Re-examination of cyst-motile relationships of Polykrikos kofoidii Chatton and Polykrikos schwartzii Bütschli (Gymnodiniales, Dinophyceae)

We have re-examined the cyst-motile relationships of two Polykrikos species; P. schwartzii and P. kofoidii (Dinophyceae), based on the literature, incubation experiments, and molecular phylogenetic analysis. The longitudinal furrows on the hypocone of P. kofoidii differentiate it from P. schwartzii. Differences in surface ornamentations on the cysts of P. schwartzii and P. kofoidii were considered as important morphological features to differentiate these two species. Many researchers accepted that the cyst of P. schwartzii was characterized by reticulate ornaments and P. kofoidii by separate, rod-like processes. However, encystment/excystment experiments carried out in previous studies clarified that the P. kofoidii cyst has coarse reticulate ornaments, not rod-like processes. Further subsequent observations on these relationships have indicated that rod-like processes develop not on cysts of P. kofoidii but on cysts of P. schwartzii. In combination with morphological observations, phylogenetic analyses of the small subunit and large subunit rDNA sequences, directly collected from vegetative cells and living cysts of P. kofoidii and P. schwartzii confirmed these findings on the cyst motile relationships and thus the criteria for the identification of cysts required revision. Furthermore morphologically intermediate forms, which sometimes occur, are identical to the cysts of P. schwartzii based on the molecular data of a single cell PCR technique for living cysts.