IDENTIFICATION OF GROUP-SPECIFIC AND STRAIN-SPECIFIC GENETIC-MARKERS FOR GLOBALLY DISTRIBUTED ALEXANDRIUM (DINOPHYCEAE) .2. SEQUENCE-ANALYSIS OF A FRAGMENT OF THE LSU RIBOSOMAL-RNA GENE

A fragment of the large-subunit (LSU) ribosomal RNA gene (rDNA) from t he marine finoflagellates Alexandrium tamarense (Lebour) Balech, A. ca tenella (Whedon et Kofoid) Balech, A. fundyense Balech, A. affine (Fuk uyo et Inoue) Balech, A. minutum Halim, A. lusitanicum Balech, and A. andersoni Balech was cloned and sequenced to assess inter- and intrasp ecific relationships. Cultures examined were from North America, weste rn Europe, Thailand, Japan, Australia, and the ballast water of severa l cargo vessels and included both toxic and nontoxic isolates. Parsimo ny analyses revealed eight major classes of sequences, or ''ribotypes, '' indicative of both species- and strain-specific genetic markers. Fi ve ribotypes subdivided members of the A. tamarense/catenella/ fundyen se species cluster (the ''tamarensis complex'') but did not correlate with morphospecies designations. The three remaining ribotypes were as sociated with cultures that clearly differ morphologically from the ta marensis complex. These distinct sequences were typified by 1) A. affi ne, 2) A. minutum and A. lusitanicum, and 3) A. andersoni. LSU rDNA fr om A. minutum and A. lusitanicum was indistinguishable. An isolate's a bility to produce toxin, or lack thereof, was consistent within phylog enetic terminal taxa. Results of this study are in complete agreement with conclusions from previous work using restriction fragment-length polymorphism analysis of small-subunit rRNA genes, but the LSU rDNA se quences provided finer-scale species and population resolution. The fi ve divergent lineages of the tamarensis complex appeared indicative of regional populations; representatives collected from the same geograp hic region were the most similar, regardless of morphotype, whereas th ose from geographically separated populations were more divergent even when the same morphospecies were compared. Contrary to this general p attern, A. tamarense and A. catenella from Japan were exceptionally he terogeneous, displaying sequences associated with Australian, North Am erican, and western European isolates. This diversity may stem from in troductions of A. tamarense to Japan from genetically divergent source s in North America and western Europe. Alexandrium catenella from Japa n and Australia appeared identical, suggesting that these two regional populations share a recent, common ancestry. One explanation for this genetic continuity was suggested by A. catenella cysts transported fr om Japan to Australia via ships' ballast water: the cysts contained LS U rDNA sequences that were indistinguishable from those of known popul ations of A. catenella in both Japan and Australia. Ships ballasted in South Korea and Japan have also fostered a dispersal of viable A. tam arense cysts to Australia, but their LSU rDNA sequences indicated they are genetically distinct from A. tamarense/catenella previously found in Australia and genetically distinct from each other, as well. Human -assisted dispersal is a plausible mechanism for inoculating a region with diverse representatives of the tamarensis complex from geographic ally and genetically distinct source populations. The D1-D2 region of Alexandrium LSU rDNA is a valuable taxonomic and biogeographic marker and a useful genetic reference for addressing dispersal hypotheses.