Abstract
Among tetrapods, the well differentiated heteromorphic sex chromosomes of birds and mammals have been highly investigated and their master sex-determining (MSD) gene, Dmrt1 and SRY, respectively, have been identified. The homomorphic sex chromosomes of reptiles have been the least studied, but the gap with birds and mammals has begun to fill. This review describes our current knowledge of reptilian sex chromosomes at the cytogenetic and molecular level. Most of it arose recently from various studies comparing male to female gene content. This includes restriction site-associated DNA sequencing (RAD-Seq) experiments in several male and female samples, RNA sequencing and identification of Z- or X-linked genes by male/female comparative transcriptome coverage, and male/female transcriptomic or transcriptome/genome substraction approaches allowing the identification of Y- or W-linked transcripts. A few putative master sex-determining (MSD) genes have been proposed, but none has been demonstrated yet. Lastly, future directions in the field of reptilian sex chromosomes and their MSD gene studies are considered.
Highlights
Sexual reproduction is ubiquitous in all vertebrates, and the formation and development of gonads, either testes or ovaries, is determined by the sequential action of members of the same genetic network
In Genetic Sex Determination (GSD), the bipotential gonad develops into either testis or ovary based on the control of a master sex-determining (MSD) gene upstream of the whole network
Genes with no SNPs in all five males were selected, and their chicken homologs mapped to the chicken genome. These genes were scattered with roughly the same density over all chicken chromosomes, except on the long arm of chicken chromosome 4 (4q) which is exceptionally enriched in such genes, suggesting this syntenic block is the homolog of the X-specific region of L. burtonis [200]. This hypothesis was confirmed by qPCR experiments, which demonstrated that genes homologous to chicken chromosome 4q genes are X-specific in L. burtonis and in L. jicari and three other Pygopodidae genera distributed throughout the Pygopodidae family
Summary
Sexual reproduction is ubiquitous in all vertebrates, and the formation and development of gonads, either testes or ovaries, is determined by the sequential action of members of the same genetic network. In Genetic Sex Determination (GSD), the bipotential gonad develops into either testis or ovary based on the control of a master sex-determining (MSD) gene upstream of the whole network This MSD gene is located on only one member of a special pair of non-identical chromosomes called sex chromosomes. ATRX is well known for its role in mammalian sexual differentiation, as demonstrated by the various degrees of gonadal dysgenesis observed in human XY mutated for ATRX, the most drastic phenotype being a complete male to female sex reversal [28] This list of species with identified MSD genes includes no reptilian species so far. Genetic manipulation of embryos was unavailable in reptiles, and as no reptiles have high economic value there was no financial interest to push the research in developing reptile-specific techniques All these reasons explain why the identification of MSD genes in reptiles still lags behind research in other groups.
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