Chang Ching-Ho, Larracuente Amanda M
Department of Biology, University of Rochester, Rochester, New York, 14627.
Evolution. 2017 May;71(5):1285-1296. doi: 10.1111/evo.13229. Epub 2017 Apr 10.
Robertsonian translocations resulting in fusions between sex chromosomes and autosomes shape karyotype evolution by creating new sex chromosomes from autosomes. These translocations can also reverse sex chromosomes back into autosomes, which is especially intriguing given the dramatic differences between autosomes and sex chromosomes. To study the genomic events following a Y chromosome reversal, we investigated an autosome-Y translocation in Drosophila pseudoobscura. The ancestral Y chromosome fused to a small autosome (the dot chromosome) approximately 10-15 Mya. We used single molecule real-time sequencing reads to assemble the D. pseudoobscura dot chromosome, including this Y-to-dot translocation. We find that the intervening sequence between the ancestral Y and the rest of the dot chromosome is only ∼78 Kb and is not repeat-dense, suggesting that the centromere now falls outside, rather than between, the fused chromosomes. The Y-to-dot region is 100 times smaller than the D. melanogaster Y chromosome, owing to changes in repeat landscape. However, we do not find a consistent reduction in intron sizes across the Y-to-dot region. Instead, deletions in intergenic regions and possibly a small ancestral Y chromosome size may explain the compact size of the Y-to-dot translocation.
罗伯逊易位导致性染色体与常染色体融合，通过从常染色体创造新的性染色体来塑造核型进化。这些易位还可使性染色体逆转回常染色体，鉴于常染色体和性染色体之间的巨大差异，这一点尤其引人关注。为了研究Y染色体逆转后的基因组事件，我们调查了果蝇中的一种常染色体 - Y易位。大约在1000万至1500万年前，祖先Y染色体与一条小常染色体（点状染色体）融合。我们使用单分子实时测序读数来组装果蝇的点状染色体，包括这种从Y到点状的易位。我们发现，祖先Y染色体与点状染色体其余部分之间的间隔序列仅约78千碱基对，且重复序列不密集，这表明着丝粒现在位于融合染色体之外而非之间。由于重复序列格局的变化，从Y到点状的区域比黑腹果蝇的Y染色体小100倍。然而，我们并未在从Y到点状的区域中发现内含子大小一致减小的情况。相反，基因间区域的缺失以及可能较小的祖先Y染色体大小或许可以解释从Y到点状易位的紧凑大小。
