Molecular genetic evidence for probable reticulate speciation in the coralgenus Madracis from a Caribbean fringing reef slope

For many corals, the existence of morphologically distinct yet sympatric po pulations/species implies reproductive isolation. Conversely, the presence of many intermediate and overlapping morphologies combined with synchronous , mass spawning suggests incomplete reproductive isolation. In Madracis (Sc leractinia: Astrocoeniina: Pocilloporidae), high levels of morphological pl asticity among the five most commonly recognized species (M. mirabilis, M. senaria, M. decactis, M. formosa and M. pharensis) on Caribbean reefs led u s to question species boundaries. Phylogenetic relationships were investiga ted at the intra-individual, inter-individual and inter-specific levels usi ng the ITSI-5.8S-ITS2 region (ca. 613 bp) of the ribosomal DNA cistron. Int er-specific divergence was ca. 6%, while intra-individual and intra-specifi c divergences ranged from 0% to 4.9% and 3.3% to 3.5%, respectively. M. sen aria and M. mirabilis formed monophyletic groups. M. formosa, M. decactis a nd M. pharensis formed a paraphyletic complex. High levels of intra-individ ual and intra-specific ITS polymorphism in the decactis-formosa-pharensis c luster may be the result of very recent speciation within the clade (i.e. m aintenance of ancestral polymorphism and incomplete lineage sorting), or th e result of repeated introgressive hybridization among the three taxa. Poly morphism parsimony of 89 sites, including nine that showed additivity, reve aled a phylogenetic topology more consistent with inter-taxal hybridization . Results are discussed in terms of weak reproductive barriers, and phyloge netic fission and fusion under Veron's model of reticulate speciation in co rals. Ecological studies involving Madracis should consider AT decactis, M. formosa and M. pharensis as a complex.