Evolutionary Study of Reef-Building Corals, Genus Acropora

Hironobu FUKAMI1 and Masayuki HATTA

1Tokyo University of Fishery

Genus Acropora shares dominant species in reef- building corals. Dozens of Acropora species spawn at the same time in the small areas. This mass-spawning gives a problem how they could speciate without geological isolation. We tried to find out the evidences for the coral evolution by molecular phylogenetic analysis and experimental cross. First we isolated the gene for mini-collagen encoding a major component of nematocyst capsule from A. donei (ref. ) and A. nasuta, and designed primers for amplification of a fragment of the gene by PCR. DNA was extracted from sperms used for cross experiments and subjected to PCR. DNA sequences obtained were analyzed by molecular phylogenetic methods to estimate genetic distances and variation, and compared with the results of cross experiments. Five conclusions were obtained; (1) two species which do not join mass-spawning are genetically distant from the others, (2) DNA polymorphism are higher in two similar corymbose shape species than the others, (3) intraspecific polymorphisms and interspecific divergence were indistinguishable in the two corymbose species and interspecific cross were observed between the two, (4) one major staghorn shape species cross-fertilized with one of the corymbose shape species at high frequency, (5) the major table shape species revealed low frequency of intraspecific fertilization suggesting it is a sibling species. It is suggested that Acropora is evolving through repeating speciation by rapid establishment of reproductive isolation and gene introgression or species fusion by interspecific fertilization, giving rise to variety of morphology.