Steinemann M, Steinemann S
Institut für Zoologie, Technische Universität Darmstadt, FR Germany.
Genetica. 2000;109(1-2):105-11. doi: 10.1023/a:1026584016524.
Y chromosome evolution is characterized by the expansion of genetic inertness along the Y chromosome and changes in the chromosome structure, especially the tendency of becoming heterochromatic. It is generally assumed that the sex chromosome pair has developed from a pair of homologues. In an evolutionary process the proto-Y-chromosome, with a very short differential segment, develops in its final stage into a completely heterochromatic and to a great extends genetically eroded Y chromosome. The constraints evolving the Y chromosome have been the objects of speculation since the discovery of sex chromosomes. Several models have been suggested. We use the exceptional situation of the neo-Y in Drosophila miranda to analyze the molecular process in progress involved in Y chromosome evolution. We suggest that the first steps in the switch from a euchromatic proto-Y-chromosome into a completely heterochromatic Y chromosome are driven by the accumulation of transposable elements, especially retrotransposons inserted along the evolving nonrecombining part of the Y chromosome. In this evolutionary process trapping and accumulation of retrotransposons on the proto-Y-chromosome should lead to conformational changes that are responsible for successive silencing of euchromatic genes, both intact or already mutated ones and eventually transform functionally euchromatic domains into genetically inert heterochromatin. Accumulation of further mutations, deletions, and duplications followed by the evolution and expansion of tandem repetitive sequence motifs of high copy number (satellite sequences) together with a few vital genes for male fertility will then represent the final state of the degenerated Y chromosome.
Y染色体的进化特征是遗传惰性沿Y染色体扩展以及染色体结构发生变化，尤其是呈现出异染色质化的趋势。一般认为，性染色体对是由一对同源染色体进化而来的。在进化过程中，具有非常短的差异区段的原始Y染色体在其最后阶段演变成完全异染色质化且在很大程度上基因受到侵蚀的Y染色体。自性染色体被发现以来，Y染色体进化所受到的限制一直是人们猜测的对象。已经提出了几种模型。我们利用米兰达果蝇（Drosophila miranda）中新Y染色体的特殊情况来分析Y染色体进化过程中正在进行的分子过程。我们认为，从常染色质的原始Y染色体转变为完全异染色质的Y染色体的最初步骤是由转座元件的积累驱动的，特别是沿着Y染色体不断进化的非重组部分插入的逆转座子。在这个进化过程中，逆转座子在原始Y染色体上的捕获和积累应该会导致构象变化，从而导致常染色质基因（无论是完整的还是已经发生突变的）相继沉默，并最终将功能上的常染色质结构域转变为遗传惰性的异染色质。随后，进一步的突变、缺失和重复的积累，以及高拷贝数串联重复序列基序（卫星序列）的进化和扩展，再加上一些对雄性生育至关重要的基因，将代表退化Y染色体的最终状态。
