Enigma of Y chromosome degeneration: neo-Y and neo-X chromosomes of Drosophila miranda a model for sex chromosome evolution.
Y chromosome degeneration is characterized by structural changes in the chromosome architecture and expansion of genetic inertness along the Y chromosome. It is generally assumed that the heteromorphic sex chromosome pair has developed from a pair of homologues. Several models have been suggested. We use the unique situation of the secondary sex chromosome pair, neo-Y and neo-X (X2), in Drosophila miranda to analyze molecular mechanisms involved in the evolutionary processes of Y chromosome degeneration. Due to the fusion of one of the autosomes to the Y chromosome (about 2 Mya), a neo-Y chromosome and a neo-X chromosome, designated X2, were formed. Thus, formerly autosomal genes are inherited now on a pair of sex chromosomes in D. miranda. Analyzing DNA sequences from the X2 and neo-Y region, we observed a massive accumulation of DNA insertions on the neo-Y chromosome. From the analysis of several insertion elements, we present compelling evidence that the first step in Y chromosome degeneration is driven by the accumulation of transposable elements, especially retrotransposons. An enrichment of these elements along an evolving Y chromosome could account for the switch from a euchromatic into a heterochromatic chromatin structure.
Institut für Genetik, Heinrich-Heine-Universität, Germany. steinem@mail.rz.uni-duesseldorf.de
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D000818:Animals; D005075:Biological Evolution; D002874:Chromosome Mapping; D004330:Drosophila / Q000235:genetics*; D004331:Drosophila melanogaster / Q000235:genetics; D005260:Female; D008297:Male; D008957:Models, Genetic; D010802:Phylogeny; D012469:Salivary Glands / Q000166:cytology; D014960:X Chromosome / Q000235:genetics*; D014998:Y Chromosome / Q000235:genetics*