BACKGROUND
chinese taihu pigs are highly prolific; the erhualian , one of the taihu pigs, is known for producing the highest recorded litter sizes in the world  <cit> .
litter size is influenced by many factors, such as the boar, season, and nutrition.
however, it has been demonstrated that these factors do not account for the prolificacy of meishan pigs, which are another breed of chinese taihu pig  <cit> .
in addition, meishan sows are little affected by the factors involved in stillbirth  <cit> .
taihu pigs express a high level of maternal heterosis in litter size when used in crosses with western pig breeds  <cit> .
studies have indicated that the large litter sizes of meishan pigs are due to genes acting in the dam  <cit> .
the er sows can give birth to more than  <dig> piglets per litter, even when the coefficient of inbreeding is as high as  <dig>   <cit> .
these findings indicate that the desirable alleles related to litter size are preponderant in taihu sows.
embryonic loss is one of the major barriers to large litter size  <cit> .
it is estimated that approximately 20-30% of embryonic death occurs during gestation days  11– <dig>  <cit> .
the embryonic survival rate does not differ among pig breeds until gd <dig>  but it is elevated on gd <dig> in meishan pigs when compared with landrace × large yorkshire  pigs  <cit> .
at this stage, the blastocysts undergo dramatic morphological changes, developing from an 11–50 mm tubular structure into a 100 mm filamentous structure.
the rapid changes in shape and size caused by the elongation of porcine blastocysts are not a result of cellular hyperplasis but cellular rearrangements and remodeling of the trophectoderm  <cit> .
these changes coincide with the synthesis and release of maternal-fetal recognition signals  and cytokines required for the establishment of pregnancy  <cit> .
porcine conceptuses initiate the secretion of estrogen on gd10- <dig>  <cit> , although meishan embryos are smaller and contain fewer cells when they initiate steroidogenesis and begin to elongate  <cit> .
meishan conceptuses also secrete less estrogen into the uterine luminal fluid and elongate to a reduced length  <cit>  and diameter  <cit>  when compared with large white conceptuses.
the level of estrogen in porcine uterine flush samples is determined primarily by the amount of estrogen secreted by the embryos  <cit> .
the estrogen level in the uterine lumen will have multiple effects on the embryonic survival rate.
firstly, the estrogen level may affect placental weight and survival of the conceptus.
when meishan gilts were treated with estrogen on gd <dig> or gd <dig>  placental weights were increased significantly ; litter size was not affected significantly  but it tended to decrease  <cit> .
however, others have shown that placental weights are negatively correlated with litter size   <cit>  and uterine capacity at gd <dig>  <cit>   in western breeds.
the non-significant result in the former study  <cit>  may have been a consequence of smaller sample size.
secondly, embryonic estrogen, as an embryo-maternal recognition signal, can change uterine secretion of histotroph  <cit> .
the lower amount in meishan embryos may cause a more gradual change of the gravid uteri, which decreases the negative impact that faster-developing embryos could have on their slower-developing littermate embryos  <cit> .
endometrial synthesis of prostaglandins  is essential for the establishment and maintenance of pregnancy in pigs  <cit> .
during maternal recognition of pregnancy around gd <dig>  pgf2α, which is synthesized mainly by the endometrium  <cit> , has a luteolytic effect, while pge <dig> can antagonize this effect  <cit> .
the secretion of pgf2α is redirected from the uterine venous drainage  during luteolysis to the uterine lumen  at the time of maternal recognition of pregnancy.
studies have shown that the pge2:pgf2α ratio is crucial for the regulation of the estrous cycle, and the establishment and maintenance of pregnancy  <cit> .
the sum of pge <dig> and pgf2α and their ratio were higher in meishan sows than that in large white pigs  <cit> .
on gd <dig>  the placenta  has not yet formed, the conceptus is free-floating and not attached to the endometrium  <cit> , hence embryonic growth and development is dependent on histotroph in the uterine lumen.
histotroph includes hormones, growth factors, and transport proteins  <cit> .
the uterine histotroph is synthesized and secreted primarily by the epithelia of the maternal uteri during early pregnancy  <cit> .
experiments have demonstrated that embryonic growth and development are affected by the environment of the uterine lumen  <cit> .
in the present study, we detected the differentially expressed genes in the endometrium of er and ll pigs on gd <dig> by digital gene expression profiling  using an illumina genome analyzer platform.
this work will be helpful for understanding the molecular basis of different prolificacy between chinese taihu and western pigs.
RESULTS
dge libraries
pools of cdna from the gd <dig> endometrium of three ll  and three er  sows were used to construct the two dge libraries.
global gene expression profiles were obtained by massive parallel sequencing using the illumina dge system.
the raw data of the dge libraries were filtered to obtain clean tags before further analysis.
the major characteristics of the two dge libraries are described in table  <dig> and additional file 1: figure s <dig>  a total of  <dig> , <dig> tags, including  <dig> , <dig> for er and  <dig> , <dig> for ll, were obtained by sequencing.
the clean tags consisted of  <dig> , <dig> in er and  <dig> , <dig> in ll, which contained  <dig>  and  <dig>  unique tags in er and ll, respectively.
in both raw tag libraries, more than 92% of the tags were detected more than once within each library.
the distribution of tags revealed that high-expression tags  represented the majority of sequences detected, whereas the low-expression tags  had the greatest sequence diversity.
tags that represented less than 3% of the total categories of tags identified in this study accounted for more than 61% of the total number of tags.
conversely, tags that represented more than 60% of the total tag types accounted for less than 7% of the total tags.
this indicates that only a small number of mrnas are expressed abundantly and that the vast majority of mrnas are present at low level.
saturation analyses of er and ll  demonstrated that the number of newly identified unique tags and genes decreased as the total number of sequencing tags increased, which shows that the dge libraries were becoming saturated, and validates the integrity of the library for use in further analysis.
1there is only adaptor but no tag sequence in the reads.
2tag mapped to gene represents the number of tags mapped to the reference library.
3unambiguous tags mapped to gene represents the number of tags mapped to a gene in the reference library.
tag mapping
three databases  were used to generate an integrated reference library for dge tag mapping and sequence annotations.
the tags in the reference library consisted of catg, the recognition site for nlaiii, in conjunction with the next  <dig> nt sequences that were created by mmei.
one mismatch was allowed for dge tag mapping to allow for potential polymorphisms between samples.
this generated  <dig>  reference tags, which corresponded to  <dig>  unambiguous reference tags in the integrated reference library.
together,  <dig> % and  <dig> % of the clean tags and  <dig> % and  <dig> % of the unique clean tags were mapped to the reference library for er and ll, respectively;  <dig> % and  <dig> % of the total clean tags and  <dig> % and  <dig> % of the unique clean tags were mapped unambiguously to the integrated reference library for er and ll, respectively.
in total,  <dig> % and  <dig> % of the unique tags were mapped to the mitochondrial genome and nuclear non-coding genome sequence, respectively.
other dge unique tags  were not mapped to the integrated reference library.
these unknown tags probably arose from incomplete reference tag libraries.
tag position analyses  indicated that the most dge tags that matched the reference tags were close to the 3′ end of the transcripts.
dge based on illumina sequencing was able to discriminate the tags from the sense and antisense strands of dna.
we found that  <dig>  genes  had antisense transcripts for er ), and  <dig>  genes  for ll .
in total,  <dig>  and  <dig>  unique tags were mapped to the non-coding nuclear genome for er  and ll , respectively, which suggests that novel transcripts may exist close to these tags.
identification and analysis of differentially expressed genes
the tag number obtained via dge reflects the level of expression of the transcripts represented by those tags.
all the clean tags were mapped to the reference sequences; the number of unambiguous clean tags for each gene was calculated and normalized to tags per million .
by comparing the normalized dge profiles between er and ll, we obtained the global transcriptional difference between er and ll.
the results showed that  <dig>  genes were significantly differentially expressed between the breeds ;  <dig>  genes were more abundantly represented and  <dig>  were less abundant in er than in ll.
there were apparent differences in the proportions of expressed genes unique to er and ll.
a total of  <dig>  genes were represented in the combined endometrial dge profiles.
the proportions of genes expressed uniquely in er and ll were  <dig> %  and  <dig> % , respectively; the remaining genes were shared by the transcriptomes.
of the total number of genes expressed,  <dig> %  and  <dig> %  had an expression level of > <dig> % in the er and ll transcriptomes, respectively.
gene ontology  and signalling pathway analysis
go is an international standard system of classification for the comprehensive description of the properties of genes and their products.
it was used to classify all the genes expressed in er and ll endometrium into one of three groups according to their biological process, cellular component, and molecular function .
go analysis of all the genes expressed in this period revealed that the overall genomic expression profiles were very similar between er and ll, but differences were detected in specific aspects such as cellular synapses and proteasome regulation.
the go annotation  indicated that the differentially expressed genes were involved in many processes, such as reproduction, growth, cellular component biogenesis and organization, biological adhesion, and immune function.
to identify the metabolic and signal transduction pathways in which the differentially expressed genes are likely to be involved, we performed pathway analysis on the basis of the kyoto encyclopedia of genes and genomes  pathway database using an ultra-geometric test.
in total,  <dig>  differentially expressed genes had kegg pathway annotations.
as shown in additional file 9: table s <dig>  the significant signaling pathways included steroid biosynthesis, oxidative phosphorylation, basal transcription factors, and the transcription machinery.
qpcr analysis
quantitative real-time rt-pcr  was performed on eight genes to confirm the patterns of differential gene expression between er and ll pigs.
the detailed information about these genes was listed in additional file 10: table s <dig>  this set included five genes that showed increased dge representation in er  and three genes with lower dge representation in er .
as shown in figure  <dig>  qpcr validated the results of the dge analysis in all cases.
expression analysis of candidate genes for embryonic survival
quantitative trait loci  related to the embryonic survival rate in pigs have rarely been reported so far  <cit> , probably because of the high costs of such experiments.
hence, candidate genes have been selected on the basis of their physiological functions and the results of candidate gene studies.
we concentrated on maternal genes related to embryonic growth and pg synthesis.
the expression levels and functions of the candidate genes are listed in table  <dig>
see additional file 7: table s <dig> for details of all the differentially expressed genes.
numbers in square brackets are the numbers of the papers in the reference list.
discussion
in this study, we generated the endometrial expression profiles and identified the genes differentially expressed in gd <dig> er and ll endometrium.
the results in this paper will be valuable for future studies on the identification of major genes for embryonic survival.
the genes for growth factors and nutrient-delivery proteins
the secretion of uteroferrin  is not responsive to the plasma levels of iron  <cit> , thus it is speculated that the iron supply to the embryos during the peri-implantation period is determined by genotypes.
retinol-binding protein  <dig>  is significantly associated with litter size in german landrace pigs   <cit> .
receptors for hb-egf  <cit> , kgf  <cit> , igf <dig>  <cit> , and igf <dig>  <cit>  are all expressed by porcine embryos on gd <dig>  studies have shown that igf <dig> promotes embryonic growth in response to the nutrient supply  <cit> , while igf <dig> may regulate the supply of maternal nutrient to conceptus  <cit> .
the expression levels of the five genes  and igf <dig> in ll versus er pigs were significantly up-regulated and down-regulated , respectively.
vallet et al.
<cit>  reported that expressions of uf and rbp were lower in pregnant meishan endometrium than in white crossbred.
the go molecular function classification showed that the differential genes were associated with growth .
the expression patterns and the physiological functions of these genes  indicated that the endometrium of er pigs had a lower growth-promoting ability to embryos than that of ll pigs.
the above results can partially explain the phenomenon that embryos in uteri of taihu sows grow slower than those in the uteri of western sows.
igf <dig> was expressed significantly higher in ll endometrium than in er endometrium .
igf <dig>  rather than igf <dig>  is known to induce estrogen synthesis by stimulating expression of aromatase in the conceptus  <cit> .
aromatase is the rate-limiting enzyme in estrogen synthesis in the pig conceptus  <cit> .
therefore, it is very likely that er embryos secret less estrogen than ll embryos, which will contribute to the higher embryonic survival rate in er pigs.
moreover, igf <dig> increases the permeability of the placenta in mice  <cit> , and thus a higher level of igf <dig> in er endometrium  may improve the placental efficiency.
the genes in the prostaglandin  synthetic pathway
pg synthesis in endometrum, especially the pge2:pgf2α ratio, is crucial for the establishment and maintenance of pregnancy in pigs  <cit> .
the high pge2:pgf2α ratio may be a beneficial factor for large litter size in meishan sows  <cit> .
the expressions of the genes, ptgs1/ptgs <dig>  pges/pges <dig> and cbr1/cbr <dig>  play critical roles in the pg synthesis.
ptgs <dig> and ptgs <dig> are rate-limiting enzymes in pg synthesis pathway  <cit> .
the expression level of ptgs <dig> is higher than that of ptgs <dig> in the endometrium of ll pigs on gd <dig> , which is consistent with other studies using western pigs  <cit> ; while the expression level of ptgs <dig> is lower than that of ptgs <dig> in er sows .
in western sows, ptgs <dig> is the primary enzyme involved in elevated pg synthesis  <cit> , whereas ptgs <dig> may perform this function in er sows according to our results.
it has been demonstrated that both the mrna and the protein of ptgs <dig> have shorter half-lives than those of ptgs <dig>  <cit> .
hence, the higher ptgs <dig> expression can contribute to the larger capacity for pg synthesis in er pigs on gd <dig>
the convert of pgh <dig> to pge <dig> in pg synthesis is catalyzed by ptges  <cit>  and ptgs <dig>  <cit> .
the higher expression of ptges and ptges <dig> in er endometrium  is helpful for the higher pge <dig>  which will be contribute to the higher ratio of pge <dig> to pgf2α on gd  <dig>
in western breeds, expression of cbr <dig> has been examined  <cit>  but cbr <dig> neglected.
although the levels of endometrial cbr <dig> on gd <dig> and gd <dig> did not differ  <cit> , the ratio of pge <dig> to pgf2α on gd <dig> was higher than that on gd <dig>  <cit> .
cbr <dig> may play a role in the conversion of pge <dig> into pgf2α according to our results and kegg pathway , and that higher expression of cbr <dig> may decrease the ratio of pge <dig> to pgf2α.
in the present study, the patterns of expression of cbr <dig> and cbr <dig> observed in the two breeds  suggest that the ratio in the endometrium of er pigs be greater than that in the endometrium of ll pigs on gd <dig>
CONCLUSIONS
in summary, we have described genes that are expressed differentially in the endometrium of er and ll pigs on gd <dig>  compared with those in the ll pigs, the gene expression profiles in the endometrium of the prolific er pigs are found to benefit for the establishment and maintenance of pregnancy, delay embryonic development and growth, and enhance uterine capacity via reduced estrogen secretion.
the gene-driven events that are characteristic of er pigs could contribute to the lower embryonic mortality and higher prolificacy of this indigenous chinese taihu breed.
the data provided by this study will be useful for porcine transcriptomic studies.

 chinese taihu pigs are highly prolific; the erhualian, one of the taihu pigs, is known for producing the highest recorded litter sizes in the world. litter size is influenced by many factors, such as the boar, season, and nutrition. however, it has been demonstrated that these factors do not account for the prolificacy of meishan pigs, which are another breed of chinese taihu pig. in addition, meishan sows are little affected by the factors involved in stillbirth. taihu pigs express a high level of maternal heterosis in litter size when used in crosses with western pig breeds. studies have indicated that the large litter sizes of meishan pigs are due to genes acting in the dam. the er sows can give birth to more than piglets per litter, even when the coefficient of inbreeding is as high as. these findings indicate that the desirable alleles related to litter size are preponderant in taihu sows. embryonic loss is one of the major barriers to large litter size. it is estimated that approximately 20-30% of embryonic death occurs during gestation days 11–. the embryonic survival rate does not differ among pig breeds until gd but it is elevated on gd in meishan pigs when compared with landrace × large yorkshire pigs. at this stage, the blastocysts undergo dramatic morphological changes, developing from an 11–50 mm tubular structure into a 100 mm filamentous structure. the rapid changes in shape and size caused by the elongation of porcine blastocysts are not a result of cellular hyperplasis but cellular rearrangements and remodeling of the trophectoderm. these changes coincide with the synthesis and release of maternal-fetal recognition signals and cytokines required for the establishment of pregnancy. porcine conceptuses initiate the secretion of estrogen on gd10-, although meishan embryos are smaller and contain fewer cells when they initiate steroidogenesis and begin to elongate. meishan conceptuses also secrete less estrogen into the uterine luminal fluid and elongate to a reduced length and diameter when compared with large white conceptuses. the level of estrogen in porcine uterine flush samples is determined primarily by the amount of estrogen secreted by the embryos. the estrogen level in the uterine lumen will have multiple effects on the embryonic survival rate. firstly, the estrogen level may affect placental weight and survival of the conceptus. when meishan gilts were treated with estrogen on gd or gd placental weights were increased significantly; litter size was not affected significantly but it tended to decrease. however, others have shown that placental weights are negatively correlated with litter size and uterine capacity at gd in western breeds. the non-significant result in the former study may have been a consequence of smaller sample size. secondly, embryonic estrogen, as an embryo-maternal recognition signal, can change uterine secretion of histotroph. the lower amount in meishan embryos may cause a more gradual change of the gravid uteri, which decreases the negative impact that faster-developing embryos could have on their slower-developing littermate embryos. endometrial synthesis of prostaglandins is essential for the establishment and maintenance of pregnancy in pigs. during maternal recognition of pregnancy around gd pgf2α, which is synthesized mainly by the endometrium, has a luteolytic effect, while pge can antagonize this effect. the secretion of pgf2α is redirected from the uterine venous drainage during luteolysis to the uterine lumen at the time of maternal recognition of pregnancy. studies have shown that the pge2:pgf2α ratio is crucial for the regulation of the estrous cycle, and the establishment and maintenance of pregnancy. the sum of pge and pgf2α and their ratio were higher in meishan sows than that in large white pigs. on gd the placenta has not yet formed, the conceptus is free-floating and not attached to the endometrium, hence embryonic growth and development is dependent on histotroph in the uterine lumen. histotroph includes hormones, growth factors, and transport proteins. the uterine histotroph is synthesized and secreted primarily by the epithelia of the maternal uteri during early pregnancy. experiments have demonstrated that embryonic growth and development are affected by the environment of the uterine lumen. in the present study, we detected the differentially expressed genes in the endometrium of er and ll pigs on gd by digital gene expression profiling using an illumina genome analyzer platform. this work will be helpful for understanding the molecular basis of different prolificacy between chinese taihu and western pigs.  dge libraries pools of cdna from the gd endometrium of three ll and three er sows were used to construct the two dge libraries. global gene expression profiles were obtained by massive parallel sequencing using the illumina dge system. the raw data of the dge libraries were filtered to obtain clean tags before further analysis. the major characteristics of the two dge libraries are described in table  and additional file 1: figure s a total of, tags, including, for er and, for ll, were obtained by sequencing. the clean tags consisted of, in er and, in ll, which contained and unique tags in er and ll, respectively. in both raw tag libraries, more than 92% of the tags were detected more than once within each library. the distribution of tags revealed that high-expression tags represented the majority of sequences detected, whereas the low-expression tags had the greatest sequence diversity. tags that represented less than 3% of the total categories of tags identified in this study accounted for more than 61% of the total number of tags. conversely, tags that represented more than 60% of the total tag types accounted for less than 7% of the total tags. this indicates that only a small number of mrnas are expressed abundantly and that the vast majority of mrnas are present at low level. saturation analyses of er and ll demonstrated that the number of newly identified unique tags and genes decreased as the total number of sequencing tags increased, which shows that the dge libraries were becoming saturated, and validates the integrity of the library for use in further analysis. 1there is only adaptor but no tag sequence in the reads. 2tag mapped to gene represents the number of tags mapped to the reference library. 3unambiguous tags mapped to gene represents the number of tags mapped to a gene in the reference library. tag mapping three databases were used to generate an integrated reference library for dge tag mapping and sequence annotations. the tags in the reference library consisted of catg, the recognition site for nlaiii, in conjunction with the next nt sequences that were created by mmei. one mismatch was allowed for dge tag mapping to allow for potential polymorphisms between samples. this generated reference tags, which corresponded to unambiguous reference tags in the integrated reference library. together, % and % of the clean tags and % and % of the unique clean tags were mapped to the reference library for er and ll, respectively; % and % of the total clean tags and % and % of the unique clean tags were mapped unambiguously to the integrated reference library for er and ll, respectively. in total, % and % of the unique tags were mapped to the mitochondrial genome and nuclear non-coding genome sequence, respectively. other dge unique tags were not mapped to the integrated reference library. these unknown tags probably arose from incomplete reference tag libraries. tag position analyses indicated that the most dge tags that matched the reference tags were close to the 3′ end of the transcripts. dge based on illumina sequencing was able to discriminate the tags from the sense and antisense strands of dna. we found that genes had antisense transcripts for er ), and genes for ll. in total, and unique tags were mapped to the non-coding nuclear genome for er and ll, respectively, which suggests that novel transcripts may exist close to these tags. identification and analysis of differentially expressed genes the tag number obtained via dge reflects the level of expression of the transcripts represented by those tags. all the clean tags were mapped to the reference sequences; the number of unambiguous clean tags for each gene was calculated and normalized to tags per million. by comparing the normalized dge profiles between er and ll, we obtained the global transcriptional difference between er and ll. the results showed that genes were significantly differentially expressed between the breeds; genes were more abundantly represented and were less abundant in er than in ll. there were apparent differences in the proportions of expressed genes unique to er and ll. a total of genes were represented in the combined endometrial dge profiles. the proportions of genes expressed uniquely in er and ll were % and %, respectively; the remaining genes were shared by the transcriptomes. of the total number of genes expressed, % and % had an expression level of > % in the er and ll transcriptomes, respectively. gene ontology and signalling pathway analysis go is an international standard system of classification for the comprehensive description of the properties of genes and their products. it was used to classify all the genes expressed in er and ll endometrium into one of three groups according to their biological process, cellular component, and molecular function. go analysis of all the genes expressed in this period revealed that the overall genomic expression profiles were very similar between er and ll, but differences were detected in specific aspects such as cellular synapses and proteasome regulation. the go annotation indicated that the differentially expressed genes were involved in many processes, such as reproduction, growth, cellular component biogenesis and organization, biological adhesion, and immune function. to identify the metabolic and signal transduction pathways in which the differentially expressed genes are likely to be involved, we performed pathway analysis on the basis of the kyoto encyclopedia of genes and genomes pathway database using an ultra-geometric test. in total, differentially expressed genes had kegg pathway annotations. as shown in additional file 9: table s the significant signaling pathways included steroid biosynthesis, oxidative phosphorylation, basal transcription factors, and the transcription machinery. qpcr analysis quantitative real-time rt-pcr was performed on eight genes to confirm the patterns of differential gene expression between er and ll pigs. the detailed information about these genes was listed in additional file 10: table s this set included five genes that showed increased dge representation in er and three genes with lower dge representation in er. as shown in figure  qpcr validated the results of the dge analysis in all cases. expression analysis of candidate genes for embryonic survival quantitative trait loci related to the embryonic survival rate in pigs have rarely been reported so far, probably because of the high costs of such experiments. hence, candidate genes have been selected on the basis of their physiological functions and the results of candidate gene studies. we concentrated on maternal genes related to embryonic growth and pg synthesis. the expression levels and functions of the candidate genes are listed in table  see additional file 7: table s for details of all the differentially expressed genes. numbers in square brackets are the numbers of the papers in the reference list. discussion in this study, we generated the endometrial expression profiles and identified the genes differentially expressed in gd er and ll endometrium. the results in this paper will be valuable for future studies on the identification of major genes for embryonic survival. the genes for growth factors and nutrient-delivery proteins the secretion of uteroferrin is not responsive to the plasma levels of iron, thus it is speculated that the iron supply to the embryos during the peri-implantation period is determined by genotypes. retinol-binding protein is significantly associated with litter size in german landrace pigs. receptors for hb-egf, kgf, igf, and igf are all expressed by porcine embryos on gd studies have shown that igf promotes embryonic growth in response to the nutrient supply, while igf may regulate the supply of maternal nutrient to conceptus. the expression levels of the five genes and igf in ll versus er pigs were significantly up-regulated and down-regulated, respectively. vallet et al. reported that expressions of uf and rbp were lower in pregnant meishan endometrium than in white crossbred. the go molecular function classification showed that the differential genes were associated with growth. the expression patterns and the physiological functions of these genes indicated that the endometrium of er pigs had a lower growth-promoting ability to embryos than that of ll pigs. the above results can partially explain the phenomenon that embryos in uteri of taihu sows grow slower than those in the uteri of western sows. igf was expressed significantly higher in ll endometrium than in er endometrium. igf rather than igf is known to induce estrogen synthesis by stimulating expression of aromatase in the conceptus. aromatase is the rate-limiting enzyme in estrogen synthesis in the pig conceptus. therefore, it is very likely that er embryos secret less estrogen than ll embryos, which will contribute to the higher embryonic survival rate in er pigs. moreover, igf increases the permeability of the placenta in mice, and thus a higher level of igf in er endometrium may improve the placental efficiency. the genes in the prostaglandin synthetic pathway pg synthesis in endometrum, especially the pge2:pgf2α ratio, is crucial for the establishment and maintenance of pregnancy in pigs. the high pge2:pgf2α ratio may be a beneficial factor for large litter size in meishan sows. the expressions of the genes, ptgs1/ptgs pges/pges and cbr1/cbr play critical roles in the pg synthesis. ptgs and ptgs are rate-limiting enzymes in pg synthesis pathway. the expression level of ptgs is higher than that of ptgs in the endometrium of ll pigs on gd, which is consistent with other studies using western pigs; while the expression level of ptgs is lower than that of ptgs in er sows. in western sows, ptgs is the primary enzyme involved in elevated pg synthesis, whereas ptgs may perform this function in er sows according to our results. it has been demonstrated that both the mrna and the protein of ptgs have shorter half-lives than those of ptgs. hence, the higher ptgs expression can contribute to the larger capacity for pg synthesis in er pigs on gd the convert of pgh to pge in pg synthesis is catalyzed by ptges and ptgs. the higher expression of ptges and ptges in er endometrium is helpful for the higher pge which will be contribute to the higher ratio of pge to pgf2α on gd in western breeds, expression of cbr has been examined but cbr neglected. although the levels of endometrial cbr on gd and gd did not differ, the ratio of pge to pgf2α on gd was higher than that on gd. cbr may play a role in the conversion of pge into pgf2α according to our results and kegg pathway, and that higher expression of cbr may decrease the ratio of pge to pgf2α. in the present study, the patterns of expression of cbr and cbr observed in the two breeds suggest that the ratio in the endometrium of er pigs be greater than that in the endometrium of ll pigs on gd  in summary, we have described genes that are expressed differentially in the endometrium of er and ll pigs on gd compared with those in the ll pigs, the gene expression profiles in the endometrium of the prolific er pigs are found to benefit for the establishment and maintenance of pregnancy, delay embryonic development and growth, and enhance uterine capacity via reduced estrogen secretion. the gene-driven events that are characteristic of er pigs could contribute to the lower embryonic mortality and higher prolificacy of this indigenous chinese taihu breed. the data provided by this study will be useful for porcine transcriptomic studies. 
our results will increase understanding of the molecular mechanisms of the low rate of embryonic loss in chinese taihu pigs, facilitate the identification of major genes that affect litter size, and be valuable for porcine transcriptomic studies. cross-breeding experiments have demonstrated that taihu sows exhibit a strong maternal effect and that their large litter sizes are mainly caused by maternal genes. a lower prenatal death rate is the major contributing factor to the prolificacy of taihu pigs. embryonic loss of taihu pigs on gd is lower than that of western pigs. here, endometrial samples were collected from pregnant erhualian sows and landrace × large white sows on gd digital gene expression profiling was used to measure the gene expression in the endometrium of the two breeds. pathway analysis revealed that the differentially expressed genes significantly enriched in kegg pathways. the dge expression profiles of erhualian and ll endometrium demonstrated differential expression of genes. erhualian pigs, one of chinese taihu pig breeds, are known to have the largest recorded litter size in the world. gene ontology analysis showed that the differential genes involved in reproduction and growth. we also suggest that the differences in the uterine environment lead to higher uterine capacity in erhualian pigs. the growth and development of porcine embryos on gestation day are dependent on histotroph secreted by endometrium.
 erhualian pigs, one of chinese taihu pig breeds, are known to have the largest recorded litter size in the world. a lower prenatal death rate is the major contributing factor to the prolificacy of taihu pigs. cross-breeding experiments have demonstrated that taihu sows exhibit a strong maternal effect and that their large litter sizes are mainly caused by maternal genes. the growth and development of porcine embryos on gestation day are dependent on histotroph secreted by endometrium. embryonic loss of taihu pigs on gd is lower than that of western pigs. here, endometrial samples were collected from pregnant erhualian sows and landrace × large white sows on gd digital gene expression profiling was used to measure the gene expression in the endometrium of the two breeds.  a total of genes were differentially expressed between the two breeds. gene ontology analysis showed that the differential genes involved in reproduction and growth. pathway analysis revealed that the differentially expressed genes significantly enriched in kegg pathways. quantitative real-time rt-pcr confirmed the differential expression of eight genes. analyses of the differentially expressed genes suggested possible reasons for the difference in embryonic survival ratio between the two breeds. specifically, these findings point to a higher ratio of pge2:pgf2α in the endometrium of erhualian pigs, which facilitates the establishment and maintenance of pregnancy. we also suggest that the differences in the uterine environment lead to higher uterine capacity in erhualian pigs.  the dge expression profiles of erhualian and ll endometrium demonstrated differential expression of genes. our results will increase understanding of the molecular mechanisms of the low rate of embryonic loss in chinese taihu pigs, facilitate the identification of major genes that affect litter size, and be valuable for porcine transcriptomic studies. 