Assessing patterns of geographic dispersal of Gelidium sesquipedale (Rhodophyta) through RAPD differentiation of populations

Randomly ampldied polymorphic DNAs (RAPDs) of bulked genomic DNA samples were used to analyse the genetlc differentiation of Gelidium sesquipedale populations. They reflect the patterns of gene flow, which in turn depend on the dispersal mechanisms of the species and on near-shore ocean currents. Fourteen populations were sampled from northern France to Morocco, covering the geographical distribution of the species. A single bulk DNA sample (from 15 individuals) was used in each population, under the assumption that the resulting patterns represent the populations' most common genetic features. To test this, we investigated the genetic variability among 5 bulk samples within a single population. Genetic distances anlong bulks were very low (average = 0.065) and were significantly lower than those observed between geographically separated populations (average = 0.241). Neighbour-joining analysis of the distance matrix of populations separated a well-supported group including populations of northern Spain and of northern France, and a less-supported group containing populations of northern Portugal. Multidimensional scaling of the genetic distance matrix revealed 2 isolated populations. Sao Rafael in southern Portugal and Algeciras in southern Spain. These patterns of genetic differentiation are discussed under the available data on the near-shore ocean currents. Results suggest that the genetic differentiatlon of G. sesqujpedale populations may be used as a biological tracer of prevailing flows and barriers of the near-shore currents. A positive correlation between geographical and genet~c distances of G. sesquipedale populations along the specles geographical distribution was found, suggesting that a continuous transport of detached fronds and their reattachment to new substrate must be an effective dispersal mechanism of the species, sustaining the gene flow among populations.