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.
Highlights
Unlike 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
Eleven of nineteen microsatellite repeat motifs were successfully mapped onto most N. kaouthia chromosomes (NKA) (Fig. 2, Additional file 1: Figure S1); only six microsatellite repeat motifs showed high amplification on the W chromosome
This study suggests the fundamental basis on which the Siamese cobra chromosome 2 and the W sex chromosome share partial synteny and some characteristics of sex chromosomes observed in other amniotes, for example the amplification of repeat sequences
Summary
Unlike 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. Differentiated sex chromosomes developed from the accumulation of sexually antagonistic alleles at loci linked to the sex-specific region. To maintain linkage of these genes, meiotic recombination between proto-sex chromosomes was suppressed around the heterologous region in one sex, which in turn promoted the loss of active genes or segmental insertions and deletions, and caused this region to extend along part or all of the chromosome. This produced chromosomal morphological differences between the X and Y (male heterogamety) or Z and W (female heterogamety) sex chromosomes. It is of great importance to understand the mechanism and role of sex chromosome differentiation with regard to sex determination
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