Sequence comparisons link toxic European isolates of Alexandrium tamarense from the Orkney Islands to toxic North American stocks

Twenty-one cultures of the Alexandrium tamarense/catenella/fundyense species- complex, isolated from a bloom at the Orkney Islands, north of Scotland, were examined morphologically and tested for toxicity using HPLC analyses. All cultures belong to the morpho-species of A. tamarense (Labour) Balech rather than A. fundyense Balech and are as toxic as the most toxic A. fundyense isolates from North America. A subset was genetically analysed using sequence data from the D1/D2 region of the LSU rRNA gene (656 nt). Genetic analyses indicated that these A. tamarense populations were related to North American isolates. At least three classes of the D1/D2 region were found in the cloned material. Sufficient base substitutions in the D1/D2 region preclude introduction into the Orkneys by human mechanisms (i.e., in ballast water or in shellfish stocks). The sequence analysis supports a dispersal hypothesis in recent evolutionary time of North American stocks of A. tamarense into Northern European waters via currents, possibly as part of the hypothesised original dispersal in the North Atlantic from the Pacific. In contrast, other Western European isolates of A. tamarense, whose D1/D2 region has been sequenced, are non-toxic and belong to a different gene pool.