Alexandrium minutum is a dinoflagellate producing recurrent toxic blooms along the French Atlantic coast. This microalga produces PSP toxins which are among the most powerful toxins in the world. Originally from the Mediterranean, this species has gradually invaded the European Atlantic coasts. In the middle of 80's, this invasive alga was observed along the French Atlantic coast, and then successively extended its distribution to England, Ireland and Denmark. Today this toxic species persists along the Brittany coast estuaries producing recurrent toxic blooms, although in some sites blooms appear to be limited by the presence of parasites. In this study we examined the processes that may affect population genetic diversity of A. minutum and the effects of local adaptation to biotic and abiotic factors. We made crossing experiments in laboratory to better understand reproduction and to determine its reproductive system. Then we used molecular tools to determine the importance of sexual reproduction in natural populations (sexual versus asexual reproduction) in French Brittany. Finally, we studied the genetic variability of annual blooms for two consecutive years in Penzé and Rance estuaries. Crossing experiments helped to better understand the gamete recognition system. Gametes recognition in this species is not a bipolar system as is usually observed in numerous organisms and involves different mating types. Moreover, our results suggest that temporary cysts are able to survive several months in cold and darkness in the laboratory conditions. Using microsatellite markers, we found that blooms have a high genetic and genotypic diversity, with no repeated multilocus genotypes. These results suggest that clonal reproduction is not a dominant process during bloom period compared to sexual reproduction. The spatio-temporal genetic analyses (between Penzé and Rance, between years, and between bloom periods) showed significant differentiation at both spatial and temporal scales. The spatial differentiation indicates a restricted gene flow between the two sites and this mainly during the blooms events. Indeed, the blooms usually develop during the neap tides are relatively isolated. These results were confirmed by the study of the genetic structure of A. minutum along the Brittany coast that reveals limited gene flow between populations. These results suggest that migration is relatively limited between estuaries. In addition, the genetic differentiation between estuaries could also be closely linked to the life history traits. Indeed, the rapid development of blooms in a new site, followed by the formation of a large number of cysts accumulated in estuaries could be a particular system characterized by high genetic buffer and limiting arrival of immigrants by competition (Monopolization hypothesis) (De Meester et al. 2002). Finally, our results suggest that the toxic blooms observed in Rance, in the Morlaix Bay and Brest estuaries correspond to different introductions processes.
