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Populations of Thalassia testudinum (Banks ex Konig) at the northern and southern limits of the west coast of Florida were compared with a Jamaican population using randomly amplified polymorphism detection (RAPD). With the exception of those from Apalachicola Bay, virtually all samples were distinct genetic individuals. Those putative clone mates that were identified often had other genets dispersed between them. Several distinct differences were observed between the northern and southern populations. The southern populations have higher percentages of the total possible number of bands present, of polymorphic bands, and of bands exclusive to a population, as well as a greater number of RAPD phenotypes and a greater mean number of differences between phenotypes. The biological phenomena that may explain these patterns include increased reproductive success, decreased inbreeding or increased population size in the southern populations. A fourth possibility is unidirectional gene flow from north to south. An analysis of molecular variance (AMOVA) was done indicating that approximately 81% of the variation is within beds, suggesting a homogeneous species. Yet the populations were clearly distinguishable from one another at microgeographic ranges. The level of genetic variation observed is characteristic of species with the same life history traits as Thalassia testudinum and was not predicted based on field observations of the species. This study, in conjunction with other molecular seagrass studies done to date, challenges our understanding of seagrass growth, reproduction, and propagation. There does not appear to be any pattern of reproductive traits, such as dioecy, monoecy, vivipary, or seed banking, that can reliably predict levels of genetic variation in a given seagrass species. |
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