Patterns of molecular evolution of an avian neo-sex chromosome.

Abstract:
Newer parts of sex chromosomes, neo-sex chromosomes, offer unique possibilities for studying gene degeneration and sequence evolution in response to loss of recombination and population size decrease. We have recently described a neo-sex chromosome system in Sylvioidea passerines that has resulted from a fusion between the first half (10 Mb) of chromosome 4a and the ancestral sex chromosomes. In the present study, we report the results of molecular analyses of neo-Z and neo-W gametologues and intronic parts of neo-Z and autosomal genes on the second half of chromosome 4a in three species within different Sylvioidea lineages (Acrocephalidea, Timaliidae and Alaudidae). In line with hypotheses of neo-sex chromosome evolution, we observe (i) lower genetic diversity of neo-Z genes comparatively to autosomal genes, (ii) moderate synonymous and weak non-synonymous sequence divergence between neo-Z and the neo-W gametologues and (iii) lower GC content on neo-W than on neo-Z gametologues. Phylogenetic reconstruction of eight neo-Z and neo-W gametologues suggests that recombination continued after the split of Alaudidae from the rest of the Sylvioidea lineages (i.e. after approximately 42.2 MYA) and with some exceptions also after the split of Acrocephalidea and Timaliidae (i.e. after approximately 39.4 36 MYA). The Sylvioidea neo-sex chromosome shares classical evolutionary features with the ancestral sex chromosomes, but as expected from its more recent origin shows weaker divergence between gametologues.