Genetic differentiation, reproductive mode, and gene flow in the brooding coral Pocillopora damicornis along the Great Barrier Reef, Australia

The widespread and morphologically variable coral Pocillopora damicornis has been reported to exhibit huge variation in life-history traits (e.g. mode of reproduction, growth rate, longevity and dispersal) both locally and regionally throughout the Indo-Pacific Ocean. Dispersal may be achieved by the settlement of sexually or asexually generated brooded planula larvae, by broadcast spawning or more locally through asexual fragmentation of large colonies. In the present study, we conducted a hierarchical survey of allozyme variation within and among reef-crest sites on 3 mid-shelf reefs separated by up to 1200 km on the Great Barrier Reef (GBR), Australia. Our objective was to use allozyme data (1) to quantify local and regional patterns of variation in P. damicornis (along the northeastern coast of Australia), (2) to determine the relative contribution of sexual versus asexual production of planulae in P. damicornis, and (3) to estimate levels of gene flow among adjacent sites (>5 km apart) and among reefs separated by 500 to 1200 km. High levels of genotypic diversity in our samples of P. damicornis imply that dispersive propagules in this species are produced sexually rather than asexually along the length of the GBR. Corals at all sites displayed the same level of multi-locus genotypic diversity expected for randomly mating, sexually derived populations, and the majority of individual colonies possessed unique 7-locus genotypes. We also detected consistent deficits of heterozygotes within each collection (from 3 local sites on each of the 3 widely spaced reefs). This pattern is consistent with the predicted effects of sexual reproduction associated with some localised dispersal of gametes or larvae (although other explanations cannot be excluded). Furthermore, each reef was genetically subdivided, suggesting that larval recruitment was localised and that these populations are slightly inbred: hierarchical analysis of the standardised genetic variances (FST)(estimated as Weir & Cockerham's Θ) revealed that, although there was only moderate variation among all 9 sites (FST = 0.055 ± 0.029), more variation was found among sites within reefs (FSL= 0.035 ± 0.04 to 0.088 ± 0.033) than among distant reefs (FLT= 0.008 ± 0.014). This homogeneity of gene frequencies across widely separated reefs implies that reefs are connected by high levels of gene flow (Nem = ca 31) and that local populations of P. damicornis separated by >1000 km can interbreed sufficiently to maintain a consistent suite of life-history characters