Increased evolutionary rate in the Z-chromosome ofMorphobutterflies and implications for speciation

The evolution of reproductive isolation between populations shapes divergence in genome structure and content: comparing the genomes of closely-related species can thus enlighten the speciation process. Comparisons of genomes of allopatric vs . sympatric species sharing similar vs . dissimilar ecological niches allows to specifically investigate the effect of reinforcement and ecological specialization on genome evolution. In the butterfly genus Morpho , several species can be found in sympatry presenting specialisation in different microhabitats and temporal niches. Here, we sequenced, assembled and annotated the genomes of 8 Morpho species and used previously published genomes of three other Morpho species to study genomic rearrangements and signatures of positive selection. We found extensive chromosomal rearrangements in the Z chromosome compared to the autosomes, particularly among closely related sympatric species occupying similar niches, pointing at the putative role of inversions in preventing gene flow at a postzygotic level. We also detected a higher proportion of genes under positive selection on the Z-chromosome compared to the autosomes, suggesting a potential role of the Z-chromosome in driving adaptive evolution in Morpho . Finally, because of the divergence in daily activities between species, we studied the evolution of eight genes involved in the circadian clock and detected a signature of positive selection on the gene Period , located in the Z chromosome. By studying the evolution of genome structure and coding sequences, our study indicates fast evolution of the Z-chromosome, partly driven by selection, throughout this genus, highlighting the putative implication of this sexual chromosome on pre and post-zygotic isolation.