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 th e chromosome architecture and expansion of genetic inertness along the Y chromosome. It is generally assumed that the heteromorphic sex chro mosome pair has developed from a pair of homologues. Several models ha ve been suggested. We use the unique situation of the secondary sex ch romosome pair, neo-Y and neo-X(X2), in Drosophila miranda to analyze m olecular mechanisms involved in the evolutionary processes of Y chromo some 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, d esignated X2, were formed. Thus, formerly autosomal genes are inherite d now on a pair of sex chromosomes in D. miranda. Analyzing DNA sequen ces from the X2 and neo-Y region, we observed a massive accumulation o f DNA insertions on the neo-Y chromosome. From the analysis of several insertion elements, we present compelling evidence that the first ste p in Y chromosome degeneration is driven by the accumulation of transp osable elements, especially retrotransposons. An enrichment of these e lements along an evolving Y chromosome could account for the switch fr om a euchromatic into a heterochromatic chromatin structure.