EvoLunch Seminar: Violaine Llaurens (College de France, FR)

Determining the evolutionary forces initiating and fuelling the speciation process is challenging, especially when population divergence occurs in sympatry. Sympatric speciation often involves complex interactions between ecological specialization generating divergent selection and the accumulation of genetic barriers to gene flow. Here we focus on a pair of sister-species of butterflies (M. achilles and M. helenor) observed in sympatry throughout the Amazonian basin, and sharing similar micro-habitat but diverging in the timing of male patrolling behaviour. Since M. helenor has a larger geographical range, we compared the temporal niche in sympatric and allopatric populations of this species and found a significant shift in patrolling hours, suggesting that the evolution of temporal niches might contribute to the speciation or reinforcement process. Using population genomics, we then reconstructed the history of speciation between these two species and investigated the heterogeneity in the level of genetic divergence throughout the genome. We found a very strong differentiation in the sexual chromosome (Z) as compared to the autosomes between species but not between populations within species. We then specifically tested for the effect of selection in such increased rate of evolution in the Z chromosomes and found significantly higher signals of positive selection in the Z vs. the autosomes in sympatric population, while such a signal was missing in the allopatric ones. Interestingly, we also noticed that the circadian gene Period, located on the Z chromosome, was found under positive selection throughout the genus Morpho. Altogether, our results raise questions on the implication of the Z chromosome in ecological specialization and speciation in sympatry.