Abstract
BackgroundRecent translocations of autosomal regions to the sex chromosomes represent important systems for identifying the evolutionary forces affecting convergent patterns of sex-chromosome heteromorphism. Additions to the sex chromosomes have been reported in the melanica and robusta species groups, two sister clades of Drosophila. The close relationship between these two species groups and the similarity of their rearranged karyotypes motivates this test of alternative hypotheses; the rearranged sex chromosomes in both groups are derived through a common origin, or the rearrangements are derived through at least two independent origins. Here we examine chromosomal arrangement in representatives of the melanica and the robusta species groups and test these alternative hypotheses using a phylogenetic approach.ResultsTwo mitochondrial and two nuclear gene sequences were used to reconstruct phylogenetic relationships of a set of nine ingroup species having fused and unfused sex chromosomes and representing a broad sample of both species groups. Different methods of phylogenetic inference, coupled with concurrent cytogenetic analysis, indicate that the hypothesis of independent origins of rearranged sex chromosomes within each species group is significantly more likely than the alternative hypothesis of a single common origin. An estimate tightly constrained around 8 My was obtained for the age of the rearranged sex chromosomes in the melanica group; however, a more loosely constrained estimate of 10–15 My was obtained for the age of the rearrangement in the robusta group.ConclusionIndependent acquisition of new chromosomal arms by the sex chromosomes in the melanica and robusta species groups represents a case of striking convergence at the karyotypic level. Our findings indicate that the parallel divergence experienced by newly sex-linked genomic regions in these groups represents an excellent system for studying the tempo of sex chromosome evolution.
Highlights
Recent translocations of autosomal regions to the sex chromosomes represent important systems for identifying the evolutionary forces affecting convergent patterns of sexchromosome heteromorphism
A fusion between an autosomal element and the sex chromosomes in D. robusta and D. sordidula, representatives of the robusta species group, and in D. melanica, D. nigromelanica, PFhigyulorgeen1ies proposed in the melanica and robusta groups Phylogenies proposed in the melanica and robusta groups
Phylogenies previously reported for species in the robusta and melanica species groups. a) Relationships within the robusta group proposed by Narayanan [33], based on polytene chromosomes; b) and c) Alternative topologies proposed by Stalker ([41, 42], respectively) for species in the melanica group
Summary
Recent translocations of autosomal regions to the sex chromosomes represent important systems for identifying the evolutionary forces affecting convergent patterns of sexchromosome heteromorphism. Convergence upon similar patterns of heteromorphism in a variety of organisms indicates that common mechanisms shape the evolution of sex chromosomes [912]; the rate at which heteromorphism develops on a newly derived sex-chromosome pair is unknown, as are the mechanisms affecting this transition. Sex chromosomes underlying recent transitions to dioecy in different plant lineages represent promising sources of model systems [8,13]. Sex-linked transmission results from this type of translocation, and the newly acquired region of the sex chromosomes differentiates through the same evolutionary pathway as did the original pair of sex chromosomes; i.e., suppression of recombination, degeneration of the heterozygous chromosomes and dosage compensation [14,15]. The sex chromosomes in humans are a mosaic comprised of an ancestral sex-linked region shared with non-eutherian mammals and a newly acquired region unique to eutherian mammals [16]
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have