Light and electron microscopical observations on the type species of Gymnodinium, G-fuscum (Dinophyceae)

Gymnodinium fuscum (Ehrenberg) Stein is the type species of Gymnodinium. one of the largest genera of dinoflagellates. It has the typical fine structure of dinoflagellates, but also possesses several unusual features. The nuclear envelope is specialized by having vesicular chambers in which the nuclear pores are located: the pusular complex includes an internal collection chamber: and a nuclear fibrous connective links the nucleus with the longitudinal microtubular root of the flagellar apparatus. A delicate horseshoe-shaped apical groove is visible by scanning electron microscopy. The same morphological features have been observed in a few other gymnodinioids. indicating a close phylogenetic relationship, and have recently been used, together with large subunit (LSU) rDNA data, in redefining the genus Gymnodinium [Daugbjerg er nl. (2000) Phycologia 39: 302-317]. However, one of the most unusual features of G. fuscum is the absence of a transverse striated flagellar root, a feature presently known only in the zoospores of the otherwise very different genus Noctiluca. This feature, together with the lack of striated collars around the flagellar canals, suggests a phylogenetically more isolated position for G. fuscum. Future investigations may reveal the need for further 'splitting' of the genus Gymnodinium.