Neo-X and Neo-Y Chromosomes in Drosophila miranda

Many eukaryotic taxa inherit a heteromorphic sex chromosome pair. It is a generally accepted hypothesis that originally homologous chromosomes develop after the occurrence of a sex differentiator into two structurally and functionally different partners (Muller, 1932; Bull, 1983; Charlesworth, 1991; Steinemann and Steinemann, 1998; Lucchesi, 1999), X and Y chromosome (or Z and W). Often the Y chromosome evolves into a heterochromatic and genetically inert chromosome (chromosome degeneration). This evolutionary process has the final consequence that the male lineage becomes hemizygous for the heterosomal genes, a situation which is compensated by a special mechanism, the gene dosage compensation. The assumption, originally made for Drosophila, was extended by Ohno (1967) for the development of mammalian sex chromosomes. The constraints evolving the structurally and functionally different X and Y chromosomes have been the object of speculation since the discovery of sex chromosomes by McClung (1901). For the evolution of sex chromosomes it is assumed that starting from the differential locus in the heterogametic sex the state of permanent heterozygosity is spread further into the flanking regions (for review Lucchesi, 1994; Rice 1996; Charlesworth, 1996).