Redefinition of the Dinoflagellate GenusAlexandriumBased onCentrodinium: Reinstatement ofGessneriumandProtogonyaulax, andEpisemicolongen. nov. (Gonyaulacales, Dinophyceae)

The genusCentrodiniumcontains oceanic and predominantly tropical species that have received little attention. Three species ofCentrodiniumwere examined using thecal plate dissociation, scanning electron microscopy, and molecular sequences. The apical horn ofCentrodinium intermediumandC. eminensis formed by the elongation of the fourth apical plate, and a second apical split into two plates. InC. punctatumtwo apical plates (2′ and 4′) almost completely encircle the apical pore plate (Po), while the contact with the plate 1′ in the ventral side is much reduced, and the plate 3′ does not reach the Po. Moreover, its left posterior lateral sulcal plate is longer than its right pair, while reversed in the typicalCentrodiniumspp. The sulcal posterior plate ofC. punctatumis located in the left-ventral side below the plates 1′′′ and 2′′′, while the sulcal posterior plate located in the right face below the plates 4′′′ and 5′′′ in the typicalCentrodiniumspp. Phylogenetic analyses based on the small and large subunit of the rRNA gene showed thatCentrodiniumspp. andAlexandrium affine/A. gaarderaeclustered as a sister clade of theAlexandrium tamarense/catenella/fraterculusgroups. The clade of the subgenusGessnerium, and the clade of the type species ofAlexandrium,A. minutum, with four divergent species, clustered in more basal positions. The polyphyly ofAlexandriumis solved with the split into four genera: (1)Alexandriumsensu stricto for the species of the clade ofA. minutumand four divergent species; (2) the reinstatement of the genusGessneriumfor the species of the clade ofA. monilatum; (3) the reinstatement of genusProtogonyaulaxfor the species of thetamarense/catenella/fraterculusgroups, and (4) the new genusEpisemicolongen. nov. forA. affineandA. gaarderae.New combinations in the generaGessnerium,Protogonyaulax,andEpisemicolonare proposed.