Degenerating Y chromosome of Drosophila miranda: a trap for retrotransposons.

In Drosophila miranda, the larval cuticle protein (Lcp) genes are located on the X2 and Y chromosomes, while in other Drosophila species the Lcp genes are inherited on the autosomes. We chose the D. miranda species as a model system to analyze the molecular bases of Y chromosome degeneration, a phenomenon observed in many species. DNA sequence analysis of the Y chromosomal Lcp gene locus reveals dense clustering of trapped retrotransposons. Once inserted at the Y chromosomal location they cannot easily be eliminated by unequal crossing-over, as recombination is a rare event in Drosophila males. In addition, we have uncovered an example of a completely inactive allele on the degenerating Y chromosome. The existence of such inactive Y-specific alleles was originally predicted in H. J. Muller's model for Y chromosome degeneration. We demonstrate that the Y chromosomal Lcp4 allele is no longer transcribed. From the divergence in DNA sequence organization of former homologous chromosome regions we conclude that changes in chromosome structure and destruction of genetic activity in degenerating Y chromosomes are based on one major mechanism, which operates by means of transposable elements. IMAGES:

在亮斑果蝇（Drosophila miranda）中，幼虫表皮蛋白（larval cuticle protein, Lcp）基因定位于X2与Y染色体；而在其他果蝇物种中，Lcp基因则以常染色体方式遗传。我们选用亮斑果蝇作为模型系统，以分析Y染色体退化的分子基础——这是在众多物种中均被观测到的现象。对Y染色体连锁的Lcp基因座进行DNA序列分析后发现，捕获的反转录转座子（retrotransposons）呈密集簇状分布。由于果蝇雄性的重组事件极为罕见，一旦这些反转录转座子插入Y染色体区域，便难以通过不等交换将其清除。此外，我们在退化的Y染色体上发现了一个完全失活的等位基因实例。这类Y连锁失活等位基因的存在，最初由H·J·穆勒提出的Y染色体退化模型所预测。我们证实，Y染色体连锁的Lcp4等位基因已不再发生转录。通过对前同源染色体区域的DNA序列组织差异进行分析，我们得出结论：退化Y染色体的结构改变与遗传活性丧失，均基于一套以转座因子（transposable elements）为作用载体的核心机制。IMAGES: