Effect of the frequency of multi-specific synchronous spawning on genetic introgression among three Acropora species

Hybridisation is an evolutionary process that generates genetic diversity in organisms. However, the relationship between reproductive features, such as spawning synchronisation and gamete compatibility, and the degree of introgression leading to hybridisation are poorly understood. The reef-building coral Acropora spp. have a complex evolutionary history, and the link between their ecology, life-history traits, and potential to hybridise is disputed. Here, we examined the relationship among the reproductive features involved in the intercrossing of three species, Acropora florida, Acropora gemmifera, and Acropora intermedia, at two sites: Akajima and the Sesoko islands in southern Japan. Although the examined species showed synchronous spawning and high rates of gamete compatibility, spawning synchronisation and gamete compatibility were less strongly associated with high rates of interbreeding among the three species. Model-based genetic clustering and site-pattern frequency-based tests with single nucleotide polymorphisms supported genetic admixture among the three species in each location. Demographic analyses using fastsimcoal implied that the admixture among the three species in each location might have occurred in the past (> 2,000 generations) and recently (< 50 generations). Furthermore, the recent admixture of these three species is potentially associated with heavy bleaching events and population declines. The principal component analysis, structure, and fastsimcoal showed that the extensive admixture of A. intermedia and A. gemmifera on Sesoko Island occurred recently. Therefore, gamete interactions that lead to hybridisation in the field must be clarified. Furthermore, the connectivity between the two locations needs to be identified; however, our results implied that population fluctuations could be associated with introgression.