Genetic Diversity and Population Connectivity of Syringodium filiforme in the Florida Keys, USA and Northeastern Subtropical Atlantic Region

Syringodium filiforme is a seagrass that populates extensive meadows in South Florida and other coastal areas throughout the tropical and subtropical Atlantic. Such meadows are habitat-forming, support an abundance of biota, and confer economic and ecological benefits. The ability of this species to colonize bare areas following disturbance has been attributed to its fast rhizome elongation rates. The central research problem addressed in this thesis is how this rapid clonal growth strategy influences sexual reproduction and overall population survival in S. filiforme, which I indirectly evaluated through genetic diversity analyses. Measures of population genetic diversity and population genetic structure provide insight into the number and distribution of genetically distinct or related individuals within and among populations, thereby indirectly estimating the prevalence of sexual reproduction. In this thesis, I assess genetic diversity, population structure and gene flow in the tropical seagrass, Syringodium filiforme, from populations across the Florida Keys as well as additional sites in South Florida and the northeastern subtropical Atlantic.
A total of 17 polymorphic microsatellite markers were developed for population genetic analysis of S. filiforme at the onset of this study. Sixteen populations from the Florida Keys and single populations from Florida Bay, Tampa Bay, Bermuda and the Bahamas were sampled and analyzed using these markers to determine how genetic diversity partitions within and among sampled populations. Allelic diversity and heterozygosity-based statistics were used to measure genetic diversity, and population structure and gene flow were analyzed using FST- and model-based methods.
The markers used in this study proved to be powerful, as the probability of genet identity (clonal membership) due to random chance was very low among the samples analyzed because of the high number of loci (17) amplified and the moderate to high allelic diversity (A = 35 – 106) detected in all populations. In most cases, observed heterozygosity exceeded expected heterozygosity, indicating heterozygote excess is common in S. filiforme. Population genetic analyses revealed that S. filiforme exhibits high clonality at the spatial scales sampled (1.5 – 5 m between ramets) where the ratio of unique genotypes to samples was always less than R = 0.62. In fact, there were only 6 unique multilocus genotypes detected among 123 shoots sampled in the Florida Bay population.
Across all populations, moderate yet significant genetic differentiation was observed (FST = 0.15, p = 0.001). The Gulf- and Atlantic-side populations exhibited relatively low within-group differentiation and consistently clustered separately, suggesting the Florida Keys archipelago presents a barrier to gene flow. The Bahamas population was most differentiated from all other populations (FST = 0.19 – 0.53), while the Bermuda population did not exhibit as great differentiation (FST = 0.14 – 0.28) though it is geographically most distant from all other populations.
The findings from this study suggest S. filiforme maintains moderate to high levels of heterozygosity and higher than expected genetic diversity even though it is a highly clonal seagrass. Population structure clearly develops across the complex topography and hydrology of the Florida Keys, in which Gulf and Atlantic populations represent distinct genetic clusters. Gulf and Atlantic genetic disjunctions have been observed for a number of other marine organisms in the southeastern U.S., suggesting the Florida peninsula represents a phylogeographic break for S. filiforme and several other taxa. In addition to geologic history, contemporary isolating mechanisms such as limited propagule exchange mediated by ocean currents may contribute to lower genetic connectivity between Florida, Bermuda and the Bahamas populations.