Mammalian Sex Chromosomes Research Articles

Correction: No Evidence for a Second Evolutionary Stratum during the Early Evolution of Mammalian Sex Chromosomes

Correction: No Evidence for a Second Evolutionary Stratum during the Early Evolution of Mammalian Sex Chromosomes

The epigenetic sole of sex and dosage compensation

Sex chromosomes in mammals, birds, reptiles and fish represent many independent evolutionary events, but there is spectacular convergence and stunning parallels. A new study details the early stages of ZW differentiation and sex determination in a flatfish and the establishment of dosage compensation and sex reversal by epigenetic mechanisms including DNA methylation.

Sex-linked and autosomal microsatellites provide new insights into island populations of the tammar wallaby

The emerging availability of microsatellite markers from mammalian sex chromosomes provides opportunities to investigate both male- and female-mediated gene flow in wild populations, identifying patterns not apparent from the analysis of autosomal markers alone. Tammar wallabies (Macropus eugenii), once spread over the southern mainland, have been isolated on several islands off the Western Australian and South Australian coastlines for between 10,000 and 13,000 years. Here, we combine analyses of autosomal, Y-linked and X-linked microsatellite loci to investigate genetic variation in populations of this species on two islands (Kangaroo Island, South Australia and Garden Island, Western Australia). All measures of diversity were higher for the larger Kangaroo Island population, in which genetic variation was lowest at Y-linked markers and highest at autosomal markers (θ=3.291, 1.208 and 0.627 for autosomal, X-linked and Y-linked data, respectively). Greater relatedness among females than males provides evidence for male-biased dispersal in this population, while sex-linked markers identified genetic lineages not apparent from autosomal data alone. Overall genetic diversity in the Garden Island population was low, especially on the Y chromosome where most males shared a common haplotype, and we observed high levels of inbreeding and relatedness among individuals. Our findings highlight the utility of this approach for management actions, such as the selection of animals for translocation or captive breeding, and the ecological insights that may be gained by combining analyses of microsatellite markers on sex chromosomes with those derived from autosomes.

Detecting Evolutionary Strata on the Human X Chromosome in the Absence of Gametologous Y-Linked Sequences

Mammalian sex chromosomes arose from a pair of homologous autosomes that differentiated into the X and Y chromosomes following a series of recombination suppression events between the X and Y. The stepwise recombination suppressions from the distal long arm to the distal short arm of the chromosomes are reflected as regions with distinct X-Y divergence, referred to as evolutionary strata on the X. All current methods for stratum detection depend on X-Y comparisons but are severely limited by the paucity of X-Y gametologs. We have developed an integrative method that combines a top-down, recursive segmentation algorithm with a bottom-up, agglomerative clustering algorithm to decipher compositionally distinct regions on the X, which reflect regions of unique X-Y divergence. In application to human X chromosome, our method correctly classified a concatenated set of 35 previously assayed X-linked gene sequences by evolutionary strata. We then extended our analysis, applying this method to the entire sequence of the human X chromosome, in an effort to define stratum boundaries. The boundaries of more recently formed strata on X-added region, namely the fourth and fifth strata, have been defined by previous studies and are recapitulated with our method. The older strata, from the first up to the third stratum, have remained poorly resolved due to paucity of X-Y gametologs. By analyzing the entire X sequence, our method identified seven evolutionary strata in these ancient regions, where only three could previously be assayed, thus demonstrating the robustness of our method in detecting the evolutionary strata.

The Evolutionary Pathway of X Chromosome Inactivation in Mammals

X chromosome inactivation is a complex process that occurs in marsupial and eutherian mammals. The process is thought to have arisen during the differentiation of mammalian sex chromosomes to achieve an equal dosage of X chromosome genes in males and females. The differences in the X chromosome inactivation processes in marsupial and eutherian mammals are considered, and the hypotheses on its origin and evolution are discussed in this review.

Dimorphic Goat Amelogenin Alleles on Sex Chromosomes for Gender Determination of Preimplantation Embryos

 Abstract—Amelogenin (AMEL) is a conserved gene located on the sex chromosomes of mammals. It is involved in the formation of enamel, which is the hard, white material that forms the protective outer layer of each tooth. In this study, we first cloned the coding sequences of the goat AMELX and AMELY transcripts from female and male lamb enamel tissues during tooth development. All 207 amino acids of the putative AMEL proteins from the cDNA coding regions of both sex chromosomes were the same, but the AMEL sequences of the 3’-untranslated regions (UTR) were different. The results showed that the nucleotide sequences of intron 5 of the goat AMELX and AMELY genes contained multiple deletions/insertions and had only 48.5% identity. Based on the dimorphic AMEL intron sequences, a set of sex-specific primers was successfully applied to goat gender determination. A high sensitivity for sex determination was reached with a minimal amount of template, such as a trace amount of genomic DNA from a single blastomere isolated from embryos at the blastula stage among different strains of goats.

Kangaroo gene mapping and sequencing: insights into mammalian genome evolution

The deep divergence of marsupials and eutherian mammals 160 million years ago provides genetic variation to explore the evolution of DNA sequence, gene arrangement and regulation of gene expression in mammals. Following the pioneering work of Professor Desmond W. Cooper, emerging techniques in cytogenetics and molecular biology have been adapted to characterise the genomes of kangaroos and other marsupials. In particular, genetic and genomic work over four decades has shown that marsupial sex chromosomes differ significantly from the eutherian XY chromosome pair in their size, gene content and activity. These differences can be exploited to deduce how mammalian sex chromosomes, sex determination and epigenetic silencing evolved.

No Evidence for a Second Evolutionary Stratum during the Early Evolution of Mammalian Sex Chromosomes

Mammalian sex chromosomes originated from a pair of autosomes, and homologous genes on the sex chromosomes (gametologs) differentiated through recombination arrest between the chromosomes. It was hypothesized that this differentiation in eutherians took place in a stepwise fashion and left a footprint on the X chromosome termed “evolutionary strata.” The evolutionary stratum hypothesis claims that strata 1 and 2 (which correspond to the first two steps of chromosomal differentiation) were generated in the stem lineage of Theria or before the divergence between eutherians and marsupials. However, this prediction relied solely on the molecular clock hypothesis between pairs of human gametologs, and molecular evolution of marsupial sex chromosomal genes has not yet been investigated. In this study, we analyzed the following 7 pairs of marsupial gametologs, together with their eutherian orthologs that reside in stratum 1 or 2: SOX3/SRY, RBMX/Y, RPS4X/Y, HSFX/Y, XKRX/Y, SMCX/Y (KDM5C/D, JARID1C/D), and UBE1X/Y (UBA1/UBA1Y). Phylogenetic analyses and estimated divergence time of these gametologs reveal that they all differentiated at the same time in the therian ancestor. We have also provided strong evidence for gene conversion that occurred in the 3′ region of the eutherian stratum 2 genes (SMCX/Y and UBE1X/Y). The results of the present study show that (1) there is no compelling evidence for the second stratum in the stem lineage of Theria; (2) gene conversion, which may have occurred between SMCX/Y and UBE1X/Y in the eutherian lineage, potentially accounts for their apparently lower degree of overall divergence.

The Male-Specific Region of the Bovine Y Chromosome Is Enriched for Testis-Expressed Noncoding RNAs.

The mammalian sex chromosomes, the X and Y, evolved from a pair of autosomes ~ 166 million years ago. Recombination suppression between these two ancestral chromosomes led to a male-specific region on the Y (MSY). Two major regions, X-degenerate and ampliconic, are usually present in MSY. The X-degenerate region contains single-copy genes evolved as a consequence of the gradual Y degeneration, whereas the ampliconic region contains multi-copy genes derived from large expansions of X-degenerate genes or genes acquired from the autosomes. Our knowledge on the gene content and its transcriptional profile of the mammalian MSY is still limited to a few species including the human, chimpanzee and rhesus monkey. The objective of this study is to investigate the transcriptional activity in the bovine MSY region based on a direct cDNA selection and Next-Generation sequencing approach. A total of ~ 6.7 million pair-end reads were obtained, ~ 3.0 million of which were aligned with the bovine MSY sequence. The Y-specific read-pairs were de novo assembled into 1438 transcripts, 260 (18%) of which comprised an open reading frame (ORF) (= 90 amino acids) and were classified as coding transcripts. The coding transcripts contain 82 novel transcripts and 178 transcripts derived from 17 protein coding genes (families), including 12 X-degenerate genes and five ampliconic gene families. Motif analysis revealed that 36 novel coding transcripts have a signature associated with either reverse transcriptase or transposase, suggesting that the transposable elements are involved in gene regulation in testes. The remaining transcripts (1177, 82%) were non-coding RNAs (ncRNAs), which are bovine (bovid)-specific and mainly ampliconic with copy numbers ranging from ~80-240. Sequence comparisons against the ncRNA databases indicated that eight ncRNAs (0.7%) had orthologs (e-value < 1e-5) in the human or mouse genome and the majority (99.3%) of the ncRNAs was novel. Two of the eight known ncRNAs belong to Piwi-interacting RNA (piRNA) and the remaining six were long ncRNA (lncRNA). A total of 50 ncRNAs were randomly selected for temporal and spatial expression profiling by RT-PCR, and the results revealed that the majority (70%) of ncRNAs were predominantly expressed across different development stages of testes, suggesting a role in testis development and spermatogenesis. We believe that the identified coding and non-coding transcripts of the bovine MSY provide an important resource to further examine the regulatory network at distinct developmental stages of spermatogenesis. This project was supported by a USDA-NIFA grant no. 2010-65205-20362 to WSL.

Deep ancestry of mammalian X chromosome revealed by comparison with the basal tetrapod Xenopus tropicalis.

BackgroundThe X and Y sex chromosomes are conspicuous features of placental mammal genomes. Mammalian sex chromosomes arose from an ordinary pair of autosomes after the proto-Y acquired a male-determining gene and degenerated due to suppression of X-Y recombination. Analysis of earlier steps in X chromosome evolution has been hampered by the long interval between the origins of teleost and amniote lineages as well as scarcity of X chromosome orthologs in incomplete avian genome assemblies.ResultsThis study clarifies the genesis and remodelling of the Eutherian X chromosome by using a combination of sequence analysis, meiotic map information, and cytogenetic localization to compare amniote genome organization with that of the amphibian Xenopus tropicalis. Nearly all orthologs of human X genes localize to X. tropicalis chromosomes 2 and 8, consistent with an ancestral X-conserved region and a single X-added region precursor. This finding contradicts a previous hypothesis of three evolutionary strata in this region. Homologies between human, opossum, chicken and frog chromosomes suggest a single X-added region predecessor in therian mammals, corresponding to opossum chromosomes 4 and 7. A more ancient X-added ancestral region, currently extant as a major part of chicken chromosome 1, is likely to have been present in the progenitor of synapsids and sauropsids. Analysis of X chromosome gene content emphasizes conservation of single protein coding genes and the role of tandem arrays in formation of novel genes.ConclusionsChromosomal regions orthologous to Therian X chromosomes have been located in the genome of the frog X. tropicalis. These X chromosome ancestral components experienced a series of fusion and breakage events to give rise to avian autosomes and mammalian sex chromosomes. The early branching tetrapod X. tropicalis’ simple diploid genome and robust synteny to amniotes greatly enhances studies of vertebrate chromosome evolution.

Expression reduction in mammalian X chromosome evolution refutes Ohno’s hypothesis of dosage compensation

Susumu Ohno proposed in 1967 that, during the origin of mammalian sex chromosomes from a pair of autosomes, per-allele expression levels of X-linked genes were doubled to compensate for the degeneration of their Y homologs. This conjecture forms the foundation of the current evolutionary model of sex chromosome dosage compensation, but has been tested in mammals only indirectly via a comparison of expression levels between X-linked and autosomal genes in the same genome. The test results have been controversial, because examinations of different gene sets led to different conclusions that either support or refute Ohno's hypothesis. Here we resolve this uncertainty by directly comparing mammalian X-linked genes with their one-to-one orthologs in species that diverged before the origin of the mammalian sex chromosomes. Analyses of RNA sequencing data and proteomic data provide unambiguous evidence for expression halving (i.e., no change in per-allele expression level) of X-linked genes during evolution, with the exception of only ∼5% of genes that encode members of large protein complexes. We conclude that Ohno's hypothesis is rejected for the vast majority of genes, reopening the search for the evolutionary force driving the origin of chromosome-wide X inactivation in female mammals.

RNF12 initiates X-chromosome inactivation by targeting REX1 for degradation

Evolution of the mammalian sex chromosomes has resulted in a heterologous X and Y pair, where the Y chromosome has lost most of its genes. Hence, there is a need for X-linked gene dosage compensation between XY males and XX females. In placental mammals, this is achieved by random inactivation of one X chromosome in all female somatic cells. Upregulation of Xist transcription on the future inactive X chromosome acts against Tsix antisense transcription, and spreading of Xist RNA in cis triggers epigenetic changes leading to X-chromosome inactivation. Previously, we have shown that the X-encoded E3 ubiquitin ligase RNF12 is upregulated in differentiating mouse embryonic stem cells and activates Xist transcription and X-chromosome inactivation. Here we identify the pluripotency factor REX1 as a key target of RNF12 in the mechanism of X-chromosome inactivation. RNF12 causes ubiquitination and proteasomal degradation of REX1, and Rnf12 knockout embryonic stem cells show an increased level of REX1. Using chromatin immunoprecipitation sequencing, REX1 binding sites were detected in Xist and Tsix regulatory regions. Overexpression of REX1 in female embryonic stem cells was found to inhibit Xist transcription and X-chromosome inactivation, whereas male Rex1(+/-) embryonic stem cells showed ectopic X-chromosome inactivation. From this, we propose that RNF12 causes REX1 breakdown through dose-dependent catalysis, thereby representing an important pathway to initiate X-chromosome inactivation. Rex1 and Xist are present only in placental mammals, which points to co-evolution of these two genes and X-chromosome inactivation.

Evolution of Genomic Structures on Mammalian Sex Chromosomes

Throughout mammalian evolution, recombination between the two sex chromosomes was suppressed in a stepwise manner. It is thought that the suppression of recombination led to an accumulation of deleterious mutations and frequent genomic rearrangements on the Y chromosome. In this article, we review three evolutionary aspects related to genomic rearrangements and structures, such as inverted repeats (IRs) and palindromes (PDs), on the mammalian sex chromosomes. First, we describe the stepwise manner in which recombination between the X and Y chromosomes was suppressed in placental mammals and discuss a genomic rearrangement that might have led to the formation of present pseudoautosomal boundaries (PAB). Second, we describe ectopic gene conversion between the X and Y chromosomes, and propose possible molecular causes. Third, we focus on the evolutionary mode and timing of PD formation on the X and Y chromosomes. The sequence of the chimpanzee Y chromosome was recently published by two groups. Both groups suggest that rapid evolution of genomic structure occurred on the Y chromosome. Our re-analysis of the sequences confirmed the species-specific mode of human and chimpanzee Y chromosomal evolution. Finally, we present a general outlook regarding the rapid evolution of mammalian sex chromosomes.

Editorial [Hot Topic: Comparative Genomics and Genome Evolution (Guest Editors: Sabyasachi Das and Masayuki Hirano)

It has become a fundamental need to have genome sequence data due to the fact that the basic process of evolution is the change in DNA sequence and genome size. Recent advances in DNA sequencing technology have facilitated the availability of quite a large number of complete genome sequences from simplest prokaryotes to higher eukaryotes. The availability of whole genome sequence data at our fingertips can provide critical insight into how various genomic compositions have contributed to a contemporary understanding of molecular evolution. For this reason, comparative evolutionary genomics has become one of the most rapidly advancing disciplines in the biological sciences. The new generation of comparative genomics offers a powerful aid to studying evolutionary changes among organisms and identifying the genes that are conserved among species, and also the genes that give each organism its own specific characteristics. Informatics related to structural and functional genomics is potentially important in understanding the emergence of new phenotypic characters necessary for the adaptation of organisms. In addition, the evolutionary perspective of genes and genomes is also helpful in understanding disease susceptibility. A large number of scientists from all over the world are involved in determining the genomic underpinnings of morphological, physiological, and behavioral changes. A lot of research has been done to find selection signals on a genome-wide scale, and also focusing on specific gene families to learn more about evolutionary mechanisms. Therefore, it is necessary to compile various current aspects of genomic and evolutionary research at some point. This prompts us to devote a special issue we have named “Comparative Genomics and Genome Evolution”. The series of review articles in this issue of Current Genomics present current advances in multiple areas of comparative genomics and molecular evolutionary studies. This special issue is comprised of eight high quality articles which have been selected through a rigorous reviewing process. Several years ago, with the ground-breaking work of Max D. Cooper and colleagues, an adaptive immune system was discovered in a representative jawless vertebrate (lamprey). The first review article describes concordance and divergence in the immunogenetic architecture between two alternative adaptive immune systems of jawed and jawless vertebrates, and discusses in detail the evolution of the jawless vertebrate immune system on the basis of currently available lamprey genomic resources. In the second review paper, Nikolas Nikolaidis and colleagues discuss the current knowledge on the genomics and evolution of the immunoglobulin mulitigene family in vertebrates. To provide further coverage and discussion of the evolutionary dynamics of multigene families, we commissioned Yoshihito Niimura to review the evolutionary genomics of the olfactory receptor multigene family on a genome-wide scale. The fourth review article presents the interesting scenario of the evolution of mammalian sex chromosomes. Yoko Satta and colleagues reviewed evolutionary aspects related to genomic rearrangements and structures of the sex chromosomes. The next review article, by Arnab Gupta and Svetlana Lutsenko, focuses on the origin and evolution of copper transporting ATPases in eukaryotic organisms. In the sixth article, Mariko Kondo and Koji Akasaka highlight the phylogenetic relationships among echinoderms and the current status of echinoderm genome analysis. The seventh article is related to the evolution of microRNAs. In this article, Zhumur Ghosh and Bibekanand Mallick describe advances of genomics, evolution, and biogenesis of microRNAs. Finally, we conclude with an interesting review article by Chitra Dutta and Sandip Paul on microbial genome signature related to different lifestyles. The authors describe how closely related microbial species are diverged on the basis of the genomic signature of ecological kinship throughout microbial evolution. Recognizing the growing interest in evolutionary genomics, we are happy to present this collection of high quality review articles for this special issue. We would like to thank all of the reviewers for their valuable comments meant to improve the quality of articles. In addition, we offer special thanks to the Editor-in-Chief, and the Current Genomics editorial/production staff, who have contributed invaluably to this project. We hope you find the issue timely, scholarly, and interesting.

Human postmeiotic sex chromatin and its impact on sex chromosome evolution

Sex chromosome inactivation is essential epigenetic programming in male germ cells. However, it remains largely unclear how epigenetic silencing of sex chromosomes impacts the evolution of the mammalian genome. Here we demonstrate that male sex chromosome inactivation is highly conserved between humans and mice and has an impact on the genetic evolution of human sex chromosomes. We show that, in humans, sex chromosome inactivation established during meiosis is maintained into spermatids with the silent compartment postmeiotic sex chromatin (PMSC). Human PMSC is illuminated with epigenetic modifications such as trimethylated lysine 9 of histone H3 and heterochromatin proteins CBX1 and CBX3, which implicate a conserved mechanism underlying the maintenance of sex chromosome inactivation in mammals. Furthermore, our analyses suggest that male sex chromosome inactivation has impacted multiple aspects of the evolutionary history of mammalian sex chromosomes: amplification of copy number, retrotranspositions, acquisition of de novo genes, and acquisition of different expression profiles. Most strikingly, profiles of escape genes from postmeiotic silencing diverge significantly between humans and mice. Escape genes exhibit higher rates of amino acid changes compared with non-escape genes, suggesting that they are beneficial for reproductive fitness and may allow mammals to cope with conserved postmeiotic silencing during the evolutionary past. Taken together, we propose that the epigenetic silencing mechanism impacts the genetic evolution of sex chromosomes and contributed to speciation and reproductive diversity in mammals.

Identification of mediator complex 26 (Crsp7) gametologs on platypus X1 and Y5 sex chromosomes: a candidate testis-determining gene in monotremes?

The basal lineage of monotremes features an extraordinarily complex sex chromosome system which has provided novel insights into the evolution of mammalian sex chromosomes. Recently, sequence information from autosomes, X chromosomes, and XY-shared pseudoautosomal regions has become available. However, no gene has so far been described on any of the Y chromosome-specific regions. We analyzed sequences derived from Y-specific BAC clones to identify genes with potentially male-specific function. Here, we report the identification and characterization of the mediator complex protein gametologs on platypus Y5 (Crspy). We also identified the X-chromosomal copy which unexpectedly maps to X1 (Crspx). Sequence comparison shows extensive divergence between the X and Y copy, but we found no significant positive selection on either gametolog. Expression analysis shows widespread expression of Crspx. Crspy is expressed exclusively in males with particularly strong expression in testis and kidney. Reporter gene assays to investigate whether Crspx/y can act on the recently discovered mouse Sox9 testis-specific enhancer element did reveal a modest effect together with mouse Sox9 + Sf1, but showed overall no significant upregulation of the reporter gene. This is the first report of a differentiated functional male-specific gene on platypus Y chromosomes, providing new insights into sex chromosome evolution and a candidate gene for male-specific function in monotremes.

Comparative analysis of a plant pseudoautosomal region (PAR) in Silene latifolia with the corresponding S. vulgaris autosome

BackgroundThe sex chromosomes of Silene latifolia are heteromorphic as in mammals, with females being homogametic (XX) and males heterogametic (XY). While recombination occurs along the entire X chromosome in females, recombination between the X and Y chromosomes in males is restricted to the pseudoautosomal region (PAR). In the few mammals so far studied, PARs are often characterized by elevated recombination and mutation rates and high GC content compared with the rest of the genome. However, PARs have not been studied in plants until now. In this paper we report the construction of a BAC library for S. latifolia and the first analysis of a > 100 kb fragment of a S. latifolia PAR that we compare to the homologous autosomal region in the closely related gynodioecious species S. vulgaris.ResultsSix new sex-linked genes were identified in the S. latifolia PAR, together with numerous transposable elements. The same genes were found on the S. vulgaris autosomal segment, with no enlargement of the predicted coding sequences in S. latifolia. Intergenic regions were on average 1.6 times longer in S. latifolia than in S. vulgaris, mainly as a consequence of the insertion of transposable elements. The GC content did not differ significantly between the PAR region in S. latifolia and the corresponding autosomal region in S. vulgaris.ConclusionsOur results demonstrate the usefulness of the BAC library developed here for the analysis of plant sex chromosomes and indicate that the PAR in the evolutionarily young S. latifolia sex chromosomes has diverged from the corresponding autosomal region in the gynodioecious S. vulgaris mainly with respect to the insertion of transposable elements. Gene order between the PAR and autosomal region investigated is conserved, and the PAR does not have the high GC content observed in evolutionarily much older mammalian sex chromosomes.

Association of Sly with sex-linked gene amplification during mouse evolution: a side effect of genomic conflict in spermatids?

In common with other mammalian sex chromosomes, the mouse sex chromosomes are enriched for genes with male-specific function such as testis genes. However, in mouse there has been an unprecedented expansion of ampliconic sequence containing spermatid-expressed genes. We show via a phylogenetic analysis of gene amplification on the mouse sex chromosomes that multiple families of sex-linked spermatid-expressed genes are highly amplified in Mus musculus subspecies and in two further species from the Palaearctic clade of mouse species. Ampliconic X-linked genes expressed in other cell types showed a different evolutionary trajectory, without the distinctive simultaneous amplification seen in spermatid-expressed genes. The Palaearctic gene amplification occurred concurrently with the appearance of Sly, a Yq-linked regulator of post-meiotic sex chromatin (PMSC) which acts to repress sex chromosome transcription in spermatids. Despite the gene amplification, there was comparatively little effect on transcript abundance, suggesting that the genes in question became amplified in order to overcome Sly-mediated transcriptional repression and maintain steady expression levels in spermatids. Together with the known sex-ratio effects of Yq/Sly deficiency, our results suggest that Sly is involved in a genomic conflict with one or more X-linked sex-ratio distorter genes. The recent evolution of the novel PMSC regulator Sly in mouse lineages has significant implications for the use of mouse-model systems in investigating sex chromosome dynamics in spermatids.

Identification of sex-specific polymorphic sequences in the goat amelogenin gene for embryo sexing1

Amelogenin (AMEL) is a conserved gene located on the sex chromosomes of mammals. It is involved in the formation of enamel, which is the hard, white material that forms the protective outer layer of each tooth. In this study, we first cloned and determined the intron sequences of the goat AMELX and AMELY genes from female and male ear tissues. The polymorphic AMEL alleles were further analyzed by PCR-based RFLP and Southern blot hybridization analyses. Results showed that intron 5 nucleotide sequences of the goat AMELY gene contains multiple deletions/insertions and shares only 48.5% identity to intron 5 of the goat AMELX gene. Based on the polymorphic AMEL intron sequences, a set of sex-specific triplex primers was designed to PCR amplify a single fragment of 264 bp from the X chromosome of female goats and 2 fragments of 264 and 206 bp from the X and Y chromosomes, respectively, of male goats. An increased sensitivity for sex determination was reached with a single blastomere at the blastula stage isolated from goat embryos. A total of 43 goat embryos were used to estimate a 100% accuracy rate of this method confirmed by chromosomal karyotyping and live births. The embryo sexing technique has been successfully applied in different strains of goats including Alpine, Saanen, Nubian, and Taiwan goats.

Hitting a Tiny Target in the Dark

A region in mammalian sex chromosomes has distinct properties that ensure proper segregation during gamete development.