Sexually dimorphic gene expression and transcriptome evolution provide mixed evidence for a fast-Z effect in...

Area of Science:

• Evolutionary Biology
• Genomics
• Molecular Evolution

Background:

• Sex chromosomes exhibit unique evolutionary dynamics compared to autosomes due to their hemizygous nature.
• Recessive mutations on sex chromosomes are more exposed to natural selection, potentially accelerating molecular evolution.
• The fast-Z effect proposes accelerated rates of adaptive substitution on Z chromosomes, but evidence is mixed across species.

Purpose of the Study:

• To investigate sex- and tissue-specific gene expression patterns in the butterfly Heliconius melpomene.
• To compare molecular evolution patterns, including divergence, polymorphism, and selection, between Z-linked and autosomal genes.
• To test the hypothesis of an accelerated rate of adaptive evolution on the Z chromosome (fast-Z effect) in Heliconius butterflies.

Main Methods:

• Improved the reference genome annotation for Heliconius melpomene.
• Analyzed RNA sequencing data from male and female abdomens and sequenced female ovary and gut tissues.
• Compared sequence divergence, polymorphism, and selection strengths for Z-linked and autosomal genes between H. melpomene and H. erato.

Main Results:

• The rate of adaptive substitutions is significantly higher for Z-linked genes than for autosomal genes.
• Contrary to expectations, male-biased genes exhibit a higher rate of adaptive evolution than female-biased genes.
• No significant increase in adaptive evolution or purifying selection was observed for genes expressed in ovary tissue.

Conclusions:

• The study provides evidence supporting an accelerated rate of adaptive evolution for Z-linked genes in Heliconius butterflies.
• Findings suggest that gene expression bias (male-biased) may play a more significant role than tissue-specific expression in Z chromosome evolution.
• Results contribute mixed evidence to the fast-Z effect literature, highlighting the complexity of sex chromosome evolution.