Faster-X Evolution in the Speciation of Heliconius Butterflies

The X and Z chromosomes have unique characteristics that lead to unique evolutionary consequences. Lepidopterans have a well-known, disproportionately large-Z effect for behavioral and morphological traits that distinguish closely related species. A potential explanation for the Large-X effect is the faster evolution of the sex chromosome (Faster-X evolution). We use whole genome re-sequencing of Heliconius erato races and of the incipient species H. himera to test for faster-Z evolution between hybridizing populations at different reproductive isolation levels, by calculating divergence and nucleotide diversity. We show evidence for Faster-Z evolution in Heliconius butterflies at the early stages of speciation and along the speciation continuum. Evidence of higher divergence and lower nucleotide diversity suggests not only selection but also nonaptive process, like demographic changes, may be driving faster-Z evolution, especially in the incipient species