Laboratory of Animal Cytogenetics and Comparative Genomics (ACCG), Department of Genetics, Faculty of Science, Kasetsart University, Bangkok, 10900, Thailand.
Animal Breeding and Genetics Consortium of Kasetsart University (ABG-KU), Bangkok, 10900, Thailand.
BMC Genomics. 2018 Dec 17;19(1):939. doi: 10.1186/s12864-018-5293-6.
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. 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.
We 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) and six microsatellite repeat motifs ((AAGG), (AGAT), (AAAC), (ACAG), (AATC), and (AAAAT)) were observed on NKAW, and most of these were also found on other amniote sex chromosomes.
The 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.
与大多数蛇类（2n=36）的染色体组成不同，眼镜蛇的核型显示出二倍体染色体数为 38 条，其中包括一条高度异染色质的 W 染色体和一条形态上差异很大的 2 号染色体。为了研究眼镜蛇、大多数蛇类和其他羊膜动物之间性染色体分化和进化的过程，我们利用荧光原位杂交（FISH）技术，构建了来自鸡和斑胸草雀文库的 43 个细菌人工染色体（BACs）的暹罗眼镜蛇（Naja kaouthia）染色体图谱，并与鸡、斑胸草雀和其他羊膜动物进行了比较。
我们制作了暹罗眼镜蛇基因组的详细染色体图谱，重点关注 2 号染色体和性染色体。蛇类和其他有鳞目爬行动物之间的暹罗眼镜蛇染色体 2（NKA2）和 NKAZ 的同线性高度保守，除了 NKA2 中发生的染色体内重排。有趣的是，12 个 BAC 与几个羊膜动物的性染色体具有部分同源性，作为重复序列等杂交信号被定位在异染色质 NKAW 上。序列分析表明，这些 BAC 中的大多数含有高比例的转座元件。此外，在 NKAW 上观察到端粒重复（TTAGGG）和 6 个微卫星重复基序（（AAGG）、（AGAT）、（AAAC）、（ACAG）、（AATC）和（AAAAT））的杂交信号，这些信号也在其他羊膜动物的性染色体上发现。
重复序列的频繁扩增可能涉及异染色质化，并促进暹罗眼镜蛇 W 性染色体的性染色体分化。重复序列也在羊膜动物的性染色体之间共享，这支持了祖先超性染色体具有部分重叠同线性的假说。或者，微卫星重复基序的扩增可能在每个谱系中独立发生，代表羊膜动物性染色体之间的趋同性染色体分化。
