The highly cross-fertile coral species, Acropora hyacinthus and Acropora cytherea, constitute statistically distinguishable lineages

A major challenge for understanding the evolutionary genetics of mass-spawning corals is to explain the maintenance of discrete morphospecies in view of high rates of interspecific fertilization in vitro and nonmonophyletic patterns in molecular phylogenies. In this study, we focused on Acropora cytherea and A. hyacinthus, which have one of the highest potentials for interspecific fertilization. Using sequences of a nuclear intron, we performed phylogenetic and nested clade analyses (NCA). Both species were polyphyletic in molecular phylogenies, but the NCA indicated that they constitute statistically distinguishable lineages. Phylogenetic analysis using an intergenic region of the mitochondrial DNA (mtDNA), was inconclusive because of low levels of variability in this marker. The position of these two species differed between the nuclear DNA (nDNA) and mtDNA phylogenies and was also at odds with a cladistic analysis based on morphology. We conclude that despite the potential for high levels of hybridization and introgression, A. cytherea and A. hyacinthus constitute statistically distinguishable lineages and their taxonomic status is consistent with the cohesion species concept

Unexpected patterns of genetic structuring among locations but not colour morphs in Acropora nasuta (Cnidaria; Scleractinia)

Symbiotic relationships have contributed greatly to the evolution and maintenance of biological diversity. On the Great Barrier Reef, species of obligate coral-dwelling fishes (genus Gobiodon) coexist by selectively recruiting to colonies of Acropora nasuta that differ in branch-tip colour. In this study, we investigate genetic variability among sympatric populations of two colour morphs of A. nasuta ('blue-tip' and 'brown-tip') living in symbiosis with two fish species, Gobiodon histrio and G. quinquestrigatus, respectively, to determine whether gobies are selecting between intraspecific colour polymorphisms or cryptic coral species. We also examine genetic differentiation among coral populations containing both these colour morphs that are separated by metres between local sites, tens of kilometres across the continental shelf and hundreds of kilometres along the Great Barrier Reef. We use three nuclear DNA loci, two of which we present here for the first time for Acropora. No significant genetic differentiation was detected between sympatric colour morphs at these three loci. Hence, symbiotic gobies are selecting among colour morphs of A. nasuta, rather than cryptic species. Significant genetic geographical structuring was observed among populations, independent of colour, at regional (i.e. latitudinal separation by < 500 km) and cross-shelf (< 50 km) scales, alongside relative homogeneity between local populations on within reef scales (< 5 km). This contrasts with the reported absence of large-scale genetic structuring in A. valida, which is a member of the same species group as A. nasuta. Apparent differences in biogeographical structuring between species within the A. nasuta group emphasize the need for comparative sampling across both spatial (i.e. within reefs, between reefs and between regions) and taxonomic scales (i.e. within and between closely related species)