and Genotypes Occur Together within Natural Sub-Populations from the Western Atlantic.

Saxitoxin (STX) is a secondary metabolite and potent neurotoxin produced by several genera of harmful algal bloom (HAB) marine dinoflagellates. The basis for variability in STX production within natural bloom populations is undefined as both toxic and non-toxic strains (of the same species) have been isolated from the same geographic locations. is a STX-producing bioluminescent dinoflagellate that blooms along the east coast of Florida as well as the bioluminescent bays in Puerto Rico (PR), though no toxicity reports exist for PR populations. The core genes in the dinoflagellate STX biosynthetic pathway have been identified, and the gene is essential for toxin production. Using as a molecular proxy for the genetic capacity of STX production, we examined + and - genotype frequency at the single cell level in populations from different locations in the Indian River Lagoon (IRL), FL, and Mosquito Bay (MB), a bioluminescent bay in PR. Multiplex PCR was performed on individual cells with -specific primers targeting the 18S rRNA gene and . The results reveal that within discrete natural populations of , both + and - genotypes occur, and the + genotype dominates. In the IRL, the frequency of the + genotype ranged from ca. 80-100%. In MB, + genotype frequency ranged from ca 40-66%. To assess the extent of variation within individual cells, amplicons from single cells representative of the different sampling sites were cloned and sequenced. Overall, two variants were consistently obtained, one of which is likely a pseudogene based on alignment with cDNA sequences. These are the first data demonstrating the existence of both genotypes in natural sub-populations, as well as presence in from PR. These results provide insights on underlying genetic factors influencing the potential for toxin variability among natural sub-populations of HAB species and highlight the need to study the genetic diversity within HAB sub-populations at a fine level in order to identify the molecular mechanisms driving HAB evolution.