Nanda Indrajit, Schories Susanne, Tripathi Namita, Dreyer Christine, Haaf Thomas, Schmid Michael, Schartl Manfred
Biocenter, Institute for Human Genetics, Am Hubland, 97074, Würzburg, Germany.
Chromosoma. 2014 Aug;123(4):373-83. doi: 10.1007/s00412-014-0455-z. Epub 2014 Mar 28.
Sex chromosomes differ from autosomes by dissimilar gene content and, at a more advanced stage of their evolution, also in structure and size. This is driven by the divergence of the Y or W from their counterparts, X and Z, due to reduced recombination and the resulting degeneration as well as the accumulation of sex-specific and sexually antagonistic genes. A paradigmatic example for Y-chromosome evolution is found in guppies. In these fishes, conflicting data exist for a morphological and molecular differentiation of sex chromosomes. Using molecular probes and the previously established linkage map, we performed a cytogenetic analysis of sex chromosomes. We show that the Y chromosome has a very large pseudoautosomal region, which is followed by a heterochromatin block (HCY) separating the subtelomeric male-specific region from the rest of the chromosome. Interestingly, the size of the HCY is highly variable between individuals from different population. The largest HCY was found in one population of Poecilia wingei, making the Y almost double the size of the X and the largest chromosome of the complement. Comparative analysis revealed that the Y chromosomes of different guppy species are homologous and share the same structure and organization. The observed size differences are explained by an expansion of the HCY, which is due to increased amounts of repetitive DNA. In one population, we observed also a polymorphism of the X chromosome. We suggest that sex chromosome-linked color patterns and other sexually selected genes are important for maintaining the observed structural polymorphism of sex chromosomes.
性染色体与常染色体在基因含量上存在差异，并且在进化的更高级阶段，在结构和大小上也有所不同。这是由于Y或W与其对应物X和Z的分化所致，这是由于重组减少以及由此产生的退化以及性别特异性和性拮抗基因的积累。孔雀鱼中发现了Y染色体进化的一个典型例子。在这些鱼类中，关于性染色体的形态和分子分化存在相互矛盾的数据。我们使用分子探针和先前建立的连锁图谱对性染色体进行了细胞遗传学分析。我们发现Y染色体有一个非常大的拟常染色体区域，其后是一个异染色质块（HCY），它将亚端粒雄性特异性区域与染色体的其余部分隔开。有趣的是，不同种群个体之间HCY的大小差异很大。在波氏翼花鳉的一个种群中发现了最大的HCY，使得Y染色体几乎是X染色体大小的两倍，也是该基因组中最大的染色体。比较分析表明，不同孔雀鱼物种的Y染色体是同源的，并且具有相同的结构和组织。观察到的大小差异是由HCY的扩展所解释的，这是由于重复DNA数量的增加。在一个种群中，我们还观察到了X染色体的多态性。我们认为，与性染色体连锁的颜色模式和其他性选择基因对于维持观察到的性染色体结构多态性很重要。
