High connectivity across environmental gradients and implications for phenotypic plasticity in a marine plant

Thalassia testudinum is prevalent throughout the western tropical Atlantic, Gulf of Mexico, and Florida. This seagrass inhabits a wide range of coastal ecosystems and published data suggest significant morphological variation in T. testudinum across acute physicochemical environmental gradients. Strong reliance on vegetative growth provokes expectations of a clonal signature in the population structure of this species. We utilize high resolution genetic data to explore the population structure of T. testudinum and evaluate the basis for this species' plasticity as a function of a phenotypic versus a genotypic response. We studied one of the largest populations of T. testudinum, the Florida Bay system, and found that the population exhibited high levels of genetic diversity suggesting strong recruitment of sexually derived propagules. Allelic richness was high (A(RIC) = 5.94 to 7.33) and expected heterozygosity was consistently high across our study subpopulations (H-e = 0.558 to 0.673). There was no evidence of inbreeding within subpopulations (F-IS = 0.02 to 0.13) and overall gene flow estimates were moderate to high (N-m = 5.71). These data support T. testudinum in Florida Bay as a single metapopulation with high genetic connectivity among subpopulations. Models of migration utilizing Bayesian modeling revealed a distinct directionality to immigration counter to models of historical formation of Florida Bay. We also found no evidence that meadows formed genetic subpopulations suggesting morphological variability observed across environmental gradients represents norms of reaction within the genetically diverse, interbreeding metapopulation. We suggest T. testudinum evolved phenotypic plasticity as a general purpose trait under natural selection.