BMC Genomics 2009, 10:1 http://www.biomedcentral.com/1471-2164/10/1 age results from the chicken [56] and the great reed war- bler [4,6], suggesting significant sex-specific heterogeneity in recombination rate on the avian Z-chromosome. Between markers SJ046 and SJ069, a small amount of recombination (recombination fraction θ = 0.02–0.05) was found to occur between Z and W chromosomes in the mapping pedigree. Z-W recombination has only once been previously reported in previous mapping efforts of birds based on SNPs (see [8]), but equivalent X-Y recom- bination has been observed in linkage mapping studies of sex chromosomes in many other species such as in salmo- nid fishes [59], rainbow trout [60], cattle [61], sheep [51,62], oyster [63], and humans [64]. However, in con- trast to increased recombination rates in heterogametic sex (male, XY) as observed in the pseudoautosomal regions of above mentioned studies, an overall 8-fold lower recombination rates between SJ048 and SJ069 were found in female (ZW) as compared to male (ZZ) Siberian jays. In addition, it seems as the pseudoautosomal region is extensive in Siberian jay (15.8 cM/23.8 cM, 66.4%). While in other species, 13.3% (20.1 cM/151 cM) were reported for the PAR in the bovine sex-average consensus map [52], 6.3% (3.5 cM/55.8 cM) in the male-specific and 45.1% (54.2 cM/121 cM) in the female-specific map of the sex chromosomes in sheep [51], and a very small pro- portion of PAR in the chicken sex chromosomal maps [56]. It is unclear why these are so, but potential explana- tions include the evolutionary strata [65], patterns of genome variability [66], distribution of sex-biased loci [67,68], chiasma interference [69] and selection [70] on the avian Z-chromosome. Since the difference was found in the recombination rates and the change of genome structure during the avian evolution between Siberian jay and other wild bird species, we would like to speculate that the pattern of genome variability and distribution of sex-biased loci may have contributed more to this obser- vation. Further investigations using more pseudoauto- somal markers are needed to verify this result and understand its origin and significance. Recombination heterogeneity When testing for sex-specific differences in recombination rates in different linkage groups and over the total auto- somal map, we found evidence for statistically significant distortions towards reduced male recombination frac- tions only in two autosomal LGs, rather than in each of the nine autosomal LGs. This indicates that the sex-related differences in recombination rates are confined to certain, specific parts of the genome. This result is not unexpected given that in many species there is a large variation in the recombination rates within and among chromosomes (see [71,72]). Similar situations of linkage group sex-spe- cific distortion have also been observed in other species for example in marsupials (e.g. [73,74]) and in various aquatic organisms (the pacific oyster [63]; the tiger puffer- fish [75]; and the turbot [76]). Sex-specific differences in recombination rates have been found in a diverse range of organisms from molluscs (e.g. [77]) and fish (e.g. [49]) to mammals (e.g. pig [78]; cattle [79]; and human [80]). However, in birds various patterns have been reported: little evidence of heterochiasmy in chicken (Gallus domesticus, [54]) and zebra finch (Taen- iopygia guttata [8]); higher rates of recombination in males than in females in linkage maps of blue tit (Parus caeruleus [81]) and collared flycatcher (Ficedula albicollis [7]); and higher rates of recombination in females than in males in maps of great tit (Parus major [81]), great reed warbler (Acrocephalus arundinaceus [4,6]) and house sparrow (Pas- ser domesticus [14]). The sex-bias (females > males) observed in Siberian jay conforms to the last pattern opposing the Haldane's prediction [82] according to which the heterogametic sex should show lower recombi- nation rates than the homogametic sex. So far the compar- ative data suggest a divergence between the genetic maps of the chicken (higher recombination rate and little differ- ence in recombination rate between sexes) and the passer- ines (lower recombination rate and significant difference in recombination rate between sexes). Interestingly, on the one hand, we note the pronouncedly different pat- terns of recombination observed in collared flycatcher [11], zebra finch [8] and Siberian jay here, all of which are passerine birds. It has been proposed that heterochiasmy is the result of sexual selection, with the sex experiencing the greater variance in reproductive success exhibiting the lower recombination rate [83]. However, this explanation may not be relevant here because both collared flycatchers [84] and zebra finches [85] are polygamous, whereas the Siberian jay is a monogamous species [34,37], so that the reproductive success should be very similar between the sexes. On the other hand, we found that the female:male recombination rate (1.21) in the best-order map was much smaller than that observed in the great reed warbler where it varied from 2.15 (microsatellites[4]) to 1.86 (AFLP markers [6]). This difference is not necessarily bio- logically meaningful as it could be attributable to less informative and smaller number of loci (albeit larger number of individuals) in the earlier studies. Further- more, since marker density in both Siberian jay and great reed warbler maps is relatively low and the sex-specific recombination rates heterogenous among and within linkage groups, the estimated female:male recombination rates among the maps may vary if different genomic regions are mapped The observed sex-specific recombination rates are poten- tially influenced by numerous factors, and at the moment, there is no consensus in respect to the relative importance of mechanisms accounting for the recombination differ- Page 11 of 17 (page number not for citation purposes)