GENETIC-STRUCTURE OF THE FLOUNDERS PLATICHTHYS-FLESUS AND P-STELLATUSAT DIFFERENT GEOGRAPHIC SCALES

The genetic structure of the flounders Platichthys flesus L. and P. st ellatus Pallas was investigated on different spatial scales through an alysis of allozyme variation at 7 to 24 polymorphic loci in samples co llected from different regions (Baltic Sea, North Sea; Brittany, Portu gal, western Mediterranean, Adriatic Sea, Aegean Sea and Japan) in 198 4 to 1987. No geographic variation was evident within a region. Some p attern of differentiation by distance was inferred within the Atlantic , while the Mediterranean comprised three geographically isolated popu lations and was itself geographically isolated from the Atlantic (fixe d allele differences at up to three loci were found among P. flesus po pulations from the Atlantic, the western Mediterranean, the Adriatic S ea, the Aegean Sea and also P. stellatus from the coast of Japan). Sea temperature during the reproductive period probably acts as a barrier to gene flow between populations. Genetic distances among European fl ounder populations (P. flesus) were higher than, or of the same magnit ude as, the genetic distance between Pacific (P. stellatus) and Europe an flounder populations, suggesting that P. flesus is paraphyletic and /or there is no phylogenetic basis to recognising P. stellatus as a di fferent species. The divergence between P. flesus and P. stellatus was thus inferred to be more recent than the divergence between the prese nt P. flesus populations from the NE Atlantic and eastern Mediterranea n. The eastern Mediterranean populations are thought to originate from the colonisation of the Mediterranean by a proto-P. flesus/P. stellat us ancestor, whereas the present western Mediterranean population has undergone a more recent colonisation event by P. flesus. Patterns of m itochondrial DNA variation, established on a smaller array of P. flesu s samples, were in accordance with the geographic patterns inferred fr om the allozyme survey, In addition, they supported the hypothesis of a two-step colonisation of the western Mediterranean. These results co ntribute to our understanding of the biogeography of the Mediterranean marine fauna, especially the group of boreal remnants to which P. fle sus belongs.