Analysis of toxic and non-toxic Alexandrium (Dinophyceae) species using ribosomal RNA gene sequences
Scholin, Christopher A.
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Sequences of small subunit (Ss) and large subunit (Ls) ribosomal RNA genes (rDNA) from the marine dinoflagellates Alexandrium tamarense, A. catenella, A. fundyense, A. affine, A. minutum, A. lusitanicum and A. andersoni were compared to assess the organisms' relationships. Cultures represent isolates from North America, Western Europe, Thailand, Japan, Australia and the ballast water of several cargo vessels, and include both toxic and non-toxic strains. An emphasis was placed on the A. tamarense/catenella/fundyense "species complex," a group of morphotypically-similar organisms found in many regions of the world. Two distinct SsrRNA genes, termed the "A gene" and the "B gene," were found in a toxic A. fundyense isolated from eastern North America. The B gene is considered to be a pseudogene. A restriction fragment length polymorphism (RFLP) assay developed to detect the A and B genes revealed five distinct groups of Alexandrium isolates. Three subdivide the A. tamarense/catenella/fundyense complex, but do not correlate with morphospecies designations. The two remaining groups are associated with cultures that clearly differ morphologically from the A. tamarense/ catenella/fundyense group: the fourth group consists of A. affine isolates, and the fifth group is represented by A. minutum, A.lusitanicum and A. andersoni. The B gene was only found in A. tamarense/catenella/ fundyense, but not in all members of this species complex. The B gene is not uniformly distributed among global populations of Alexandrium. All A. tamarense/catenella/fundyense isolates from North America harbor this gene, but it has also been found in some A. tamarense from scattered locations in Japan, as well as in A. tamarense from the ballast water of one cargo vessel which was on a defined run from Japan to Australia. The B gene may be endemic to North American populations of A. tamarense/catenella/fundyense. It is possible that in the recent past North American A. tamarense were introduced to Japanese waters, and cysts of these organisms have been transported from Japan to Australia. A subset of isolates examined using the the RFLP assay were also compared by cloning and sequencing a fragment of their LsrDNA. Eight major classes of LsrDNA sequences, termed "ribotypes," were identified. Five ribotypes subdivide members of the A. tamarense/catenella!fundyense complex; all isolates containing the B gene cluster as one ribotype. The three remaining ribotypes are typified by: 1) A. affine; 2) A. minutum and A. lusitanicum; and, 3) A. andersoni. LsrDNAs from A. minutum and A. lusitanicum are indistinguishable. A. minutum!lusitanicum/andersoni may represent another Alexandrium species complex, analogous to the A. tamarense/catenella/fundyense group. An organisms' ability to produce toxin appears to be correlated with its LsrDNA phylogenetic lineage. Ribotypes ascribed by the LsrDNA sequences are in complete agreement with, and offer a finer-scale resolution of, groups defined by SsrDNA restriction patterns. The SsrDNA RFLP groups and LsrDNA ribotypes are useful species- and population-specific markers. Alexandrium tamarense/catenella/fundyense exist as geneticallydistinct "strains" (populations), not three genetically-distinct species: representatives collected from the same geographic region appear the most similar, regardless of morphotype, whereas those from geographicallyseparated populations are more divergent even when the same morphospecies are compared. Contrary to this general pattern, A. tamarense/catenella from Japan were found to be exceptionally heterogeneous. Ballast water samples show that viable cysts (resting spores) of toxigenic A. tamarense/catenella are being discharged into Australian ports from multiple, genetically-distinct source populations. The rDNA sequences were also used to test theories accounting for the evolution and global dispersal of A. tamarense/catenella/fundyense. Results suggest a monophyletic radiation of these organisms from a common ancestor that included, or gave rise to, multiple morphotypes. Populations appear to have diverged as a result of vicariance (geographic isolation). The co-occurrence of genetically-distinct strains of these organisms is an indication of dispersal. An example of this is seen in Japan where an introduction of North American A. tamarense appears likely. Determining the timing of dispersal events is problematic if based strictly on rDNA sequence similarities, since these molecules undergo change on a scale of millions of years.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution November 1992
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