Ancient Hybridisation Fuelled Diversification in <i>Acropora</i> Corals

Introgression is the infiltration or flow of genes from one species to another through hybridisation followed by backcrossing. This may lead to incorrect phylogenetic reconstruction or divergence-time estimation. Acropora is a dominant genus of reef-building corals; however, whether this group has an introgression history before their diversification remains unclear, and previous divergence-time estimates of Acropora have not considered the impact of introgression. In this study, we broke through the limitation of a few genes and a few species and proved the existence of ancient introgression in the evolution of Acropora from whole-genome protein-coding sequences. We inferred 21.9% of all triplet loci (homologous loci from three different species) with a history of introgression and a series of introgression events with a genetic material contribution of up to 30.9% before diversification. Furthermore, 7756 nuclear loci were clustered into three groups using a multidimensional scaling algorithm, the heterogeneity of which resulted in different phylogenetic relationships. The diversification time of Acropora was estimated to be middle to late Miocene when we retained only the gene group with the lowest degree of introgression. The collision of Australia with the Pacific arcs and the Southeast Asian margin in the early Miocene, and a series of cooling events in the middle to late Miocene, may provide geographical and climatic conditions for the diversification of Acropora, respectively. Therefore, our results indicate that at the genome-wide level, ancient introgressive hybridisation may have promoted the radiation evolution of Acropora. Based on our results, the influence of introgression should be taken into account when reconstructing phylogenetic relationships and evaluating divergence time.