Faster adaptation but slower divergence of X chromosomes under paternal genome elimination
Robert B Baird, Thomas J. Hitchcock, Jan Ševčı́k, Katy M. Monteith, Andy Gardner, Laura Ross, Andrew J. Mongue
Abstract
Differences in transmission and ploidy between sex chromosomes and autosomes drive divergent evolutionary trajectories, with sex chromosomes generally evolving faster. Because sex-linked genes are transmitted less frequently, they are under less efficient selection. Conversely, exposure of recessive mutations on haploid sex chromosomes creates more efficient selection. In most systems, these effects occur simultaneously and are confounded. The fly families Sciaridae (fungus gnats) and Cecidomyiidae (gall midges) have X0 sex determination, but males transmit only maternally inherited chromosomes. This phenomenon results in equal transmission of the X and autosomes, allowing the effect of haploid selection to be studied in isolation. We discover that, unlike well-studied systems, X chromosomes diverge more slowly than autosomes in these flies. Using population genomic and expression data, we show that despite the X evolving more adaptively, stronger purifying selection explains slower divergence. Our findings demonstrate the utility of non-Mendelian inheritance systems for understanding fundamental evolutionary processes. X chromosomes evolve faster than autosomes, but confounding factors make this a difficult phenomenon to study. Utilising the unusual sex determination system of Sciaridae flies, this study finds a slower evolution of the X chromosomes which appears to be driven by strong purifying selection.