Brigantedinium majusculum is the cyst of Protoperidinium sinuosum (Protoperidiniaceae, Dinophyceae)

Two morphologically distinct species Brigantedinium majusculum and Trinovantedinium applanatum have been suggested as a cyst stage of Protoperidinium pentagonum; however, the cyst-theca relationships are still virtually undefined. In this study, we re-examined the motile stage of B. majusculum via thecal plate analysis together with a molecular phylogenetic analysis based on partial large subunit ribosomal DNA gene sequences. The morphological features of motile cells germinated from B. majusculum were identical to those of Pr. sinuosum, and the molecular phylogenetic analysis showed that the germinated cells and B. majusculum are closely related to Pr. pentagonum and Pr. latissimum as a sister group. Additionally, the high sequence divergences were observed between the germinated cells and B. majusculum as well as Pr. pentagonum and Pr. latissimum (18.0%-19.5% according to P-values and 20.7%-22.9% according to Kimura-2-parameter values). Based on these results, our study indicates that B. majusculum should be accepted as the cyst of Pr. sinuosum.