Mitochondrial, but not rDNA, genes fail to discriminate dinoflagellate species in the genus Ostreopsis

The marine dinoflagellate genus Ostreopsis includes species producing potent toxic compounds, such as paliytoxin and palytoxin analogs, which might cause problematic blooms in the Mediterranean Sea and other tropical or temperate areas. Phylogenetic and phylogeographical studies based on sequences of ribosomal genes, revealed the existence of distinct species and, within them, clades in relation to geographical distribution potentially representing new or cryptic species. The morphological variability of Ostreopsis complicates the identification of species; thus, molecular analyses of isolates or field samples can be helpful. The aim of this study was to improve the characterization of Ostreopsis species and investigate the geographical distribution by using large dataset composed of both new and old sampled isolates. To determine if mitochondrial genes can be used to identify Ostreopsis species, we designed new primers sets then amplified and sequenced representative regions of the COI (cytochrome c oxydase 1) and cob (cytochrome b) genes. Phylogenetic analyses of the resulting mitochondrial DNA (mtDNA) and existing or new ribosomal DNA (rDNA) sequence data showed little divergence in the mtDNA sequences among Ostreopsis species indicating that neither the COI or cob genes are phylogenetically informative. In contrast, the ribosomal gene phylogeny indicated the existence of distinct Ostreopsis species. A network of haplotypes (based on ITS-5.8S rDNA) inferred from O. cf. ovata isolates collected worldwide revealed that Atlantic/Mediterranean and Indo/Pacific areas might host two separated large populations. In conclusion, it appears that rDNA gene sequences provide an effective molecular means of distinguishing the phylogenetic and phylogeographical relationships among Ostreopsis species.