Patterns of replication in the neo-sex chromosomes of Drosophila nasuta albomicans

Drosophila nasuta albomicans (with 2n = 6), contains a pair of metacentric neo-sex chromosomes. Phylogenetically these are products, of centric fusion between ancestral sex (X, Y) chromosomes and an autosome (chromosome 3). T he polytene chromosome complement of males with a neo-X- and neo-Y-chromoso mes has revealed asynchrony in replication between the two arms of the neo- sex chromosomes. The arm which represents the ancestral X-chromosome is fas ter replicating than the arm which represents ancestral autosome. The latte r arm of the neo-sex chromosome is synchronous with other autosomes of the complement. We conclude that one arm of the neo-X/Y is still mimicking the features of an autosome while the other arm has the features of a classical X/Y-chromosome, This X-autosome translocation differs from the other evolu tionary X-autosome translocations known in certain species of Drosophila.