Evolution of gonad transcriptomes and gamete-recognition genes in sea stars

Evolved differences in life history traits, including fertilization ecology and mating systems, are expected to affect the strength of sexual selection acting on gamete-recognition genes (GRGs) responsible for gamete compatibility and fertilization success. The evolution of life history traits such as internal fertilization of eggs and mating system traits such as self-fertilization is expected to weaken the effects of sexual selection (due to the resolution of sperm competition among males and sexual conflicts between males and females). To assess these expectations, I compared the responses to selection of GRGs and other genes expressed in the gonads from multiple species of sea stars with different life histories. I first developed a bioinformatic protocol to reconstruct the transcriptomes of gonads from RNA-seq libraries using the data from the crown-of-thorns sea star Acanthaster cf. solaris and used that protocol to characterize GRGs and gene expression. I then compared GRGs in two recently diverged species with contrasting mating systems. I found little evidence of positive selection in the GRGs of the outcrossing species (Cryptasterina pentagona). Instead, I found evidence of relaxed selection in the self-fertilizing and hermaphroditic species (Cryptasterina hystera). I also found evidence of selection in non-GRG-genes linked to abiotic stressors, DNA regulation, polyspermy, and egg retention. In the last chapter, I compared the selection on female GRGs and other ovary genes using a phylogenetically broad sample of sea star species with two modes of reproduction. I found evidence of rapid evolution acting on female GRGs and of a stronger response to selection on female GRGs from sea stars with expected stronger sexual selection (gonochoric, broadcast spawning, planktonic fertilization) compared to species with derived life history traits associated with weaker sexual selection (hermaphroditic, benthic fertilization, brood protection). In summary, these results support the expectation of rapid evolution and strong selection on GRGs compared to other parts of animal genomes. GRG evolution likely contributes to the speciation process as a mechanism of reproductive incompatibility. And when selection targets GRG, life history traits can affect the response to selection.