Phylogeny and biogeography of the toxic dinoflagellate Alexandrium

Abstract

The incidence and known distribution of paralytic shellfish poisoning (PSP) have both increased dramatically in recent decades. A concurrent rise in bloom frequency and geographic range of PSP toxin-producing Alexandrium dinoflagellates explains the increase in PSP, but the reasons for changes in Alexandrium occurrence are unknown. This thesis explores the phylogeny, taxonomy, and biogeography of Alexandrium in light of this recent expansion. Alexandrium phylogeny was reconstructed through rDNA sequence analysis and compared to traditional morphological taxonomy. Alexandrium split into two groups, termed the α and ß clades. Interspecific relationships did not correlate with the morphological traits traditionally used to identify and group species, although other traits appeared phylogenetic ally conserved. The ability to produce toxins has been acquired and/or lost multiple times during Alexandrium evolution. Because most PSP events are caused by either the tamarensis or minutum complexes, the phylogeny, species definitions, and biogeography of each complex was examined. The morphospecies of the tamarensis complex, A. catenella, A. tamarense, and A. fundyense, did not represent valid species by the phylogenetic, biologic or morphological species concepts. Instead, five cryptic species were identified through phylogeny and mating incompatibility. A. universa and A. toxipotens contain all toxic strains, while A. mediterra, A. tamarensis and A. tasmanense contain only non-toxic isolates. Within the minutum group, A. lusitanicum and A. angustitabulatum were also not distinct species based on morphology and phylogeny while A. insuetum and A. tamutum were clearly distinct. Three new minutum group species were identified on the basis of morphology, phylogeny and prior research. Unlike the pattern found for the tamarensis complex, toxic and non-toxic A. minutum strains cannot be segregated based upon LSU sequences. The reconstructed biogeography of the tamarensis and minutum complexes indicate that both natural dispersal and human-assisted transportation of Alexandrium have caused the geographic spread. Human-assisted transport of toxic A. catenella-type cells from Asia to the Thau Lagoon, France, was demonstrated in chapter iv. This thesis demonstrates the importance of human action in the recent PSP increase, better defines species boundaries and provides an invaluable genetic database for tracking future Alexandrium spread and distinguishing between harmful and non-toxic Alexandrium blooms.