Evolution of a recent neo-Y sex chromosome in a laboratory population of Drosophila

In many species of animals, one of the sexes has a chromosome that is struc turally and functionally different from its so-called homologue. Convention ally, it is called Y chromosome or W chromosome depending on whether it is present in males or females respectively. The corresponding homologous chro mosomes are called X and Z chromosomes. The dimorphic sex chromosomes are b elieved to have originated from undifferentiated autosomes. In extant speci es it is difficult to envisage the changes that have occurred in the evolut ion of dimorphic sex chromosomes. In our laboratory, interracial hybridizat ion between two Drosophila chromosomal races has resulted in the evolution of a novel race, which we have called Cytorace 1. Here we record that in th e genome of Cytorace 1 one of the autosomes of its parents is inherited in a manner similar to that of a classical Y chromosome. Thus this unique Cyto race 1 has the youngest neo-Y sex chromosome (5000 days old; about 300 gene rations) and it can serve as a 'window' for following the transition of an autosome to a Y sex chromosome.