Recognizing dinoflagellate species using ITS rDNA sequences

Dinoflagellate taxonomy is based primarily on morphology and morphometric data that can be difficult to obtain. In contrast, molecular data can be rapidly and cost-effectively acquired, which has led to a rapid accumulation of sequence data in GenBank. Currently there are no systematic criteria for utilizing taxonomically unassigned sequence data to identify putative species that could in turn serve as a basis for testable hypotheses concerning the taxonomy, diversity, distribution, and toxicity of these organisms. The goal of this research was to evaluate whether simple, uncorrected genetic distances (p) calculated using ITS1/5.8S/ITS2 (ITS region) rDNA sequences could be used to develop criteria for recognizing putative species before formal morphological evaluation and classification. The current analysis used sequences from 81 dinoflagellate species belonging to 14 genera. For this diverse assemblage of dinoflagellate species, the within-species genetic distances between ITS region copies (p=0.000-0.021 substitutions per site) were consistently less than those observed between species (p=0.042-0.580). Our results indicate that a between-species uncorrected genetic distance of p >= 0.04 could be used to delineate most free-living dinoflagellate species. Recently evolved species, however, may have ITS p values < 0.04 and would require more extensive morphological and genetic analyses to resolve. For most species, the sequence of the dominant ITS region allele has the potential to serve as a unique species-specific DNA barcode that could be used for the rapid identification of dinoflagellates in field and laboratory studies.