Increased Evolutionary Rate in the Z chromosome of Sympatric and Allopatric Species of Morpho Butterflies.

Divergent evolution of genomes among closely related species is shaped by both neutral processes and ecological forces, such as local adaptation and reinforcement. These factors can drive accelerated evolution of sex chromosomes relative to autosomes. Comparative genomic analyses between allopatric and sympatric species with overlapping or divergent ecological niches offer insights into reinforcement and ecological specialization on genome evolution. In the butterfly genus Morpho, several species coexist in sympatry, with specialization across forest strata and temporal niches. We analyzed the genomes of eight Morpho species, along with previously published genomes of three others, to compare chromosomal rearrangements and signs of positive selection in the Z chromosome vs. autosomes. We found extensive chromosomal rearrangements in Z chromosome, particularly in sympatric species with similar ecological niches, suggesting a role for inversions in restricting gene flow at a postzygotic level. Z-linked genes also exhibited significantly higher dN/dS ratios than autosomal genes across the genus, with pronounced differences in closely related species living in sympatry. Additionally, we examined the evolution of eight circadian clock genes, detecting positive selection in Period, located on the Z chromosome. Our findings suggest that the Z chromosome evolves more rapidly than autosomes, particularly among closely related species, raising questions about its role in prezygotic and postzygotic isolation mechanisms.