First evidence for backcrossing of F₁ hybrids in Acropora corals under sperm competition

Acropora is a species-rich genus of reef-building corals with highly diverse morphologies. Hybridization among intercrossing species potentially influences species diversity within Acropora. However, the mechanisms that allow hybridization/backcrossing remain unknown. Although we tested a limited number of species, we hypothesized that Acropora gametes in the Indo-Pacific may preferentially fertilize conspecific gametes despite their compatibility with heterospecific gametes, leading to infrequent hybridization between potentially intercrossing species. In this study, F₁ hybrids of Acropora florida and A. intermedia showed specific fertilization trends. For example, sperm had the ability to backcross with the parental species even in the presence of sperm from the parental species. Also, eggs of the hybrids produced from A. florida eggs and A. intermedia sperm ("FLOint") exhibited self-fertilization. Since a low ratio of hybridization between A. florida and A. intermedia is predicted, the population size of hybrids should be small. Therefore, self-fertilization would facilitate reproduction of the hybrid in nature, while remaining sperm could outcompete parental species sperm to backcross with eggs. Although we succeeded in breeding two colonies of hybrids, it is reasonable to speculate that hybrids show a high tendency to choose the most efficient sexual reproduction tactics.