High levels of gene flow and low population genetic structure related to high dispersal potential of a tropical marine angiosperm

Marine ecosystems are traditionally considered to be highly connected, caused by long-distance movements of propagules in an open fluid medium. But this principle is not universal, and populations of many marine organisms have restricted gene-flow, including ones with drifting propagules. Direct tracking of marine free-floating propagules over large distances is virtually impossible, but long-distance dispersal (LDD) can be deduced from their drifting time and transport rates. Alternatively, LDD can be estimated as genetic exchange with the use of allelic frequency data obtained with genetic markers. The present study compares both approaches of LDD estimation for the tropical seagrass Thalassia testudinum. Fruit dispersal potential was measured in Puerto Morelos reef lagoon, Mexico. Between 10 and 15% of the fruits floated to the surface (floating potential <1 to 10 d), with a derived potential dispersal of <1 to 360 km. The remainder of the fruits dehisced in situ followed by limited seed dispersal (<1 to 10 m). Genetic diversity was characterized using 6 microsatellite loci for 16 populations distributed along a 1350 km long stretch of the Mexican coast, revealing a total of 58 alleles. AMOVA showed that only 9.75% of the total variation was accounted for by genetic differences between the populations. Isolation by distance was significant, and revealed a panmictic area of 350 km. The results of both approaches of LDD estimation coincided well and the maximum displacement of the fruits of this marine angiosperm exceeded, by 1 to 2 orders of magnitude, the reported seed dispersal by terrestrial seed plants.