Pfiesteria

Dinoflagellates- Alexandrium Karlodinium Pfiesteria Prorocentrum

Fig 1. Photomicrographs of Pfiesteria spp. (A) Cell division in Pfiesteria shumwayae. (B) Cell division in Pfiesteria piscicida. Scale bars, 10 µm. (Parrow et al. 2002).

Many observations and experiments on dinoflagellate life cycles have come from work on laboratory cultures. Culturing is important for initial life cycle determinations, since some life cycle processes (especially those in the sexual cycle) may require days, weeks or even months for completion. Thus, culture studies are typically the best first step toward determining the life cycle processes of a dinoflagellate; from those insights, work with field populations can provide confirmatory and additional findings in some cases. Cultured populations of Pfiesteria spp. and related species have been examined for the processes of asexual cell reproduction and sexuality using light (Fig. 1) and scanning electron microscopy (Fig. 2). Single-cell isolations and light microscopic observations of living cells also revealed the occurrence of nuclear cyclosis in the zygotes of Pfiesteria spp. and a related species. Nuclear cyclosis is a protracted swirling of the zygotic chromosomes thought to occur prior to meiotic division (Fig. 3 video link; Parrow and Burkholder 2003b). By isolating individual sexual cells and following their development, we were able to examine the pattern of apparent meiotic division in Pfiesteria piscicida and related cryptoperidiniopsoids (Fig. 4).

Fig 2. Scanning electron micrographs of Pfiesteria shumwayae sexual cells. (A) Fusing gametes. (B) Planozygote with a single transverse and two longitudinal flagella. (C) Planozygote, lateral view. Scale bars, 5 µm. Parrow and Burkholder 2003a.

Fig 3. Click on the image above to view nuclear cyclosis.

Fig 4. Light micrographs of live cryptoperidiniopsoid dinoflagellate sexual cycle cells from drop microculture isolations: (A) Fusing gametes. (B, C) Resulting early planozygotes. (D, E, F) Older planozygotes with pigmented food vacuoles from feeding. (G-I) In series: (G) Zygotic cyst with nuclear cyclosis. (H) 3 h later, zygote division has occurred. (I) Excystment, the two flagellated offspring of zygote division. (J) The two offspring of zygote division, both feeding on a cryptophyte prey cell. Scale bars, 10 µm. From Parrow and Burkholder (2004).

Fig 5. Pfiesteria shumwayae cell with stained DNA. The chromosomes within the nucleus are visible as fluorescent green rod-shaped structures. Scale bar, 5 µm.

Flow cytometry has been used to measure the cellular DNA content (genome size), and population DNA distribution in Pfiesteria spp. (Parrow and Burkholder 2002). Flow cytometric cell cycle determinations were made by staining cells with a stoichiometric DNA fluorophore (Fig. 5), and then measuring DNA fluorescence. By measuring the DNA content of individual cells in populations, information can be gained about the proportions of cells occupying different nuclear phases of the asexual and sexual cycles (Fig. 6). Examination of how the cell cycle is affected under growth-limiting conditions yields important information about cell cycle control points that regulate basic reproductive biology (Parrow et al. 2002).

Fig 6. Flow cytometric DNA measurements of Pfiesteria spp., showing subpopulations of differing relative DNA content. (a-c) Vegetative cells in G1 cell cycle phase and gametes have 1C DNA, whereas vegetative cells in G2 cell cycle phase (preparing to divide) and planozygotes have 2C relative DNA. (d-f) Cells with 3C and 4C DNA have also been detected. Beads are an internal fluorescence standard. From Parrow et al. (2002).

These microscopic studies and quantitative flow cytometric DNA analyses of cultured populations have demonstrated that Pfiesteria spp. and cryptoperidiniopsoids have a haplontic life cycle, with zygotic meiosis (Fig. 7), as is thought to be the case with most dinoflagellates. Although many features of the life cycle of these dinoflagellates have been demonstrated, there are still many aspects that await further study (Elbrachter 2003). Such life cycle determinations will lead to a much better understanding of the occurrence, distribution, and basic ecology of these and other dinoflagellates.

Fig 7. Schematic diagram of a haplontic life cycle with zygotic meiosis.

References Elbrächter, M. 2003. Dinophyte reproduction: progress and conflicts. Journal of Phycology 39, 629-32.

Parrow, M.W. and Burkholder, J.M. 2002. Flow cytometric determination of zoospore DNA content and population DNA distribution in cultured Pfiesteria spp. (Pyrrhophyta). Journal of Experimental Marine Biology and Ecology 271, 140-155.

Parrow, M.W. and Burkholder, J.M. 2003a. Reproduction and sexuality in Pfiesteria shumwayae (Dinophyceae). Journal of Phycology 39, 697-711.

Parrow, M.W. and Burkholder, J.M. 2003b. Estuarine heterotrophic cryptoperidiniopsoids (Dinophyceae): life cycle and culture studies. Journal of Phycology 39, 678-696.

Parrow, M.W. and Burkholder, J.M. 2004. The sexual life cycles of Pfiesteria piscicida and cryptoperidiniopsoids (Dinophyceae). Journal of Phycology, 40, 664-673

Parrow, M., Burkholder, J.M., Deamer, N.J. and Zhang, C. 2002. Vegetative and sexual reproduction in Pfiesteria spp. (Dinophyceae) cultured with algal prey, and inferences for their classification. Harmful Algae 1, 5-33.