Examination of species boundaries in the Acropora cervicornis group (Scleractinia, Cnidaria) using nuclear DNA sequence analyses

Although Acropora is the most species-rich genus of the scleractinian (ston y) corals, only three species occur in the Caribbean: A. cervicornis, A. pa lmata and A. prolifera. Based on overall coral morphology, abundance and di stribution patterns, it has been suggested that A. prolifera may be a hybri d between A. cervicornis and A. palmata. The species boundaries among these three morphospecies were examined using DNA sequence analyses of the nucle ar Pax-C 46/47 intron and the ribosomal DNA internal Transcribed Spacer (IT S1 and ITS2) and 5.8S regions. Moderate levels of sequence variability were observed in the ITS and 5.8S sequences (up to 5.2% overall sequence differ ence), but variability within species was as large as between species and a ll three species carried similar sequences. Since this is unlikely to repre sent a shared ancestral polymorphism, the data suggest that introgressive h ybridization occurs among the three species. For the Pax-C intron, A. cervi cornis and A. palmata had very distinct allele frequencies and A. cervicorn is carried a unique allele at a frequency of 0.769 (although sequence diffe rences between alleles were small). All A. prolifera colonies examined were heterozygous for the Pax-C intron, whereas heterozygosity was only 0.286 a nd 0.333 for A. cervicornis and A. palmata, respectively. These data suppor t the hypothesis that A. prolifera is the product of hybridization between two species that have a different allelic composition for the Pax-C intron, i.e. A. cervicornis and A. palmata. We therefore suggest that A: prolifera is a hybrid between A. cervicornis and A. palmata, which backcrosses with the parental species at low frequency.