Observations of asexual and sexual processes in Protoperidinium depressum (Dinophyceae)

Abstract

Life cycle stages of the heterotrophic dinoflagellate Protoperidinium depressum were determined for a culture isolated from the west coast of Ireland. Division, sexuality, DNA content, gamete morphology, and identity of morphologically distinct life‐history stages were revealed for the first time using a suite of morphological and molecular tools. Asexual division occurred by eleutheroschisis within a temporary, immotile cyst, yielding two daughter cells. Morphological features, including apical and antapical horns, were not fully formed upon emergence from the division cyst, but developed some time after cell division. Daughter cells were half to two‐thirds the size of parent cells and swam attached in an epitheca‐to‐hypotheca orientation before separating. Stages indicating a completed life cycle, including gametes, planozygotes, and nuclear cyclosis were observed. Small cells, believed to be gametes, formed in repeated clonal and non‐clonal re‐isolations of the culture. In addition to their small size (40–50 μm), blunt apical horn, small antapical horns, and lack of pink pigmentation, these putative gametes had a thecal plate morphology different from that described for P. depressum. The LSU rDNA sequence (D1–D3 region) of small cells was, however, the same as that of P. depressum. Gametes were never witnessed to feed, but did form fusing, isogamous pairs. When isolated to well plates, gamete pairs failed to complete fusion to form a planozygote. Thus, the morphology of the initial planozygote remains uncertain. Large cells believed to be planozygotes had “typical”P. depressum morphology, but possessed two longitudinal flagella and had 2C DNA based on DAPI. Nuclear cyclosis was documented in one such tri‐flagellated, motile cell. The cell remained motile after the completion of nuclear cyclosis, but did not progress to the next life stage. Dormant hypnozygotes were not observed.