Life Cycle Stages and Evidence of Sexual Reproduction in the Marine Dinoflagellate Prorocentrum minimum (Dinophyceae, Prorocentrales)1

Prorocentrum minimum is a potentially toxic marine dinoflagellate that often forms massive blooms in estuarine and coastal sea waters. In this study, the life cycle of P. minimum was investigated and sexual reproduction in culture was described for the first time. Morphology of the mitotic stages was revised and several distinguishing features from sexual steps were described. The sexual reproductive stages were observed in the stationary culture and compared with a well-studied closely related species, Prorocentrum micans. Prorocentrum minimum has a haplontic life cycle and homothallic sexual process. The gametes were isogamous and morphologically indistinguishable from the vegetative cells. Unlike P. micans, P. minimum isogametes fused, but did not conjugate, partially reorganizing their cell coverings. Newly formed planozygotes were distinguished by their irregular shape and a large asymmetrically located nucleus. No long-term resting cyst stages (hypnozygotes) were documented. The late planozygotes underwent meiosis and formed tetrads of cells. The second meiotic division could be delayed or arrested in one of the daughter nuclei leading to formation of trinucleate cells with three pairs of flagella. So, similar to P. micans, P. minimum may have two possible scenarios of sexual division: (a) formation of a four-cell stage through two successive divisions or (b) asynchronous divisions of the zygote. Changes in the DNA content were confirmed by quantitative image cytometry.