The life history of the toxic marine dinoflagellate Protoceratium reticulatum (Gonyaulacales) in culture

Abstract

Asexual and sexual life cycle events were studied in cultures of the toxic marine dinoflagellate Protoceratium reticulatum. Asexual division by desmoschisis was characterized morphologically and changes in DNA content were analyzed by flow cytometry. The results indicated that haploid cells with a C DNA content occurred only during the light period whereas a shift from a C to a 2C DNA content (indicative of S phase) took place only during darkness. The sexual life cycle was documented by examining the mating type as well as the morphology of the sexual stages and nuclei. Gamete fusion resulted in a planozygote with two longitudinal flagella, but longitudinally biflagellated cells arising from planozygote division were also observed, so one of the daughter cells retained two longitudinal flagella while the other daughter cell lacked them. Presumed planozygotes (identified by their longitudinally biflagellated form) followed two life-cycle routes: division and encystment (resting cyst formation). Both the division of longitudinally biflagellated cells and resting cyst formation are morphologically described herein. Resting cyst formation through sexual reproduction was observed in 6.1% of crosses and followed a complex heterothallic pattern. Clonal strains underwent sexuality (homothallism for planozygote formation and division) but without the production of resting cysts. Ornamental processes of resting cysts formed from the cyst wall under an outer balloon-shaped membrane and were fully developed in <1h. Obligatory dormancy period was of ∼4 months. Excystment resulted in a large, rounded, pigmented, longitudinally biflagellated but motionless, thecate germling that divided by desmoschisis. Like the planozygote, the first division of the germling yielded one longitudinally biflagellated daughter cell and another without longitudinal flagella. The longitudinal biflagellation state of both sexual stages and of the first division products of these cells is discussed.