Chromosome map of the Siamese cobra: did partial synteny of sex chromosomes in the amniote represent “a hypothetical ancestral super-sex chromosome” or random distribution?

Abstract

BackgroundUnlike the chromosome constitution of most snakes (2n=36), the cobra karyotype shows a diploid chromosome number of 38 with a highly heterochromatic W chromosome and a large morphologically different chromosome 2. To investigate the process of sex chromosome differentiation and evolution between cobras, most snakes, and other amniotes, we constructed a chromosome map of the Siamese cobra (Naja kaouthia) with 43 bacterial artificial chromosomes (BACs) derived from the chicken and zebra finch libraries using the fluorescence in situ hybridization (FISH) technique, and compared it with those of the chicken, the zebra finch, and other amniotes.ResultsWe produced a detailed chromosome map of the Siamese cobra genome, focusing on chromosome 2 and sex chromosomes. Synteny of the Siamese cobra chromosome 2 (NKA2) and NKAZ were highly conserved among snakes and other squamate reptiles, except for intrachromosomal rearrangements occurring in NKA2. Interestingly, twelve BACs that had partial homology with sex chromosomes of several amniotes were mapped on the heterochromatic NKAW as hybridization signals such as repeat sequences. Sequence analysis showed that most of these BACs contained high proportions of transposable elements. In addition, hybridization signals of telomeric repeat (TTAGGG)n and six microsatellite repeat motifs ((AAGG)8, (AGAT)8, (AAAC)8, (ACAG)8, (AATC)8, and (AAAAT)6) were observed on NKAW, and most of these were also found on other amniote sex chromosomes.ConclusionsThe frequent amplification of repeats might involve heterochromatinization and promote sex chromosome differentiation in the Siamese cobra W sex chromosome. Repeat sequences are also shared among amniote sex chromosomes, which supports the hypothesis of an ancestral super-sex chromosome with overlaps of partial syntenies. Alternatively, amplification of microsatellite repeat motifs could have occurred independently in each lineage, representing convergent sex chromosomal differentiation among amniote sex chromosomes.Electronic supplementary materialThe online version of this article (10.1186/s12864-018-5293-6) contains supplementary material, which is available to authorized users.

Dynamics of telomere length in captive Siamese cobra (Naja kaouthia) related to age and sex

“…Specific amplification of telomeric repeats was also found on the W chromosome of Siamese cobra (Singchat et al, 2018), suggesting that co-opted telomeric DNA amplifications on sex chromosomes drive the increase of telomere length in early life history.…”

“…Sex determination systems may also affect telomere length and life span. However, no complete linkage homology is shown between most mammal/reptile XY and bird/reptile ZW chromosomes (Ezaz, Srikulnath, & Graves, 2017;Singchat et al, 2018). Reptiles display considerable diversity in their sex chromosomes with both male and female heterogamety (XX/XY and ZZ/ZW), even within the same taxa (Olmo & Signorino, 2005).…”

“…Reptiles display considerable diversity in their sex chromosomes with both male and female heterogamety (XX/XY and ZZ/ZW), even within the same taxa (Olmo & Signorino, 2005). However, no complete linkage homology is shown between most mammal/reptile XY and bird/reptile ZW chromosomes (Ezaz, Srikulnath, & Graves, 2017;Singchat et al, 2018). Differences in telomere attrition have been found in mammals and XY reptiles of male heterogamety with age-related sex disparity.…”

“…The present study makes use of precise individual age records from actively maintained populations of Siamese cobra (Naja kaouthia) to improve our understanding of age-and sex-related telomere dynamics in ZZ/ ZW species. Siamese cobras that are also ZZ/ZW species are sexually dimorphic in relation to body dimensions (head length and tail length), and males are significantly larger than females when they reach their 2nd or 3rd year and become sexually active during their 3rd or 4th year of life (Chaitae, 2011;Meesook, 2008;Singchat et al, 2018). Most females mate every year before migrating to feeding grounds according to their annual estradiol cycle (Tumkiratiwong, Meesuk, Chanhome, & Aowphol, 2012).…”

The rise and fall of differentiated sex chromosomes in geckos

“…However, certain genome sections might tend to be recruited for the function of sex chromosomes nonrandomly more often than other parts due to the content of genes involved in gonad differentiation which are a basis for the evolution of sex‐determining genes (Marshall Graves & Peichel, ; O'Meally et al, ; see recent discussion on the limited options in sex‐determining genes in vertebrates by Bertho et al, ). The recently suggested hypothesis considers amniote sex chromosomes sharing repeat sequences or containing genes linked to avian ZW and mammalian XY chromosomes as partially homologous remnants of an “ancestral super‐sex chromosome” (Ezaz et al, ; Singchat et al, ). However, this view does not explain why the same genome parts expected to be parts of the ancestral sex chromosomes are sometimes involved in sex chromosomes under male heterogamety, while in other cases under female heterogamety, and does not offer a plausible scenario for the evolution of sex‐determining genes.…”