Gene flow and genetic diversity of turtle grass, Thalassia testudinum, banks ex könig, in the lower Florida Keys

Abstract

Turtle grass ( Thalassia testudinum, Banks ex König) is the dominant seagrass in the very productive and valuable coastal seagrass community. The present study investigated genetic diversity and gene flow in turtle grass collected at 18 sites in the lower Florida Keys. Fourteen allozyme loci were resolved, of which 5 (ADH-2, GPI, LAP, 6-PDGH, and PGM-2) were variable. The mean heterozygosity for the 18 turtle grass sites was 0.027. The majority of genetic diversity measured occurred within populations, with a small proportion of genetic diversity measured among populations ( G ST =0.050). The number of migrants each generation ( N m), an estimate of gene flow, was calculated using Wright's F-statistics. The F-statistics yielded an N m =24.8 for sites within the same area, N m =3.9 for adjacent sites (within 4 km), and an N m =1.0 among all 18 collection sites. These results indicated strong gene flow for sites that were adjacent, but not at a distance >4 km. Three Mantel tests were performed using a genetic distance matrix and three different geographic matrices to test which model of gene flow (isolation by distance, stepping stone, or island model) was appropriate for turtle grass. A stepping stone model closely predicted ( p=0.0015) turtle grass gene flow; physical adjacency (stepping stone model) had a greater effect than geographic distance. Turtle grass sampled on two coral reef sites had significantly different allele frequencies at the ADH-2 locus from turtle grass on sites in the mangroves. The majority of other studies dealing with seagrass species have found little-to-no genetic variation; however, the present study has documented genetic variation in turtle grass at five allozyme loci. This variation suggests that sexual reproduction may significantly contribute to T. testudinum's genetic structure and evolution. However, overall genetic diversity was relatively low across all sites, indicating a trend towards genetic uniformity of turtle grass in the lower Florida Keys. This genetic uniformity may have contributed to the large turtle grass die-offs in recent years.