification of rflp, rapd and aflp markers respectively, linked to the shell thickness locus, an important economic trait which exhibits monofactorial inheritance. however, most of the traits of economic interest in oil palm exhibit quantitative inheritance. in this area, rance et al., expanding on the genetic map developed by mayes et al., reported the detection of qtls associated with vegetative and yield components of oil palm. the work reported above represents important developments in the application of mas in oil palm breeding programmes. despite the advances being made and the progress achieved in genetic mapping of oil palm, only a limited number of economically important traits have been tagged to date. furthermore, none has been reported for fatty acid composition. this is probably because of the lower variability for most fatty acids within the e. guineensis populations. in this study, we hoped to exploit the use of complementary dna probes as rflp markers for linkage map construction. the cdna clones represent gene fragments that occur in the expressed regions of the genome. their identity can be determined via sequencing and such sequences are known as expressed sequence tags. the usefulness of ests as markers has been demonstrated in several plant species. ests help to map known genes apart from providing anchor probes for comparative mapping. furthermore, mapping ests closely linked to or co-segregating with a trait allows the gene for that trait to be identified by the candidate gene approach. this could eventually expedite the application of mas in oil palm breeding programmes. the strategy adopted in this research was to capitalize on the differences between the two species of oil palm and use an interspecific hybrid for the analysis of qtls associated with palm oil fatty acid composition. this study employed both dominant and co-dominant markers to generate a linkage map. the map was subsequently used to locate qtls associated with the fatty acid composition.  marker screening a total of polymorphic aflp loci were scored in the progeny by using the aflp primer pairs. generally, for the majority of the segregating markers scored, were in the pseudo-testcross configuration where either the male parent was heterozygous, and the fragment was absent in the female parent or vice versa. * five rflp markers detected two loci each ** four ssr primers detected two loci each # three of the ssr markers showing 1:1:1: segregation ratio were grouped here for simplicity of presentation a refers to the number of markers having each segregation pattern among the progeny of the up × t interspecific cross a total of cdna probes from various cdna libraries were tested for their ability to detect segregation in the progeny using the rflp approach. of the probes screened, showed polymorphisms with at least one restriction enzyme, were monomorphic and gave no clear banding pattern. the percentage of polymorphic probes identified was similar to the rate of 25% polymorphic rflp probes reported previously by mayes et al. for oil palm. out of the rflp probes showing polymorphism, were inherited from the male e. guineensis parent. five of these probes revealed two polymorphic loci each, giving a total of polymorphic loci. the rflp probes used in this study appeared to have mainly scanned the homozygous regions of the e. oleifera parental palm that were not segregating in the mapping progeny, thus reducing the number of polymorphic probes revealed. among the ssr primer pairs developed in the course of this study, nine were informative and segregating in the mapping population. of the single-locus ssr primer pairs reported by billotte et al., seven segregated in the mapping population. six segregated in the male e. guineensis parental gametes only, while one segregated in the female e. oleifera gametes. three of the five est-ssrs tested showed polymorphism in the mapping population. all three informative primer pairs segregated only in the male parent e. guineensis gametes. four of the informative ssr primers segregating in the male gametes revealed two loci each. information on the informative ssr primer pairs is provided in tables and * genbank of the markers identified segregating in the mapping population, were segregating in the gametes of the male parent, nigerian e. guineensis and were segregating in the gametes of the female parent, the colombian e. oleifera. this indicated that the male e. guineensis parent is more heterozygous than the female parent, e. oleifera. as such, sufficient markers could only be generated to enable development of a genetic linkage map for the male parent. it is therefore concluded that it would be more appropriate to analyze this cross as a "one-way pseudo-testcross" in which the male, e. guineensis is considered to be the heterozygous parent and the colombian e. oleifera, the homozygous parent. linkage analysis only markers showing "type b, e, f and g" profiles were used for linkage analysis. markers showing "type c" profile with a 3: segregation ratio were not employed as the recombination frequencies obtained with such markers are typically inaccurate. in the initial attempt, markers were shortlisted to generate a linkage map for the male t parent. fourteen markers had to be removed from the analysis as they showed very significant distortion. in addition, markers with more than missing data points were also removed. finally, markers were used for map construction. both the independence lod and recombination frequency methods agreed with respect to the grouping of markers in the linkage groups. however, of the markers remained unlinked. these unlinked markers could be sampling parts of the genome where there are few other markers, in which case they would be very valuable in the future. in the initial map constructed, markers of two linkage groups exhibited irregular patterns. in order to improve the map order, the total number of recombinations for each palm across linkage groups was evaluated. out of the palms used in the analysis, eight palms with relatively high recombination frequencies were identified. these eight palms were then removed from the analysis and map construction was repeated for all groups as before using the remaining palms and the markers that were shortlisted. in the second attempt, similarly, markers had to be removed from the analysis as they showed very significant distortion. in addition, markers with more than missing data points had to be removed and hence markers were finally used for map construction. the same markers that were unlinked in the previous attempt remained unlinked in this effort. the new map order was generally similar to the order produced previously and the "plausible position analysis" showed that marker order of all groups showed a regular pattern and all markers were indeed located at their "best estimated position". a graphical representation of the genetic linkage map obtained is shown in figures and in total, markers mapped in linkage groups. the average number of markers per linkage group was the total genetic distance covered by the markers was cm, with an average interval of cm between adjacent markers. the map distance of the tenera t parental palm was close to the tenera map distance of cm reported by billotte et al.. excluding the two smallest groups which had three and four markers respectively, the length of individual linkage groups varied from cm to cm, with an average of cm. the average length of the linkage groups is close to the expected size of 100– cm found in most agricultural crops. the markers were well distributed over all the linkage groups. there was only one interval of cm in group there were no gaps larger than cm in any of the other groups. this indicates that the map is relatively homogeneous with regards to marker distribution and will be useful for tagging traits of economic interest for the purpose of marker-assisted selection. of the rflp loci used for linkage analysis, were successfully mapped. the rflp loci were generated from independent cdna probes. the rflp markers were generally well distributed throughout the linkage groups. there were certain instances where two rflp markers were not interrupted by aflp loci, which in fact tended to flank the rflp markers. however, there were many regions where both marker systems intermingled and as such, probably do not at this stage represent distinct regions. twenty-four of the rflp sequences had significant similarity with genbank accessions, particularly to genes from rice and arabidopsis. however, five of these matched with unknown or hypothetical proteins. the location of some putative genes were determined on the present linkage map. # putative gene identity was inferred from homology search using blastx. * the rflp markers concerned detected more than one segregating loci fifteen of the ssr loci segregating in the male parent gametes were successfully mapped. due to the low number of ssr markers employed, only ten of the groups had at least one ssr marker each. nevertheless, the presence of ssr markers in these groups together with the rflp markers makes it more convenient for genetic map integration or comparison. development of additional ssrs from the existing ests collection is in progress, and it is anticipated that the est-ssrs will assist with map saturation in the future. the proportion of markers exhibiting distorted segregation ratio in this study was about 21%. this was slightly higher than that reported for oil palm previously and other species, such as eucalyptus and apricot. however, the segregation distortion was much lower than those observed for roses and coffee. nevertheless, 79% of the markers segregated in the expected ratios, indicating that a majority of the markers were inherited in a stable mendelian manner. groups and in particular had a large percentage of distorted markers. quantitative traits a major objective of this study is to map qtls associated with iodine value and fatty acid composition in oil palm. generally, all of the traits showed a pattern of continuous distribution around the mean, although some traits did not follow a perfect normal distribution. the frequency distribution of iv, c16: c18: c18: c18: and c18: did not differ significantly from normality. this agrees with the co-dominant theory of inheritance for the fatty acids as proposed by ong et al.. however, the frequency distribution of c14: and c16: showed deviation from normality. deviation of a trait from a perfect normal distribution has been observed in qtl analysis experiments. the correlation coefficients between the various traits and their values were computed and provided in table as expected, the iv content is positively correlated with the unsaturated fatty acids c18: and c18: the results also indicate that the saturated fatty acids c14: and c16: are negatively correlated with iv, c18: and c18: the results obtained here are as anticipated and similar to those reported previously. however, c18: showed no significant correlation to c16: and c18: weak correlation between c16: and c18: has also been reported previously for rapeseed. nevertheless, perez-vich et al. had reported that the c18: and c18: contents were negatively correlated in sunflower. the lack of correlation of c18: to c18: could be due to the low levels of inherent c18: in oil palm including the interspecific hybrids. note: correlation carried out using pearson correlation test implemented via the spss software package. ** correlation is significant at the level * correlation is significant at the level qtl analysis at a genomic wide significant threshold of p < and p <, significant qtls were detected for iv, c14:, c16:, c16:, c18:, c18: and c18: using the interval mapping approach. significant qtls were not detected for c18: the lod score profiles obtained are shown in figure of lod peak a loci flanking the likelihood peak of a qtl in the subsequent multiple-qtl model analysis, the significant qtls for iv, c16: and c18: were maintained on group however, additional qtls for c14: and c18: were also revealed on group. all five qtls showed similar shaped lod profiles suggesting that the same qtl is influencing the five traits. the qtls mapped on group for iv, c16: and c18: explain a significant proportion of the variation observed for the traits, that is % for iv, % for c16: and % for c18: the variation explained for c14: and c18: on group was % and % respectively, indicating that it was a minor qtl influencing these two traits. the qtl for unsaturation had an opposite effect to the qtl for saturated fatty acids, suggesting that the alleles at this qtl locus affect the saturated and unsaturated fatty acids differently. of lod peak the mean value of the quantitative trait for palms having the locus was higher compared to palms not having the locus the mean value of the quantitative trait for palms having the locus was lower compared to palms not having the locus the above analysis was possible as markers linked to the qtls were in the pseudo-testcross configuration a loci flanking the likelihood peak of a qtl p: significance level significant qtls for c14: and c18: were located on group which explained % and % of the variation respectively. another major qtl located on group was that for c16: which explained % of the variation. a minor qtl for c18: was also located around the same region on group, explaining about % of the variation respectively. the lod profiles of the qtls were also very similar, indicating that the same qtl is influencing the traits concerned on group in contrast to what was observed in group the minor qtl for c18: on group was in the same direction with c18: similar results were also observed by zhao et al. for rapeseed and could be an indication of a pleiotropic effect of a single qtl. mqm analysis revealed a third minor qtl on group for c18: the minor qtl detected for c14: on group through interval mapping was found to be not significant in the mqm analysis, and as such, was not considered as a locus influencing c14: in this study. with respect to c18: only a single qtl was detected on group, which explained about % of the variation observed. the rank sum test of kruskal-wallis was subsequently used to confirm whether the individual markers linked to the qtls were actually significant. the kruskal-wallis test is regarded as the non-parametric equivalent to the one-way analysis of variance, and the results are summarized in table for all traits, the markers flanking the qtl were also significant for the presence of a segregating qtl in the kruskal-wallis test. the kruskal-wallis test provides further confirmation of the marker-trait association, and indicates that the results of the qtl analysis were not influenced by segregation distortion or non-normal distribution of certain traits. segregation of markers associated with qtls this study also correlated the actual segregation of rflp and ssr markers and the traits of interest in the mapping population. the rflp and ssr markers were chosen, as they are practical for application in plant breeding and had significant lod scores for the traits concerned. since the pseudo-testcross strategy was used in map construction, palms in the mapping population were separated as either having the band present or absent for a particular marker associated with the qtl. the trait values were averaged and compared between palms with the "aa" and "ab" genotypes. the results obtained are summarized in table as is shown for the rflp marker cb75a, there was a significant difference for iv between palms having the "aa" and "ab" genotypes. the absence of the cb75a band resulted in high levels of iv, in other words, high levels of unsaturation of the oil. similar results were observed for c18: the rflp probe cb75a was also associated with the qtl for c16:. in a similar analysis, there were significant differences in the c16: content between palms having the "ab" and "aa" genotypes. in this case, the presence of the cb75a band is correlated with a higher level of the saturated fatty acid c16: the results are interesting as the presence of the cb75a band points to a higher level of saturated fatty acid, lower levels of unsaturation and vice versa, in this particular mapping population. the sequence of the rflp probe cb75a was however not associated with any of the genes in the fatty acid pathway. means of the genotypes "aa" and "ab" were compared using the independent t test via the spss statistical package. the means of the different genotypes for the markers associated with each trait were found to be significantly different aa: band absent ab: band present in a similar way, the traits c16: and c18: were negatively correlated and the qtls overlapped in the same position on group the presence of the ssr allele, p4t, which is located about cm from the estimated position of the qtl, resulted in high levels of c18: but reduced levels of c16: discussion the facs of palm oil and palm kernel oil render these oils applicable to both edible and non-edible uses. however, to venture into new markets, at least in the malaysian palm oil industry, the focus is to change the oil towards a higher unsaturated fac content particularly oleic acid, at the expense of saturated fatty acids such as palmitic acid. an oil with such properties has the potential to open up wider markets for palm oil in the salad oil sector, especially in the cooler climatic regions of the world where currently rapeseed, sunflower and soyabean are preferred. in addition, it is envisaged that such an oil could be industrially useful for producing chemical derivatives, which could serve as alternatives to petrochemical feedstock as well as be a potentially cheaper feedstock for production of biofuels by virtue of having a lower cloud point. the ultimate objective is to try to produce breeding lines that can produce oil with iv content of above palmitic acid content of below 25% and oleic acid content of 60%, without sacrificing the palm oil yield per unit area. this will ensure that maximum benefit could be achieved from diversifying away from the present commercial planting material that has a higher saturated fatty acid profile and into a more liquid oil without sacrificing the inherent high oil yield potential of the crop. two approaches are being taken to achieve this objective: i) genetic engineering of oil palm and ii) using the more conventional breeding approach of interspecific hybrid breeding. the work carried out in this study was also intended to develop probes to help expedite the latter approach, which is not complicated by issues of bio-safety and bioethics. the mapping population chosen for this purpose met two important criteria; segregating for the trait of interest and is relevant in the long term breeding scheme or strategy to improve the oil quality trait. although the female parent was mostly homozygous for the loci analyzed, the male parent, e. guineensis fortunately was highly heterozygous, hence contributing to a significant level of genetic variability that was exploited for qtl analysis. in fact, it has been reported that the range of fatty acid composition observed in nigerian based materials such as the male parent palm t used in this study, extends beyond that of the breeding materials currently in use. this suggests that the nigerian based e. guineensis materials are more suited for breeding oil palms with improved fatty acid composition. the variation captured in the male parent palm t can also prove useful for selection within e. guineensis, which can directly affect desirable changes in fatty acid composition in hybrids created subsequently. nevertheless, it is also acknowledged that to fully exploit the value of oil palm interspecific hybrids and to capture the variation between the two parents, backcross populations have to be analyzed in the future. the genetic map constructed had an excess of linkage groups in relation to the haploid chromosome number despite the relatively high number of markers used. failure to obtain the basic chromosome number despite applying high numbers of markers has also been reported for other species. the reason for this could perhaps be due to the relatively small sample size of the f progeny used in this study. another possible explanation is the lack of polymorphic markers in particular chromosomal regions, which could be due to the marker systems being employed and/or presence of large homozygous regions in the genome of the female e. oleifera parental palm used to create the interspecific hybrid population used in this study. furthermore, it has to be stressed that very strict criteria were used to carry out map construction in this study. only markers that fit extremely well in a linkage group were retained. markers that caused even a slight friction were discarded in order not to compromise the subsequent qtl analysis. this also explains why only markers were successfully ordered in the genetic map. the genetic map reported in this study depicts the mapping of expressed genes. the sequences of the rflp markers mapped in this study have been submitted to genbank. since the rflp markers were well distributed across the linkage groups, they can be used as potential anchor markers for integration or comparison of maps of different populations. as the oil palm est database is growing rapidly, additional probes either as rflp markers or est-ssr markers will be placed on the genetic map concerned. more importantly, the growing oil palm est database will allow the selective mapping of genes associated with the fatty acid composition pathway. the use of allele specific markers linked to genes underlying the synthesis of seed oils has been demonstrated in brassica. in this study, qtls were detected for iv and the six components of the fatty acid composition in four different linkage groups. for c18: two qtls were detected, one major qtl in group and a minor qtl in group which collectively explained % of the total phenotypic variation. two qtls were detected for c14: and three for c18: explaining % and % of the total phenotypic variation observed respectively. one qtl each was detected for iv, c16: and c18: for the first time, this study has revealed qtls associated with fac in oil palm. the traits were largely controlled by a limited number of genomic regions with large effects. qtls for five traits were located in group all traits showed similar shaped lod profiles suggesting that the same qtl is influencing all five traits. the fact that four of the traits are significantly correlated further supports this assumption. furthermore, looking at the pathway for fatty acid biosynthesis where c16: is in fact elongated to c18: by the enzyme β-ketoacyl acp synthase ii, and c18: is subsequently desaturated by Δ9-stearoyl acp desaturase to form c18: supports the fact that the same locus could be influencing these traits. also considering that iv is a measure of unsaturation of oils and fats, c18: is the most abundant unsaturated fatty acid while c16: is the most abundant saturated fatty acid in palm oil, it is only logical to assume that the same genomic region is influencing these traits in oil palm. qtls for c14: c16: c18: and c18: were located on group the stearoyl acp desaturase enzyme, although highly specific to the conversion of c18: to c18: is also known to sometimes act on c16: as a poor substrate and convert it to c16:. this probably explains the strong negative correlation between c18: and c16: and why the same qtl may be influencing the traits. as expected, the effect of the qtl for c16: and c18: is also in the opposite direction. the likelihood profile for the qtls affecting the two traits in group is also very similar, adding further strength to the argument that the same locus is influencing both traits. previously, a single preliminary qtl was reported for iv in oil palm in a similar population consisting of only palms. the lod peak of reported is not significant at the threshold level employed in the present study. since no other similar work especially for fac has been reported for oil palm, it was not possible to carry out a direct comparison with findings from other research groups. however, a comparison with other crops is possible. for example, in maize, alrefai et al. detected qtls associated with c16: only. similarly, mangolin et al. detected qtls distributed in eight chromosomes for kernel oil content in maize. the low number of qtls detected in this study, were however in agreement with the work by somers et al. and jourdren et al., who found that a few qtl loci could explain a large proportion of the phenotypic variation associated with one of the fatty acid components, c18: in brassica napus. furthermore, the same genomic region influencing two or more fatty acid components have also been reported for sunflower and brassica napus. however, the experience in soybean was different where panthee et al. reported lack of common markers associated with c16: c18: and c18: although the same genomic region appears to be influencing the three 18-carbon unsaturated fatty acids. nevertheless, it is important to note that the differences in qtls mapped in this research cannot be directly compared to those reported above because of the different crop, type of markers, mapping population structure and the density of the genetic maps used in the analysis. the population size employed is another major factor that may explain differences in studies on qtl analysis, as the population size can affect the power to detect qtls. the population size for qtl analysis in this study was palms, smaller than that reported for some annual crops, which also makes direct comparison with other studies more difficult. it is also noted that qtls could not be detected for c18: the small population size employed limited power to detect qtls of smaller effect. analysis of further populations, particularly backcrosses derived from the same cross, may yield qtls for this trait. the qtls identified in this study will provide breeders with a valuable tool to manipulate the fac content in oil palm. for example, absence of the rflp probe cb75a could be indicative of palms having oil with higher unsaturation level. the absence of the rflp marker resulted in an increase of about % above the family mean for iv, and a decrease of about % below the family mean for c16: content. if the marker/qtl linkage holds across different pedigrees, this rflp marker could be used to enrich for genotypes with higher levels of unsaturation. the association reported here was found only in a particular mapping population and as such may not yet be applicable for molecular breeding. many researchers have pointed out that associations established in one cross may not hold true in other crosses. however, grattapaglia et al. were of the opinion that substantial linkage disequilibrium can be maintained for marker/traits associations established in a single cross. the linkages established however can only be defined as "confirmed linkages" once they have been confirmed in a further sample, preferably by an independent group of investigators. nevertheless, it is heartening to note that qtls for fatty acid composition have generally been validated across populations, even those associated with minor qtls. although the efforts in malaysia are largely directed towards decreasing levels of saturation, increasing levels of certain saturated fatty acids can also have some economic benefits. in this respect, there is interest in increasing the stearic acid content, which can give rise to new applications such as cocoa butter substitution and personal care products. this is also partly motivated by the substantial price differential between cocoa butter and commodity oils. like most plant oils, the oil palm has low stearate content of less than 5%. the ssr marker p4t could play an important role in mas for high stearate palms. the presence of the ssr alleles resulted in an increase of % above the family mean for c18: content. an important point to note is that the saturated fatty acids, e.g. c16: and c18: are negatively correlated with total unsaturation. furthermore, the qtls for saturated and unsaturated fatty acids are largely in the opposite direction. as such, it is unlikely that a particular palm for high unsaturation and c18: can be bred. it may be more practical to select separately palms for high saturated and unsaturated oils. rajanaidu et al. reported that repeatability of measurements for fac is high indicating that a single measurement is sufficient to describe the fatty acid content of a bunch. rajanaidu et al. also predicted high heritability for most of the fatty acid traits in oil palm. arasu et al. reported that genotype × environment interaction was not detected for any of the fatty acid traits in the e. guineensis nigerian germplasm populations analyzed. as such, good repeatability, high heritability and minimum g × e interaction suggest that fac content is actually amenable to improvement with simple selection procedures. fac can be improved rapidly using the strategy pointed out by hospital et al.. after having established the linkages, the genetic gain can be accelerated by scoring for markers associated with the qtls for two generations without phenotypic observation. if the marker/qtl linkage holds true, rance et al. predicted that such a strategy could reduce the generation time by almost half for oil palm as the crosses can be made right after flowering, without having to wait for the fruits to be formed and analyzed.  in this study, the qtls were only detected for the male e. guineensis parent, t as such, we cannot conclude if the marker/qtl linkages will hold true for e. oleifera. nevertheless, we believe that the linkages established between the marker and qtls could be followed in backcross populations, which usually involve backcrossing the f hybrid to the e. guineensis parent. the qtls identified in this study would also be potentially useful in exploiting the huge e. guineensis germplasm that malaysia has accumulated. qtls with favorable alleles can be identified in the germplasm collection for incorporation into the existing breeding programmes. the high phenotypic variation explained by most of the qtl improves confidence in their application for mas. nevertheless, certain drawbacks should also be pointed out. there is always a possibility of linkage drag occurring, especially when involving germplasm collections and in oil palm interspecific hybrids, where unfavorable alleles such as that responsible for low yield are also incorporated together with the favorable alleles for higher unsaturation. however, as pointed out by rance et al., this can be minimized by selecting for qtls with small confidence interval that defines a very narrow region. 
the trait associated with oil quality, measured in terms of its fatty acid composition, is an important agronomic trait that can eventually be tracked using molecular markers. significant qtl for c14: c16: c18: and c18: content was detected around the same locus on group thus revealing another major locus influencing fatty acid composition in oil palm. this study describes the first successful detection of qtls for fatty acid composition in oil palm. one genomic region on group appears to be influencing iv, c14: c16: c18: and c18: content. at a 5% genome-wide significance threshold level, qtls associated with iodine value, myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid and linoleic acid content were detected. a framework map was generated for the male parent, t using joinmap ver. marker assisted selection is well suited to a perennial crop like oil palm, in which the economic products are not produced until several years after planting. a map was constructed using aflp, rflp and ssr markers for an interspecific cross involving a colombian elaeis oleifera and a nigerian e. guinneensis. interval mapping and multiple-qtl model mapping were used to detect quantitative trait loci controlling oil quality. additional qtl for c18: was detected on group a minor qtl for c18: was detected on group this will speed up the production of new and improved oil palm planting materials.
 marker assisted selection is well suited to a perennial crop like oil palm, in which the economic products are not produced until several years after planting. the use of dna markers for selection in such crops can greatly reduce the number of breeding cycles needed. with the use of dna markers, informed decisions can be made at the nursery stage, regarding which individuals should be retained as breeding stock, which are satisfactory for agricultural production, and which should be culled. the trait associated with oil quality, measured in terms of its fatty acid composition, is an important agronomic trait that can eventually be tracked using molecular markers. this will speed up the production of new and improved oil palm planting materials.  a map was constructed using aflp, rflp and ssr markers for an interspecific cross involving a colombian elaeis oleifera and a nigerian e. guinneensis. a framework map was generated for the male parent, t using joinmap ver.. in the paternal map, markers could be ordered in linkage groups. interval mapping and multiple-qtl model mapping were used to detect quantitative trait loci controlling oil quality. at a 5% genome-wide significance threshold level, qtls associated with iodine value, myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid and linoleic acid content were detected. one genomic region on group appears to be influencing iv, c14: c16: c18: and c18: content. significant qtl for c14: c16: c18: and c18: content was detected around the same locus on group thus revealing another major locus influencing fatty acid composition in oil palm. additional qtl for c18: was detected on group a minor qtl for c18: was detected on group  this study describes the first successful detection of qtls for fatty acid composition in oil palm. these qtls constitute useful tools for application in breeding programmes. 