Novel microsatellite markers for Pyura chilensis reveal fine-scale genetic structure along the southern coast of Chile

Studying the geographic scale of gene flow and population structure in marine populations can be a powerful tool with which to infer patterns of larval dispersal averaged across generations. Here, we describe the development of ten novel polymorphic microsatellite markers for an important endemic ascidian, Pyura chilensis, of the southeastern Pacific, and we report the results from fine-scale genetic structure analysis of 151 P. chilensis individuals sampled from five sites constituting approximate to 80km of coastline in southern Chile. All microsatellite markers were highly polymorphic (number of alleles ranged from 12 to 36). Our results revealed significant deviations from Hardy-Weinberg equilibrium (HWE) for most loci, suggesting the presence of either null alleles or deviations from random mating within sampled sites. However, we found a significantly higher spatial autocorrelation and higher mean pairwise relatedness among individuals sampled from the same sites than would be expected if samples were randomly distributed across all sites; this suggests that spatial configuration and reproduction might not be random within sites. Our results indicate the presence of a weak but significant genetic structure between sites (overall F-ST=0.015, p<0.001). Despite the short pelagic larval duration of this species, geographic distance does not appear to correlate with genetic distances between sites. From the results gathered here, it seems possible that genetic structure at this spatial scale is driven to some extent by local population dynamics (deviations from random mating and/or a large proportion of larvae settling in proximity of relatives), yet infrequent long-distance dispersal events might also be responsible for the relatively weak spatial heterogeneity between sites. Overall, our results both highlight the utility of this new marker set for population genetic studies of this species and provide new evidence regarding the complexity of the small-scale population structure of this species.