Genetic relatedness does not retain spatial pattern across multiple spatial scales: dispersal and colonization in the coral, Pocillopora damicornis

Abstract

Patterns of isolation by distance are uncommon in coral populations. Here, we depart from historical trends of large-scale, geographical genetic analyses by scaling down to a single patch reef in Kāne'ohe Bay, Hawai'i, USA, and map and genotype all colonies of the coral, Pocillopora damicornis. Six polymorphic microsatellite loci were used to assess population genetic and clonal structure and to calculate individual colony pairwise relatedness values. Our results point to an inbred, highly clonal reef (between 53 and 116 clonal lineages of 2352 genotyped colonies) with a much skewed genet frequency distribution (over 70% of the reef was composed of just seven genotypes). Spatial autocorrelation analyses revealed that corals found close together on the reef were more genetically related than corals further apart. Spatial genetic structure disappears, however, as spatial scale increases and then becomes negative at the largest distances. Stratified, random sampling of three neighbouring reefs confirms that reefs are demographically open and inter-reef genetic structuring was not detected. Attributing process to pattern in corals is complicated by their mixed reproductive strategies. Separate autocorrelation analyses, however, show that the spatial distribution of both clones and nonclones contributes to spatial genetic structure. Overall, we demonstrate genetic structure on an intrareef scale and genetic panmixia on an inter-reef scale indicating that, for P.damicornis, the effect of small- and large-scale dispersal processes on genetic diversity are not the same. By starting from an interindividual, intrareef level before scaling up to an inter-reef level, this study demonstrates that isolation-by-distance patterns for the coral P.damicornis are limited to small scales and highlights the importance of investigating genetic patterns and ecological processes at multiple scales.