Hybridization, reinforcement selection and sex-dependent reproductive character displacement of sperm and egg recognition proteins

The establishment of reproductive isolation between species via gametic incompatibility initially requires within-species selection for variation in reproductive compatibility. We investigate how the generation of within-species variation in sperm and egg recognition proteins, potentially via sexual conflict, influences reproductive isolation between two partially sympatric sea urchin species; the North American west coast Mesocentrotus franciscanus and the circumpolar Strongylocentrotus droebachiensis. Barriers to hybridization are stronger when eggs are given a choice of conspecific versus heterospecific sperm and the variation in hybridization among crosses can be explained by whether the sperm or egg protein variant is ancestral or derived. Derived proteins can be recognized as different and prevent hybridization. Examination of the allele frequencies of these proteins in M. franciscanus in and out of sympatry with S. droebachiensis along the west coast of North America reveals evidence of reinforcement selection and reproductive character displacement in eggs but not sperm, which likely reflects the differential cost of hybridization for males and females.

Methods
These data are the results of fertilization crosses between the sea urchins, Mesocentrotus franciscanus and Strongylocentrotus droebachiensis.  Tests were both choice (sperm from both males together) and no choice experiments. Data were analyzed as a function of the sire's sperm bindin genotype and the dame's EBR1 genotype.  Genotyping was conducted using PCR and Sangar sequencing.  Patterns of allele frequencies for these loci were investigated over the species range for M. franciscanus to investigate evidence of reproctive character displacement. These data files contain information of the genotypes of the parents in the cross and the fraction of eggs fertilized by conspecific or heterospecific sperm.  Fasta files of the haplotypes found for each locus are also presented.