High rates of evolution preceded shifts to sex-biased gene expression in Leucadendron, the most sexually dimorphic angiosperms

Differences between males and females are usually more subtle in dioecious
plants than animals, but strong sexual dimorphism has evolved convergently
in the South African Cape plant genus Leucadendron. Such sexual dimorphism
in leaf size is expected largely to be due to differential gene expression
between the sexes. We compared patterns of gene expression in leaves among
ten Leucadendron species across the genus. Surprisingly, we found no
positive association between sexual dimorphism in morphology and the
number or the percentage of sex-biased genes. Sex bias in most sex-biased
genes evolved recently and was species-specific. We compared rates of
evolutionary change in expression for genes that were sex-biased in one
species but unbiased in others and found that sex-biased genes evolved
faster in expression than un-biased genes. This greater rate of expression
evolution of sex-biased genes, also documented in animals, might suggest
the possible role of sexual selection in the evolution of gene expression.
However, our comparative analysis clearly indicates that the more rapid
rate of expression evolution of sex-biased genes predated the origin of
bias, and shifts towards bias were depleted in signatures of adaptation.
Our results are thus more consistent with the view that sex bias is simply
freer to evolve in genes less subject to constraints in expression level.