Sexual selection on non-ornamental traits is underpinned by evidence of
genetic constraints on sex-biased expression in dusky pipefish
Abstract
Species without dimorphic secondary sex characteristics easily visible
to humans, such as difference in size or morphology, are expected to
experience low levels of sex-specific selection. However, monomorphism
in classic visible traits could be a result of genetic or physiological
constraints that prevent the sexes from reaching divergent fitness
optima. Additionally, biochemical and molecular work has revealed a
variety of less easily observed phenotypes that none-the-less exhibit
profound dimorphism. Sex-specific selection could act on these more
subtle, less visible, traits. We investigate sex-specific selection in
the polygynandrous dusky pipefish (Syngnathus floridae), which lacks
size, color, and morphological dimorphism. Using experimental breeding
populations, we revealed that although males and females have similar
opportunities for sexual selection, only males experience significant
sexual selection pressures on body size. We also investigated patterns
of sex-biased and sex-specific gene expression in gonads, livers, and
gills, and tested whether genes with highly divergent expression
patterns between the sexes are more likely to be tissue specific, and
therefore relieved of genetic constraints. We. Sex-bias in gene
expression was widespread, although the reproductive organs had the most
sex-biased and sex-specific genes. Sex-specific selection on gene
expression in gills was primarily related to immune response, whereas
the liver and gonads had a wide variety of cellular processes, as well
as reproductive proteins, showing sex-biased expression. These
sex-biased genes are likely less constrained by pleiotropy, as they were
more organ-specific in their expression patterns. Altogether, we find
evidence for ongoing and historical sex-specific selection in the dusky
pipefish.