Hybridization Patterns Reveal Spatial Variation in Reproductive Isolation Between Two Closely Related Abalone Species

Empirical studies demonstrating ecological speciation in marine invertebrates are limited. Two large abalone species— Haliotis discus discus and H. madaka —in the Japanese archipelago are considered to be involved in this process. Previous population genomic studies have indicated that hybridization between these two species continues because of incomplete reproductive isolation. However, owing to the limited number of samples in the previous study, the nature of hybridization remains unclear. In this study, we performed population genomic analyses of these two species using samples from multiple geographical regions. Analysis of 122 species‐diagnostic single‐nucleotide polymorphism loci revealed that observed heterozygosity was significantly higher in the H. madaka population than that in the H. discus discus population. ADMIXTURE analyses revealed biased introgression from H. discus discus into H. madaka populations, with the extent of introgression varying among regions. Most individuals with genomic introgression were classified as backcrosses, with no individuals demonstrating patterns consistent with F1 hybrids, indicating a bimodal hybrid zone. These findings suggest that selection may act against early‐generation hybrids and that ecological speciation proceeds under geographically variable selective pressures in marine environments.