MOLECULAR EVOLUTION OF THE ALEXANDRIUM-TAMARENSE SPECIES COMPLEX (DINOPHYCEAE) - DISPERSAL IN THE NORTH-AMERICAN AND WEST PACIFIC REGIONS

Hypotheses concerning the molecular evolution, population structure an d dispersal of the toxic dinoflagellates Alexandrium tamarense (Lebour ) Balech, A. catenella (Whedon et Kofoid) Balech and A. fundyense Bale ch (the 'tanarensis species complex') are examined in light of previou s reports that compared their small and large-subunit ribosomal RNA ge nt: (SSU and LSU rDNA) sequences. Forty-eight cultures from North Amer ica, western Europe. Japan: Australia and Thailand were analysed by a restriction fragment length polymorphism (RFLP) assay of SSU rDNA. and 34 of those by sequencing a fragment of LSU rDNA. Results indicate th at the tamarensis species complex comprises at least 5 genetically dis tinct evolutionary lineages ('ribotypes') whose phylogenetic relations hips reflect geographic populations, not morphospecies. We believe thi s pattern reveals a monophyletic radiation from an ancestor that inclu ded or gave rise to multiple morphotypes. Accumulated mutations in des cendants' SSU and LSU rDNA are suggested to reflect the prolonged geog raphic isolation and independent evolution of distinct populations. No vel SSU rDNA data are presented in support of this hypothesis. Given t he proposed evolutionary framework and other historical considerations , we interpret the genetic diversity of Japanese A. tamarense/catenell a as indicative of dispersed populations from genetically distinct sou rces. The possibility that A. catenella was introduced to Australia fr om an Asian source is also considered. In both cases, however, rDNA da ta alone are insufficient to distinguish whether this occurred thousan ds of years ago by natural immigrations or as a result of recent human activity (ballast water transport or relays of shellfish stocks). The uncertainty of dispersal timing stems from the relatively slow rate a t which rDNA evolves and lack of fossil evidence. Ballast water sample s show that viable toxigenic Alexandrium cysts have undergone human-as sisted transoceanic transport, illustrating how a region could be 'see ded' with genetically distinct A. tamarense and A. catenella from a va riety of regional populations.