Common mechanisms of Y chromosome evolution

Y chromosome evolution is characterized by the expansion of genetic inertne ss along the Y chromosome and changes in the chromosome structure, especial ly the tendency of becoming heterochromatic. It is generally assumed that t he sex chromosome pair has developed from a pair of homologues. In an evolu tionary process the proto-Y-chromosome, with a very short differential segm ent, develops in its final stage into a completely heterochromatic and to a great extends genetically eroded Y chromosome. The constraints evolving th e 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 in Drosophila miranda to analyze the molecular process in progress involved in Y chromosome evolution. We suggest that the first ste ps in the switch from a euchromatic proto-Y-chromosome into a completely he terochromatic Y chromosome are driven by the accumulation of transposable e lements, especially retrotransposons inserted along the evolving nonrecombi ning part of the Y chromosome. In this evolutionary process trapping and ac cumulation of retrotransposons on the proto-Y-chromosome should lead to con formational changes that are responsible for successive silencing of euchro matic genes, both intact or already mutated ones and eventually transform f unctionally euchromatic domains into genetically inert heterochromatin. Acc umulation 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 ferti lity will then represent the final state of the degenerated Y chromosome.