Abstract
Many marine species have widespread geographic ranges derived from their evolutionary and ecological history particularly their modes of dispersal. Seagrass (marine angiosperm) species have ranges that are unusually widespread, which is not unexpected following recent reviews of reproductive strategies demonstrating the potential for long‐distance dispersal combined with longevity through clonality. An exemplar of these dual biological features is turtle grass (Thalassia testudinum) which is an ecologically important species throughout the tropical Atlantic region. Turtle grass has been documented to have long‐distance dispersal via floating fruits and also extreme clonality and longevity. We hypothesize that across its range, Thalassia testudinum will have very limited regional population structure due to these characteristics and under typical models of population structure would expect to detect high levels of genetic connectivity. There are very few studies of range‐wide genetic connectivity documented for seagrasses or other sessile marine species. This study presents a population genetic dataset that represents a geographic area exceeding 14,000 km2. Population genetic diversity was evaluated from 32 Thalassia testudinum populations sampled across the Caribbean and Gulf of Mexico. Genotypes were based on nine microsatellites, and haplotypes were based on chloroplast DNA sequences. Very limited phylogeographic signal from cpDNA reduced the potential comparative analyses possible. Multiple analytical clustering approaches on population genetic data revealed two significant genetic partitions: (a) the Caribbean and (b) the Gulf of Mexico. Genetic diversity was high (H E = 0.641), and isolation by distance was significant; gene flow and migration estimates across the entire range were however modest, we suggest that the frequency of successful recruitment across the range is uncommon. Thalassia testudinum maintains genetic diversity across its entire distribution range. The genetic split may be explained by genetic drift during recolonization from refugia following relatively recent reduction in available habitat such as the last glacial maxima.
Highlights
Genetic connectivity calculated as the relative number of migrants between population pairs indicate that nearby populations are more interconnected than distant ones, with some significant migration between the Yucatan Peninsula and the Florida Keys (Figure 3 based on Nm Figure 3 and GST Supporting Information Figure S6 in Appendix S1, Tables S13 and S14 in Appendix S2)
We investigated the hypothesis that the biological traits of Thalassia testudinum provide for genetic connectivity over large distances leading to low levels of genetic structure across the extant range
The findings presented in this study deliver significant insight into the genetic diversity, clonality, connectivity, and overall range-wide population structure of a important tropical seagrass species
Summary
Sample permits were issued by SAGARPA for Mexico, and Ministerio del Ambiente for Costa Rica; specific permission was not required for the other (CARICOMP) sampling sites. Structure analysis and plotting ∆K describe maximum genetic structure at two regional clusters, subdividing the populations into three (K = 3) resulted in above average structure, but less markedly (Supporting Information Figures S1, S2, and Table S3 in Appendix S1). Genetic connectivity calculated as the relative number of migrants between population pairs indicate that nearby populations are more interconnected than distant ones, with some significant migration between the Yucatan Peninsula and the Florida Keys (Figure 3 based on Nm Figure 3 and GST Supporting Information Figure S6 in Appendix S1, Tables S13 and S14 in Appendix S2). A Bayesian approach to infer migration into the sampled populations in the most recent generations (BayesAss analysis) shows that successful settlement of new or second-generation individuals is usually below 1% and never surpasses 2% of sampled MLGs (Supporting Information Table S12 in Appendix S2). Comparing the outcomes from five different measures of connectivity and population distinctiveness, as relative migration based on Nm (Figure 3) and GST (Supporting Information Figure S6 in Appendix S1), BayesAss (Supporting Information Table S12 in Appendix S2), network analysis (Supporting Information Figure S5 in Appendix S1) and PCA (Supporting Information Figure S4 in Appendix S1) demonstrates that the broad scale processes dominate the data whichever method of analysis utilized
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