Contrasting Patterns of Molecular Evolution of the Genes on the New and Old Sex Chromosomes of Drosophila miranda

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Abstract
In organisms with chromosomal sex determination, sex is determined by a set of dimorphic sex chromosomes that are thought to have evolved from a set of originally homologous chromosomes. The chromosome inherited only through the heterogametic sex (the Y chromosome in the case of male heterogamety) often exhibits loss of genetic activity for most of the genes carried on its homolog and is hence referred to as degenerate. The process by which the proto-Y chromosome loses its genetic activity has long been the subject of much speculation. We present a DNA sequence variation analysis of marker genes on the evolving sex chromosomes (neo-sex chromosomes) of Drosophila miranda. Due to its relatively recent origin, the neo-Y chromosome of this species is presumed to be still experiencing the forces responsible for the loss of its genetic activity. Indeed, several previous studies have confirmed the presence of some active loci on this chromosome. The genes on the neo-Y chromosome surveyed in the current study show generally lower levels of variation compared with their counterparts on the neo-X chromosome or an X-linked gene. This is in accord with a reduced effective population size of the neo-Y chromosome. Interestingly, the rate of replacement nucleotide substitutions for the neo-Y linked genes is significantly higher than that for the neo-X linked genes. This is not expected under a model where the faster evolution of the X chromosome is postulated to be the main force driving the degeneration of the Y chromosome.