Research School of Biology, Australian National University, Acton, ACT 0200, Australia
Chromosome Res. 2010 Nov;18(7):787-800. doi: 10.1007/s10577-010-9152-9. Epub 2010 Aug 24.
Snake sex chromosomes provided Susumo Ohno with the material on which he based his theory of how sex chromosomes differentiate from autosomal pairs. Like birds, snakes have a ZZ male/ZW female sex chromosome system, in which the snake Z is a macrochromosome much the same size as the bird Z. However, the gene content shows clearly that the snake and bird Z chromosomes are completely non-homologous. The molecular aspect of W chromosome degeneration in snakes remains largely unexplored. We used comparative genomic hybridization to identify the female-specific region of the W chromosome in representative species of Australian snakes. Using this approach, we show that an increasingly complex suite of repeats accompanies the evolution of W chromosome heteromorphy. In particular, we found that while the python Liasis fuscus exhibits no sex-specific repeats and indeed, no cytologically recognizable sex-specific region, the colubrid Stegonotus cucullatus shows a large domain on the short arm of the W chromosome that consists of female-specific repeats, and the large W of Notechis scutatus is composed almost entirely of repetitive sequences, including Bkm and 18S rDNA-related elements. FISH mapping of both simple and complex probes shows patterns of repeat amplification concordant with the size of the female-specific region in each species examined. Mapping of intronic sequences of genes that are sex-linked in both birds (DMRT1) and snakes (CTNNB1) reveals massive amplification in discrete domains on the W chromosome of the elapid N. scutatus. Using chicken W chromosome paint, we demonstrate that repetitive sequences are shared between the sex chromosomes of birds and derived snakes. This could be explained by ancestral but as yet undetected shared synteny of bird and snake sex chromosomes or may indicate functional homology of the repeats and suggests that degeneration is a convergent property of sex chromosome evolution. We also establish that synteny of snake Z-linked genes has been conserved for at least 166 million years and that the snake Z consists of two conserved blocks derived from the same ancestral vertebrate chromosome.
蛇的性染色体为 Susumo Ohno 提供了材料，他基于这些材料提出了性染色体如何从常染色体对中分化的理论。与鸟类一样，蛇具有 ZZ 雄性/ZW 雌性性染色体系统，其中蛇的 Z 是一个与鸟类 Z 大小相同的大染色体。然而，基因内容清楚地表明，蛇和鸟类的 Z 染色体完全没有同源性。蛇的 W 染色体退化的分子方面在很大程度上仍未得到探索。我们使用比较基因组杂交技术鉴定了澳大利亚蛇代表性物种的 W 染色体的雌性特异性区域。通过这种方法，我们表明，随着 W 染色体异型性的进化，越来越复杂的重复序列伴随着进化。特别是，我们发现，虽然 Python Liasis fuscus 没有表现出性别特异性重复，实际上也没有细胞学上可识别的性别特异性区域，但 Colubrid Stegonotus cucullatus 在 W 染色体的短臂上显示出一个由雌性特异性重复组成的大区域，而 Notechis scutatus 的大 W 几乎完全由重复序列组成，包括 Bkm 和 18S rDNA 相关元件。简单和复杂探针的 FISH 图谱显示，重复扩增的模式与每个被检查物种的雌性特异性区域的大小一致。在鸟类（DMRT1）和蛇（CTNNB1）中与性别连锁的基因的内含子序列的映射揭示了在 elapid N. scutatus 的 W 染色体上离散区域的大量扩增。使用鸡 W 染色体涂染，我们证明了鸟类和衍生蛇的性染色体之间存在重复序列共享。这可以通过鸟类和蛇的性染色体的祖先但尚未检测到的共享同线性来解释，或者可能表明重复序列的功能同源性，并表明退化是性染色体进化的一个趋同特性。我们还确定蛇 Z 连锁基因的同线性至少在 1.66 亿年前就已经得到了保守，并且蛇的 Z 由来自同一祖先脊椎动物染色体的两个保守块组成。
