A faster-X effect for gene expression in Drosophila embryos. Alex T. Kalinka¹, Melek A. Kayserili¹, Dave T. Gerrard², Pavel Tomancak¹. 1) Max Planck Institute, Dresden, Germany; 2) University of Manchester, Manchester, UK.

   The X chromosome is present as a single copy in the heterogametic sex, and this hemizygosity is expected to drive unusual patterns of evolution on the X relative to the autosomes. Theory suggests that the single copy of the X in males could facilitate faster evolution of the X, although this faster evolution could be either adaptive or non-adaptive. We measured gene expression across the chromosomes in several Drosophila species, and also in several inbred strains of D. melanogaster for both embryos and adults. We found that gene expression is evolving significantly faster between species in the embryos (an average excess of ~20%), yet harbours significantly less variation within inbred strains (on average ~10% lower). Furthermore, expression divergence of genes on Muller's D element is significantly greater along the branch leading to the obscura sub-group, in which this element segregates as a neo-X chromosome. In adults, the excess of X chromosome divergence is much lower than in the embryos, yet they also harbour significantly lower levels of gene expression variation on the X in inbred strains. The X chromosome also appears to fix mutations at a lower rate in mutation accumulation lines, suggesting that random genetic drift is not acting more strongly on the X. Overall, our results are consistent with there being an excess of adaptive evolution on the X chromosome in Drosophila embryos, and highlight the importance of biological context for understanding how chromosomes evolve in different species.