Résumé
Sex chromosomes have different evolutionary properties compared to
autosomes due to their hemizygous nature. In particular, recessive
mutations are more readily exposed to selection, which can lead to faster
rates of molecular evolution. Here, we report patterns of gene expression
and molecular evolution for a group of butterflies. First, we improve the
completeness of the Heliconius melpomene reference annotation, a
neotropical butterfly with a ZW sex determination system. Then, we analyse
RNA from male and female whole abdomens and sequence female ovary and gut
tissue to identify sex and tissue specific gene expression profiles in H.
melpomene. Using these expression profiles we compare: 1) sequence
divergence and polymorphism; 2) the strength of positive and negative
selection; and 3) rates of adaptive evolution, for Z and autosomal genes
between two species of Heliconius butterflies, H. melpomene and H. erato.
We show that the rate of adaptive substitutions is higher for Z than
autosomal genes, but contrary to expectation, it is also higher for male
biased than female biased genes. Additionally, we find no significant
increase in the rate of adaptive evolution or purifying selection on genes
expressed in ovary tissue, a heterogametic specific tissue. Our results
contribute to a growing body of literature from other ZW systems that also
provide mixed evidence for a fast-Z effect where hemizygosity influences
the rate of adaptive substitutions.