Phylogenetic analysis of toxic Alexandrium (Dinophyceae) isolates from South Africa: implications for the global phylogeography of the Alexandrium tamarense species complex

摘要

Current knowledge on global phylogeographic patterns and dispersal routes of dinoflagellates in the Alexandrium tamarense species complex is largely based on isolates from the northern hemisphere. The paucity of molecular data available has limited the development of evolutionary hypotheses for southern hemisphere Alexandrium Populations. To address this shortcoming, we present morphological, toxicity and molecular data from three Alexandrium isolates from South African coastal waters. Morphological descriptions indicate that these isolates correspond to the species A. catenella and A. tamiyovanichi, making this the first recorded presence of A. tamiyovanichi in African waters. Both species produce paralytic shellfish poisoning toxins, with the total average cellular toxicity of the A. catenella isolates [9.77 fmol saxitoxin (STX) eq cell(-1)] exceeding that of A. tamiyavanichi by a factor of 35. Complex toxin profiles showed high proportions of B I and C1+C2 congeners (> 20% molar percentage each) in both species, with STX representing 22 % molar percentage in A. catenella and neosaxitoxin 31 % in A. tamiyavanichi. Nuclear ribosomal large subunit (LSU) sequences from these isolates were generated and the D1-D2 LSU regions used in neighbour-joining, maximum-parsimony and Bayesian inference phylogenetic analyses with 190 additional isolate sequences from around the world. All three South African isolates belong to the A. tamarense species complex, but have different phylogenetic affinities: A. tamiyavanichi was included in the Tropical Asian clade, and A. catenella in the North American clade, a ribotype with a cosmopolitan distribution. The geographic distribution of Alexandrium populations in South African waters is discussed in relation to regional biogeographic patterns. We suggest that the A. tamiyavanichi population is derived from West Pacific ancestral stocks via a West Pacific-East-South African dispersal route. Despite high genetic similarity between the A. catenella isolates and relatives in the North American clade, historical evidence suggests that human-mediated introduction is unlikely. Elucidating the evolutionary history giving rise to the current distribution patterns of isolates in the North American clade requires molecular markers with greater resolution than the commonly used D1-D2 LSU ribosornal DNA region.