Sexual life cycle events in Pfiesteria piscicida and cryptoperidiniopsoid heterotrophic dinoflagellates were determined by following the development of isolated gamete pairs in single-drop microcultures with cryptophyte prey. Under these conditions, the observed sequence of zygote formation, development, and postzygotic divisions was similar in these dinoflagellates. Fusion of motile gamete pairs each produced a rapidly swimming uninucleate planozygote with two longitudinal flagella. Planozygotes enlarged as they fed repeatedly on cryptophytes. In < 12 h in most cases, each planozygote formed a transparent-walled nonmotile cell (cyst) with a single nucleus. Zygotic cysts did not exhibit dormancy under these conditions. In each taxon, dramatic swirling chromosome movements (nuclear cyclosis) were found in zygote nuclei before division. In P piscicida, nuclear cyclosis occurred in the zygotic cyst or apparently earlier in the planozygote. In the cryptoperidiniopsoids, nuclear cyclosis occurred in the zygotic cyst. After nuclear cyclosis, a single cell division occurred in P piscicida and cryptoperidiniopsoid zygotic cysts, producing two offspring that emerged as biflagellated cells. These two flagellated cells typically swam for hours and fed on cryptophytes before encysting. A single cell division in these cysts produced two biflagellated offspring that also fed before encysting for further reproduction. This sequence of zygote development and postzygotic divisions typically was completed within 24 h and was confirmed in examples from different isolates of each taxon. Some aspects of the P piscicida sexual life cycle determined here differed from previous reports.
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