Fertilization and Zygote Development in the Binucleate Dinoflagellate Peridinium balticum (Pyrrhophyta)

Abstract

Peridinium balticum (Pyrrhophyta) exists as a symbiosis between a nonphotosynthetic dinoflagellate host and a chlorophyll c-containing alga. It is hypothesized that P. balticum is an evolutionary link between primitive nonphotosynthetic and advanced photosynthetic dinoflagellates. This study documents pre- and postfertilization events of sexual reproduction in this unusual dinoflagellate for the first time. Light microscopy and TEM observations showed that gametes resemble vegetative cells except in the organization of their chloroplasts. Fusion of gametes occurred in a specific orientation, i.e., apical to sulcal area. The presence of an intact membrane between fusing pairs prior to plasmogamy was suggestive of enzymatic digestion of plates during fertilization. Rupture of this membrane triggers plasmogamy and karyogamy of the host, followed by that of the algal symbiont. A discussion of the cellular processes involved in gamete formation, fertilization, and zygote development is presented. The results of this investigation demonstrate that a synchronous sexual reproduction cycle has evolved for the symbiont and its dinoflagellate host.