Bachtrog D, Charlesworth B
Institute of Cell, Animal and Population Biology, University of Edinburgh, Edinburgh EH9 3JT, UK.
Curr Biol. 2000 Sep 7;10(17):1025-31. doi: 10.1016/s0960-9822(00)00656-4.
In many species, sex is determined by a system involving X and Y chromosomes, the latter having lost much of their genetic activity. Sex chromosomes have evolved independently many times, and several different mechanisms responsible for the degeneration of the Y chromosome have been proposed. Here, we have taken advantage of the secondary sex chromosome pair in Drosophila miranda to test for the effects of evolutionary forces involved in the early stages of Y-chromosome degeneration. Because of a fusion of one of the autosomes to the Y chromosome, a neo-Y chromosome and a neo-X chromosome have been formed, resulting in the transmission of formerly autosomal genes in association with the sex chromosomes.
We found a 25-fold lower level of variation at microsatellites located on the neo-Y chromosome compared with homologous loci on the neo-X chromosome, or with autosomal and X-linked microsatellites. Sequence analyses of the region flanking the microsatellites suggested that the neo-sex chromosomes originated about 1 million years ago.
Variability of the neo-Y chromosome of D. miranda is substantially reduced below expectations at mutation-drift equilibrium. Such a reduction is predicted by theories of the degeneration of the Y chromosome. Another possibility is that there is little or no mutation at microsatellite loci on a non-recombining chromosome such as the neo-Y, but this seems inconsistent with other data.
在许多物种中，性别由涉及X和Y染色体的系统决定，后者已失去大部分遗传活性。性染色体已独立进化多次，并且已经提出了几种导致Y染色体退化的不同机制。在这里，我们利用了米兰达果蝇中的次生性染色体对来测试参与Y染色体退化早期阶段的进化力量的影响。由于一条常染色体与Y染色体融合，形成了一条新Y染色体和一条新X染色体，导致以前的常染色体基因与性染色体相关联传递。
我们发现，与新X染色体上的同源位点、常染色体和X连锁微卫星相比，位于新Y染色体上的微卫星变异水平低25倍。微卫星侧翼区域的序列分析表明，新性染色体起源于约100万年前。
米兰达果蝇新Y染色体的变异性在突变-漂变平衡下大幅降低至预期水平以下。Y染色体退化理论预测了这种降低。另一种可能性是，在诸如新Y这样的非重组染色体上，微卫星位点几乎没有或没有突变，但这似乎与其他数据不一致。
