Changes in spawning time led to the speciation of the broadcast spawning corals Acropora digitifera and the cryptic species Acropora sp. 1 with similar gamete recognition systems

Abstract

Multi-species spawning is reported in the coral genus Acropora, but hybridization in nature rarely occurs because of the incompatibility of gametes and the timing of spawning. However, the evolutionary relationships between gamete compatibility and spawning time are obscure. Investigations of gamete compatibility in sister species that spawn at different times may provide clues to answering this question. Acropora sp. 1 has been defined as a cryptic species of Acropora digitifera, and they are morphologically similar, but spawn in different months, suggesting that they are either a cryptic species or a different species. We examined the morphology and conducted crossing experiments using cryopreserved sperm. The morphologies (branch length, branch width, and outer diameter of axial corallites) of A. digitifera and Acropora sp. 1 differed significantly. A phylogenetic tree of partial Pax-C nuclear sequences from A. digitifera and Acropora sp. 1 shows that they are monophyletic and closely related genetically, based on F ST values and P-distance. These results imply that these two species originated recently from a common ancestor. In addition, cryopreserved sperm from both A. digitifera and Acropora sp. 1 showed bidirectional inter-crossing (cryopreserved sperm of A. digitifera and eggs of Acropora sp. 1 from Sesoko: 32.1 ± 6.7 %, control-conspecific cryopreserved sperm and eggs: 46.1 ± 10.6 %; cryopreserved sperm of Acropora sp. 1 and eggs of A. digitifera from Oku: 63.3 ± 16.6 %, control: 83.6 ± 6.0 %). The results suggest that the gametes of these two species are compatible and that the pre-zygotic isolation mechanism is relaxed because their gametes do not interact. Overall, these two species should be classified as distinct species, and changes in spawning time are related to speciation in a similar gamete recognition system.