Integrative taxonomic analyses reveal that rapid genetic divergence drives Acropora speciation

Reef-building corals provide the structural basis for one of Earth's most spectacular and diverse but increasingly threatened ecosystems. The reef-building coral genus Acropora may have undergone substantial speciation during the Pleistocene climate and sea-level changes. Here, we aimed to evaluate the speciation history of four morphologically similar tabular Acropora species (Acropora aff. hyacinthus, A. cf. bifurcata, A. cf. cytherea, and A. cf. subulata) using an integrative approach with morphology, genetic, and reproduction methodology. Extensive morphological analyses showed that these four species are distinct and exhibited high gamete incompatibility, preventing hybridization. Furthermore, population structure and principal component analyses with SNPs (>60,000) indicated that these species were genetically distinct, and the ABBA-BABA test did not support introgression among these species. Many of their coding and noncoding RNA sequences showed high genetic variance at loci with high Fst values along the genome. Comparison of these orthologs with those of other Acropora species suggested that many of these genes are under positive selection, which could be associated with spawning time, gamete, and morphological divergence. Our findings show that the speciation of tabular Acropora occurred without hybridization, and the divergence accompanying the rapid evolution of genes in species-rich Acropora could be associated with speciation.