Genetic control of multiple births in low ovulating mammalian species

In mammals, litter size is a highly variable trait. Some species such as humans or cattle are monotocous, with one or sometimes two newborns per birth, whereas others, the polytocous species such as mice or pigs, are highly prolific and often produce a dozen newborns at each farrowing. In monotocous species, however, two or three newborns per birth may sometime be unwanted. In more polytocous species such as sheep or pigs, litter size is studied in order to increase livestock prolificacy. By contrast, twinning rates in humans or cattle may increase birth difficulties and health problems in the newborns. In this context, the aim of our review was to provide a clearer understanding of the genetic and physiological factors that control multiple births in low-ovulating mammalian species, with particular focus on three species: sheep, cattle, and humans, where knowledge of the ovulation rate in one may enlighten findings in the others. This article therefore reviews the phenotypic and genetic variability observed with respect to ovulation and twinning rates. It then presents the QTL and major genes that have been identified in each species. Finally, we draw a picture of the diversity of the physiological mechanisms underlying multiple ovulation. Although several major genes have been discovered in sheep, QTL detection methods in humans or cattle have suggested that the determinism of litter size is complex and probably involves several genes in order to explain variations in the number of ovulations.     

Experimental demonstration that pre- and post-conceptional mechanisms influence sex ratio in mouse embryos

Previously we have demonstrated in two monotocous species (bovine and sheep), a relationship between time of insemination, moment of ovulation, and embryo sex ratio. Here, we have analyzed in a polytocous specie (mice) if in addition to pre-conceptional mechanisms, also post-conceptional ones affect the offspring sex ratio. To verify this hypothesis we carried out two experiments. In the first experiment, we analyzed the effect of mating dynamics on the sex ratio of mice with synchronic male and female embryo development. Females were mated before and after ovulation and sacrificed 13 days later for sex determination of embryos and reabsorptions. A decreased litter size, and an increased offspring sex ratio in matings occurring later in oestrus, supported the view that a biased sex ratio may occur as the result of behavioral differences between the populations of X- or Y-bearing spermatozoa. In the second experiment, embryos developmentally synchronic and asynchronic with the recipient female endometrium were transferred, and again, 13 days later, females were sacrificed for sex determination of embryos and reabsorptions. The male proportion per litter found, indicated that our developmentally asynchronic transfers favored a sex ratio disbalance at birth. When combined, these results become the first experimental evidence supporting the view that both pre- and post-conceptional mechanisms of sex ratio distortion in polytocous species are not mutually exclusive and both may explain, under different conditions, sex ratio deviations at birth.     

A genome‐wide linkage analysis for reproductive traits in F2 Large White × Meishan cross gilts

Female reproductive performance traits in pigs have low heritabilities thus limiting improvement through traditional selective breeding programmes. However, there is substantial genetic variation found between pig breeds with the Chinese Meishan being one of the most prolific pig breeds known. In this study, three cohorts of Large White × Meishan F₂ cross‐bred pigs were analysed to identify quantitative trait loci (QTL) with effects on reproductive traits, including ovulation rate, teat number, litter size, total born alive and prenatal survival. A total of 307 individuals were genotyped for 174 genetic markers across the genome. The genome‐wide analysis of the trait‐recorded F₂ gilts in their first parity/litter revealed one QTL for teat number significant at the genome level and a total of 12 QTL, which are significant at the chromosome‐wide level, for: litter size (three QTL), total born alive (two QTL), ovulation rate (four QTL), prenatal survival (one QTL) and teat number (two QTL). Further support for eight of these QTL is provided by results from other studies. Four of these 12 QTL were mapped for the first time in this study: on SSC15 for ovulation rate and on SSC18 for teat number, ovulation rate and litter size.     

Proteolytic degradation of insulin-like growth factor binding proteins by ovarian follicles: a control mechanism for selection of dominant follicles

This review summarizes evidence for the role of proteolytic enzymes that degrade and inactivate insulin-like growth factor binding proteins (IGFBP) during follicular development in mammals. In some species (e.g., bovine), evidence indicates that decreases in IGFBP-4 and -5 levels in estrogen-dominant preovulatory follicles are likely due, in part, to increased protease activity, whereas lower levels of IGFBP-2 are not due to increased proteolysis. Increased IGFBP-4 and -5 protease along with lower amounts of IGFBP-4 binding activity and greater amounts of free IGF-I are some of the earliest developmental changes documented in bovine growing antral follicles. This protease activity has recently been ascribed to serine metalloprotease(s), including pregnancy-associated plasma protein-A (PAPP-A), which was first detected in human follicular fluid nearly 20 yr ago. Other recent studies verified the presence of PAPP-A mRNA in granulosa cells of humans, monkeys, cattle, mice, and pigs. Increases in the amount of PAPP-A mRNA in granulosa cells during follicular development occurs in some but not all species, indicating that other proteases or protease inhibitors may be involved in IGFBP degradation. Whether the hormonal control of PAPP-A production/activity by the ovary differs between monotocous and polytocous animals will require further study. These protease-induced decreases in IGFBP-4 and -5 likely cause increased levels of bioavailable (or free) IGFs that stimulate steroidogenesis and mitogenesis in developing dominant follicles, which ultimately prepare the follicle(s) and oocyte(s) for successful ovulation and fertilization.     

绵羊产羔性状主效基因检测研究

以绵羊BMP15基因和BMPR-IB基因为候选基因,以湖羊、中国美利奴单胎品系、中国美利奴肉用和毛用多胎品系为研究对象,采用PCR-RFLP方法对候选基因进行单核苷酸多态性（SNP）位点检测和基因型分析,同时研究基因对绵羊产羔数的影响。对BMP15基因进行SNP检测,结果未发现多态性位点;对BMPR-IB基因进行多态性检测,结果发现了一个A746 G SNP位点。依据A746 G SNP位点进行基因型分析,结果在各品种（系）羊中发现了3种基因型,即BB、B+和++。等位基因型频率在各品种（系）间差异极显著（P<0.001）,在湖羊中以BB基因型为主,在中国美利奴单胎品系中以++基因型为主, 而在中国美利奴肉用和毛用多胎品系中以B+基因型为主。BMPR-IB A746G位点的变异明显影响绵羊的产羔数,与++基因型母羊相比, BB和B+基因型母羊产羔数明显较多。研究结果同时表明,利用BMPR-IB基因型可以很好的预测母羊的产羔数。研究获得的这些结果强烈表明BMPR-IB为影响绵羊的产羔数的主效基因,可以用于对绵羊产羔数的选择。Abstract: The current study was designed to detect SNPs within BMP15 and BMPR-IB gene and investigate the effect of the genes on sheep litter size. Four sheep lines, HU-Yang, Chinese M erino monotocous, Chinese Merino multiparous for wool production and Chinese Merino multiparous for mutton production, were used in this study. Litter sizes were recorded for each ewe in the four lines. Primers for BMP15 and BMPR-IB gene were designed from database sheep sequence and polymorphisms were detected by PCR-RFLP method. The results showed that there was no polymorphism with BMP15 gene among the four lines, and there was an A / G SNP with BMPR-IB gene at base 746 among the four lines. Three types of genotype (BB, B+ and ++), based on A / G locus, were found within each line. The frequencies of genotypes were significantly different among the lines (P<0.001), with BB genotype primarily existing in HU-Yang, ++ genotype in Chinese Merino monotocous line, and B+ genotype in Chinnese Merino multiparous lines. The A / G mutation influence significantly the sheep litter sizes, and the BB and B+ ewes had significant higher litter sizes than ++ ewes. The results of present study showed simultaneously that the genotype of BMPR-IB was a perfect predictor of the sheep litter sizes. These results intensively indicated that BMPR-IB is a major gene to affect litter size in sheep, and could be used as the molecular genetic marker to select litter size in sheep.     

Identification of Prolificacy‐Related Differentially Expressed Proteins from Sheep (Ovis aries) Hypothalamus by Comparative Proteomics

Reproduction, as a physiologically complex process, can significantly affect the development of the sheep industry. However, a lack of overall understanding to sheep fecundity has long blocked the progress in sheep breeding and husbandry. In the present study, the aim is to identify differentially expressed proteins (DEPs) from hypothalamus in sheep without FecB mutation in two comparison groups: polytocous (PF) versus monotocous (MF) sheep at follicular phase and polytocous (PL) versus monotocous (ML) sheep at luteal phase. Totally 5058 proteins are identified in sheep hypothalamus, where 22 in PF versus MF, and 39 proteins in PL versus ML are differentially expressed, respectively. A functional analysis is then conducted including Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analysis to reveal the potential roles of these DEPs. The proteins ENSOARP00000020097, ENSOARP00000006714, growth hormone (GH), histone deacetylase 4 (HDAC4), and 5′‐3′ exoribonuclease 2 (XRN2) in PF versus MF, and bcl‐2‐associated athanogene 4 (BAG4), insulin‐like growth factor‐1 receptor (IGF1R), hydroxysteroid 11‐beta dehydrogenase 1 (HSD11B1), and transthyretin (TTR) in PL versus ML appear to modulate reproduction, presumably by influencing the activities of gonadotropin‐releasing hormone (GnRH). This study provides an alternative method to identify DEPs associated with sheep prolificacy from the hypothalamus. The mass spectrometry data are available via ProteomeXchange with identifier PXD013822.     

Mapping quantitative trait loci affecting female reproductive traits on porcine chromosome 8

An understanding of the genetic control of porcine female reproductive performance would offer the opportunity to utilize natural variation and improve selective breeding programs through marker-assisted selection. The Chinese Meishan is one of the most prolific pig breeds known, farrowing three to five more viable piglets per litter than the European Large White breed. This difference in prolificacy is attributed to the Meishan's superior prenatal survival levels. The present study utilized a three-generation cross in which the founder grandparental animals were purebred Meishan and Large White pigs in a scan for quantitative trait loci (QTL) on porcine chromosome 8 (SSC8) associated with reproductive performance. Reproductive traits, including number of corpora lutea (ovulation rate), teat number, litter size, and prenatal survival, were recorded for as many as 220 F2 females. Putative QTL for the related traits of litter size and prenatal survival were identified at the distal end of the long arm of SSC8. A physiological candidate gene, SPP1, was found to lie within the 95% confidence interval of these QTL. A suggestive QTL for teat number was revealed on the short arm of SSC8. The present study demonstrates, to our knowledge, the first independent confirmation of QTL for fecundity on SSC8, and these QTL regions provide a crucial starting point in the search for the causal genetic variants.     

Systems genetics approach reveals candidate genes for parasite resistance from quantitative trait loci studies in agricultural species

A systems genetics approach combining pathway analysis of quantitative trait loci (QTL) and gene expression information has provided strong evidence for common pathways associated with genetic resistance to internal parasites. Gene data, collected from published QTL regions in sheep, cattle, mice, rats and humans, and microarray data from sheep, were converted to human Entrez Gene IDs and compared to the KEGG pathway database. Selection of pathways from QTL data was based on a selection index that ensured that the selected pathways were in all species and the majority of the projects overall and within species. Pathways with either up- and down-regulated genes, primarily up-regulated genes or primarily down-regulated genes, were selected from gene expression data. After comparing the data sets independently, the pathways from each data set were compared and the common set of pathways and genes was identified. Comparisons within data sets identified 21 pathways from QTL data and 66 pathways from gene expression data. Both selected sets were enriched with pathways involved in immune functions, disease and cell responses to signals. The analysis identified 14 pathways that were common between QTL and gene expression data, and four directly associated with IFNγ or MHCII, with 31 common genes, including three MHCII genes. In conclusion, a systems genetics approach combining data from multiple QTL and gene expression projects led to the discovery of common pathways associated with genetic resistance to internal parasites. This systems genetics approach may prove significant for the discovery of candidate genes for many other multifactorial, economically important traits.     

Mutations in the genes for oocyte-derived growth factors GDF9 and BMP15 are associated with both increased ovulation rate and sterility in Cambridge and Belclare sheep (Ovis aries)

Belclare and Cambridge are prolific sheep breeds, the origins of which involved selecting ewes with exceptionally high litter size records from commercial flocks. The variation in ovulation rate in both breeds is consistent with segregation of a gene (or genes) with a large effect on this trait. Sterile ewes, due to a failure of normal ovarian follicle development, occur in both breeds. New naturally occurring mutations in genes for the oocyte-derived growth factors growth differentiation factor 9 (GDF9) and bone morphogenetic protein 15 (BMP15) are described. These mutations are associated with increased ovulation rate in heterozygous carriers and sterility in homozygous carriers in both breeds. This is the first time that a mutation in the gene for GDF9 has been found that causes increased ovulation rate and infertility in a manner similar to inactivating mutations in BMP15, and shows that GDF9 is essential for normal folliculogenesis in sheep. Furthermore, it is shown, for the first time in any species, that individuals with mutations in both GDF9 and BMP15 have a greater ovulation rate than sheep with either of the mutations separately.     

Genome Assembly Anchored QTL Map of Bovine Chromosome 14

Bovine chromosome 14 (BTA14) has been widely explored for quantitative trait loci (QTL) and genes related to economically important traits in both dairy and beef cattle. We reviewed more than 40 investigations and anchored 126 QTL to the current genome assembly (Btau 4_0). Using this anchored QTL map, we observed that, in dairy cattle, the region spanning 0 – 10 Mb on BTA14 has the highest density QTL map with a total of 56 QTL, mainly for milk production traits. It is very likely that both somatic cell score (SCS) and clinical mastitis share some common QTL in two regions: 61.48 Mb - 73.84 Mb and 7.86 Mb – 39.55 Mb, respectively. As well, both ovulation rate and twinning rate might share a common QTL region from 34.16 Mb to 65.38 Mb. However, there are no common QTL locations in three pregnancy related phenotypes: non-return rate, pregnancy rate and daughter pregnancy rate. In beef cattle, the majority of QTL are located in a broad region of 15 Mb – 45 Mb on the chromosome. Functional genes, such as CRH, CYP11B1, DGAT1, FABP4 and TG, as potential candidates for some of these QTL, were also reviewed. Therefore, our review provides a standardized QTL map anchored within the current genome assembly, which would enhance the process of selecting positional and physiological candidate genes for many important traits in cattle.     

Adaptive evolution of mammalian aromatases: lessons from Suiformes

Estrogen synthesis evolved in chordates to control reproduction. The terminal enzyme in the cascade directly responsible for estrogen synthesis is aromatase cytochrome P450 (P450arom) encoded by the CYP19 gene. Mammals typically have a single CYP19 gene but pigs, peccaries and other Suiformes have two or more resulting from duplication in a common ancestor. Duplication of CYP genes in the steroid synthetic cascade has occurred for only one other enzyme, also terminal, 11beta-hydroxylase P450 (P450c11). P450arom and P450c11 share common substrates and even physiological functions as possible remnants from a common P450 progenitor, perhaps an ancestral P450arom, which is supported by phylogenetic analysis. Conserved tissue-specific expression patterns of P450arom paralogs in placenta and gonads of pigs and peccaries suggest how functional adaptation may have proceeded divergently and influenced adopted reproductive strategies including ovulation rate and litter size. Data suggest that the porcine placental paralog evolved catalytically to protect female conceptuses from testosterone produced by male siblings; the gonadal paralog to synthesize a novel, nonaromatizable testosterone metabolite (1OH-testosterone) that may increase ovulation rate. This would represent a coevolution facilitating litter bearing as pigs diverged from peccaries. Evidence of convergence between the peccary CYP19 genes and lower tissue expression may therefore represent initiation of loss of the functional paralogs. Studies on the Suiforme aromatases provide insights into the evolution of the steroidogenic cascade and metabolic pathways in general, how it translates into physiological adaptations (altered reproductive strategies for instance), and how duplicated genes become stabilized or disappear from genomes.     

Biological and Environmental Influences on Parturition Date and Birth Mass of a Seasonal Breeder

Natal features (e.g. Julian birth date and birth mass) often have fitness consequences and can be influenced by endogenous responses by the mother to seasonal fluctuations in nutritional quality and photoperiodic cues. We sought to further understand the biological and environmental factors that influence the natal features of a polytocous species in an environment with constant nutritional resources and limited seasonal variation. During a 36-year study we assessed the influence of biological factors (maternal age and litter type [i.e., litter size and sexual composition]) and environmental factors (total precipitation and mean maximum temperature during months encompassing conception, the last trimester of gestation, and the entire length of gestation) on Julian birth date and birth mass using linear-mixed effects models. Linear and quadratic functions of maternal age influenced both natal features with earliest Julian birth dates and heaviest birth masses occurring at prime-age and older individuals, which ranged from 5–9 years of age. Litter type influenced Julian birth date and birth mass. Interestingly, environmental factors affected Julian birth date and birth mass even though mothers were continuously allowed access to a high-quality diet. Random effects revealed considerable variation among mothers and years. This study demonstrates that, in long-lived polytocous species, environmental factors may have a greater influence on natal features than previously supposed and the influence from biological factors is also complex. The documented responses to environmental influences provide unique insights into how mammalian seasonal reproductive dynamics may respond to current changes in climate.     

Swine generated by somatic cell nuclear transfer have increased incidence of intrauterine growth restriction (IUGR)

While somatic cell nuclear transfer (SCNT) has been successful in several species, many pregnancies are lost and anomalies are found in fetal and perinatal stages. In this study SCNT and artificial inseminations (AI) populations were compared for litter size, average birth weight, piglets alive at birth, stillborn, mummies, dead at the first week, intrauterine growth restriction (IUGR) and large for gestational age (LGA). Twenty-three SCNT litters (143 individuals) were compared to 112 AI litters (1300 individuals). Litter size average was 11.5 for AI and 6.2 for SCNT. Litter weight and average birth weight adjusted by litter size were significantly (p < 0.05) higher in AI than in SCNT litters. The SCNT population had a significant (p < 0.01) increase in the number of IUGRs per litter with LSmeans 7.2 +/- 1.4 versus 19.4 +/- 3.5 and means 8.0 +/- 10.8 versus 15.5 +/- 24.5 for AI and SCNT, respectively. Additionally, there was a trend for higher postnatal mortality and stillbirths in the SCNT population. These findings demonstrate that there are some differences between SCNT-derived and AI litters. SCNT-derived pigs are excellent models to study epigenetic factors and genes involved in IUGRs, and to develop effective means to improve fetal growth in humans and animals.     

Comparative mapping of bovine chromosome 27 with human chromosome 8 near a dairy form QTL in cattle

In the absence of a complete and annotated bovine genome sequence, detailed human-bovine comparative maps are one of the most effective tools for identification of positional candidate genes contributing to quantitative trait loci (QTL) in cattle. In the present study, eight genes from human chromosome 8 were selected for mapping in cattle to improve breakpoint resolution and confirm gene order on the comparative map near the 40 cM region of the BTA27 linkage map where a QTL affecting dairy form had previously been identified. The resulting map identified ADRB3 as a positional candidate gene for the QTL contributing to the dairy form trait based on its estimated position between 40 and 45 cM on the linkage map. It is also a functional candidate gene due to its role in fat metabolism, and polymorphisms in the ADRB3 gene associated with obesity and metabolic disease in humans, as well as, carcass fat in sheep. Further studies are underway to investigate the existence of polymorphisms in the bovine ADRB3 gene and their association with traits related to fat deposition in cattle.     

Combined approaches to reveal genes associated with litter size in Yunshang black goats

Intensive artificial selection has been imposed in Yunshang black goats, the first black specialist mutton goat breed in China, with a breeding object of improving reproductive performance, which has contributed to reshaping of the genome including the characterization of SNP, ROH and haplotype. However, variation in reproductive ability exists in the present population. A WGS was implemented in two subpopulations (polytocous group, PG, and monotocous group, MG) with evident differences of litter size. Following the mapping to reference genome, and SNP calling and pruning, three approaches – GWAS, ROH analysis and detection of signatures of selection – were employed to unveil candidate genes responsible for litter size. Consequently, 12 candidate genes containing OSBPL8 with the minimum P-value were uncovered by GWAS. Differences were observed in the pattern of ROH between two subpopulations that shared similar low inbreeding coefficients. Two ROH hotspots and 12 corresponding genes emerged from ROH pool association analysis. Based on the nS_L statistic, 15 and 61 promising genes were disclosed under selection for MG and PG respectively. Of them, some promising genes participate in ovarian function (PPP2R5C, CDC25A, ESR1, RPS26 and SERPINBs), seasonal reproduction (DIO3, BTG1 and CRYM) and metabolism (OSBPL8, SLC39A5 and SERPINBs). Our study pinpointed some novel promising genes influencing litter size, provided a comprehensive insight into genetic makeup of litter size and might facilitate selective breeding in goats.     

Genome‐wide association study for reproductive traits in a Large White pig population

Using the PorcineSNP80 BeadChip, we performed a genome‐wide association study for seven reproductive traits, including total number born, number born alive, litter birth weight, average birth weight, gestation length, age at first service and age at first farrowing, in a population of 1207 Large White pigs. In total, we detected 12 genome‐wide significant and 41 suggestive significant SNPs associated with six reproductive traits. The proportion of phenotypic variance explained by all significant SNPs for each trait ranged from 4.46% (number born alive) to 11.49% (gestation length). Among them, 29 significant SNPs were located within known QTL regions for swine reproductive traits, such as corpus luteum number, stillborn number and litter size, of which one QTL region associated with litter size contained the ALGA0098819 SNP for total number born. Subsequently, we found that 376 functional genes contained or were near these significant SNPs. Of these, 14 genes—BHLHA15, OCM2, IL1B2, GCK, SMAD2, HABP2, PAQR5, GRB10, PRELID2, DMKN, GPI, GPIHBP1, ADCY2 and ACVR2B—were considered important candidates for swine reproductive traits based on their critical roles in embryonic development, energy metabolism and growth development. Our findings contribute to the understanding of the genetic mechanisms for reproductive traits and could have a positive effect on pig breeding programs.     

Genetic variation in the mannosidase 2B2 gene and its association with ovulation rate in pigs

Ovulation rate is an important phenotypic trait that is a critical component of litter size in pigs. Despite being moderately heritable in pigs, selection for increased ovulation rate is difficult because it is difficult to measure and is a sex-limited trait. A QTL for ovulation rate residing on the p-terminal end of pig chromosome 8 has been detected in a Meishan-cross resource population. Comparative analysis of this region yielded a positional candidate gene, mannosidase 2B2 (MAN2B2), for this QTL. The entire coding region of MAN2B2 was resequenced in the Meishan and White Composite founder animals of the resource population to identify SNPs. Eleven polymorphisms that alter the protein product of MAN2B2 were discovered and tested for statistical associations with ovulation rate in three generations of the resource population. The polymorphism located at position 1574 of the mRNA (D28521:c.1574A>G) was the most significant polymorphism tested (P = 0.00005) where the additive effect of the c.1574A allele was estimated to be -0.89 ova. This polymorphism was determined to be more significantly associated with ovulation rate than the breed-specific analysis conducted during the line-cross QTL discovery. The c.1574A>G marker was not associated with ovulation rate in an occidental population. Therefore, either MAN2B2 has a unique epistatic interaction within the Meishan-cross population or the c.1574A>G SNP is in linkage disequilibrium with the actual causative genetic variation in the Meishan-cross population.     

影响二花脸猪高产仔性能的生理及遗传机制研究进展

二花脸猪是我国太湖流域以产仔数高而闻名于世界的优质猪种。已有的研究发现,二花脸猪排卵数高、胚胎死亡率低和子宫容积大是其高产性能的重要生理特征,通过候选基因法鉴别到其高产的重要候选基因——促卵泡素β亚基(follicular-stimulating hormone beta, FSH?),采用QTL定位方法在猪第2、6、7、8、12、13和15号染色体上定位到影响其产仔性状的QTL。但是,目前影响二花脸猪产仔数的主效基因尚不清楚。本文主要介绍了二花脸猪的高排卵数、低胚胎死亡率和高子宫容积特性对高产仔性能形成的生理作用;概括了基于传统方法鉴别出的影响二花脸猪高产仔性能的候选基因和QTL,以及基于组学方法鉴别出的二花脸猪高产候选基因;探讨了如何利用基因组、转录组、蛋白质组和功能组等多组学手段深入研究二花脸猪高产性能的成因,以期为深入解析二花脸猪高产性能的分子遗传机制提供参考。     

Fec基因及BMPR-IB基因的突变特性与生物学意义

FecB基因位于Booroola绵羊的常染色体上,具有提高排卵率和产羔数等生物学作用。FecB基因已被定位在绵羊6号染色体6q23~q31的狭窄区域内,并且已从分子水平上找到了控制Booroola绵羊排卵数的主效基因。本文详细阐述了近年来对FecB基因的定位及分子生物学作用机制方面的研究进展。     

Improving the safety of embryo technologies: possible role of genomic imprinting

Although developments in mammalian in vitro embryo technologies have allowed many new clinical and agricultural achievements, their application has been hindered by limitations in the developmental potential of resulting embryos. Low efficiencies of development to the pre-implantation blastocyst stage have been consistently observed in most species, including humans, rabbits, pigs and ruminants. Furthermore, in cattle and sheep a wide range of congenital abnormalities currently termed "Large Offspring syndrome" (LOS) are commonly observed as a result of several embryo culture and manipulation procedures. This paper reviews the hypothesis that at least some of the problems associated with embryo technologies may result from disruptions in imprinted genes. Several imprinted genes (i.e. genes which express only the maternal or paternal allele) are known to have significant effects on fetal size and survival in other species and are possible candidates for involvement in livestock LOS. Major changes in putative imprinting mechanisms such as DNA methylation of imprinted genes occur in the mouse embryo during pre-implantation development. Alterations in DNA methylation are stabley transmitted through repeated cell cycles such that changes in the embryo may still act at the fetal stages. Thus any disruption in establishment and/or maintenance of imprinting during the vulnerable periods of embryo culture or manipulation is a plausible candidate mechanism for inducing fetal loss and Large Offspring Syndrome. Identification of these disruptions may provide crucial means to improve the success of current procedures.