BACKGROUND The discovery of differentially organized sex chromosome systems suggests that heteromorphic sex chromosomes evolved from a pair of homologous chromosomes. Whereas karyotypes are highly conserved in alethinophidian snakes, the degeneration status of the W chromosomes varies among species. The Z and W chromosomes are morphologically homomorphic in henophidian species, whereas in snakes belonging to caenophidian families the W chromosomes are highly degenerated. Snakes therefore are excellent animal models in which to study sex chromosome evolution. Herein, we investigated the differentiation processes for snake sex chromosomes using both coding and repetitive sequences. We analyzed phylogenetic relationships of CTNNB1 and WAC genes, localized to the centromeric and telomeric regions, respectively, of the long arms on snake sex chromosomes, and chromosome distribution of sex chromosome-linked repetitive sequences in several henophidian and caenophidian species. RESULTS Partial or full-length coding sequences of CTNNB1 and WAC were identified for Z homologs of henophidian species from Tropidophiidae, Boidae, Cylindrophiidae, Xenopeltidae, and Pythonidae, and for Z and W homologs of caenophidian species from Acrochordidae, Viperidae, Elapidae, and Colubridae. Female-specific sequences for the two genes were not found in the henophidian (boid and pythonid) species examined. Phylogenetic trees constructed using each gene showed that the Z and W homologs of the caenophidian species cluster separately. The repetitive sequence isolated from the W chromosome heterochromatin of the colubrid Elaphe quadrivirgata and a microsatellite motif (AGAT)8 were strongly hybridized with W chromosomes of the viperid and colubrid species examined. CONCLUSION Our phylogenetic analyses suggest that the cessation of recombination between the Z and W homologs of CTNNB1 and WAC predated the diversification of the caenophidian families. As the repetitive sequences on the W chromosomes were shared among viperid and colubrid species, heterochromatinization of the proto-W chromosome appears to have occurred before the splitting of these two groups. These results collectively suggest that differentiation of the proto-Z and proto-W chromosomes extended to wide regions on the sex chromosomes in the common ancestor of caenophidian families during a relatively short period.

中文翻译：

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从两个配子基因的发散模式和性染色体连锁重复序列的染色体分布推断出蛇的性染色体进化。

背景技术差异性性染色体系统的发现表明异型性染色体是从一对同源染色体进化而来的。核型在拟南芥蛇中高度保守，而W染色体的退化状态在物种之间有所不同。Z和W染色体在hephiphidian种中形态上是同态的，而在属于caenophidian家族的蛇中，W染色体高度退化。因此，蛇是研究性染色体进化的优秀动物模型。在这里，我们调查了使用编码和重复序列的蛇性染色体的分化过程。我们分析了CTNNB1和WAC基因的系统发生关系，分别位于着丝粒和端粒区域，长臂在蛇性染色体上的分布，以及几种杂种和食巢动物中性染色体连锁重复序列的染色体分布。结果鉴定了CTNNB1和WAC的部分或全长编码序列，分别用于特罗比科，Bo科、,科，ida科，X科和蟒科的嗜蝇科物种的Z同源物，以及from科，Vi科和E科的食虫科的Z和W同源物。 。这两个基因的雌性特异性序列未在所研究的三栖类（boid和pythonid）物种中找到。使用每个基因构建的系统发育树显示，该eno虫物种的Z和W同源物分别聚类。从colubrid象形象Elaphe quadrivirgata的W染色体异染色质分离的重复序列和微卫星基序（AGAT）8与所研究的vi和colubrid菌种的W染色体强烈杂交。结论我们的系统进化分析表明，CTNNB1和WAC的Z和W同源物之间的重组停止在食巢动物家族的多样化之前发生。由于W染色体上的重复序列在活跃的和共生的物种之间共享，因此原W染色体的异染色质似乎在这两组分裂之前就已经发生了。这些结果共同表明，在Caenophidian家族的共同祖先中，Proto-Z和Proto-W染色体的分化扩展到了性染色体上的较宽区域。