Toward an Assessment on the Taxonomy of Dinoflagellates That Produce Calcareous Cysts (Calciodinelloideae, Dinophyceae): A Morphological and Molecular Approach

In recent years an unified classification system for both fossil cysts and extant dinoflagellate species has been proposed. This classification has prompted investigations aimed at testing the phylogenetic validity of the distinctive morphological characters of both vegetative cells and cysts. We have focused on a group of dinoflagellates that produce calcareous resting cysts, which are abundant in marine neritic and oceanic waters and form wide fossil deposits. Extant species are included in the genera Scrippsiella, Ensiculifera, and “ Pentapharsodinium ” (subfamily Calciodinelloideae), which are distinguished by the number and/or shape of the cingular plates of their planktonic vegetative stages. On the other hand, the classification of fossil cysts is based on the morphology of the calcareous covering and on the orientation of crystals. In this study we combine the information derived from morphological traits of both motile stages and cysts with that obtained from nucleotide sequence analysis of the ribosomal DNA internal transcribed spacers (ITS). We also describe the vegetative cell produced by the calcareous cyst Calcigonellum infula and a new variety, Scrippsiella trochoidea var. aciculifera var. nov. Molecular analyses confirm the monophyletic origin of the genus Scrippsiella, whereas the “ Pentapharsodinium ” and Ensiculifera species are grouped together in another monophyletic cluster. The coupled morphological and molecular approach supports the taxonomic value of some of the characters of the planktonic stage, such as the cingular plate number. On the other hand, it questions the validity of other morphological characters of both vegetative and encysted stages. Our data provide a phylogenetic classification of these dinoflagellates; however, they also open a debate that would imply a redefinition of the characters to be used for the circumscription of the subfamily Calciodinelloideae.