Extraordinary Dinoflagellates : Past and Present

International Symposium on "Dawn of a New Natural History - Integration of Geoscience and Biodiversity Studies". 5-6 March 2004. Sapporo, Japan.

This paper highlights the astonishing diversity of both form and function in a group of protists that were once obscure, but which now attract considerable attention from cell biologists, evolutionists, ecologists, aquaculturists and even public health authorities. They surprise and confuse all those who learn about them. For example, having only 50% of the species photosynthetic, they have been claimed to be plants (algae) by botanists and animals (protozoa) by zoologists and were separately classified as such, but both were wrong: they are flagellated protists belonging to the Alveolates and evolutionarily diverged in the Neoproterozoic, long before true animals and plants. However, they left little trace until the Mesozoic when, according to the record left by their highly resistant resting cysts, they radiated rapidly. The most likely explanation for this is the development of shallow continental shelves. Unlike many other groups they show little signs of extinction at the Cretaceous/Tertiary boundary, most major dinoflagellate lineages having persisted through more than 100 million years of Earth's history. As zooxanthellae they have contributed significantly to the success of reef-building corals. Present-day dinoflagellates are unusual in many ways. Their chromosomes are not constructed like all other eukaryotes, lacking histones. The nuclear envelope does not break down during mitosis and the spindle is extranuclear. They have multiple chloroplast types. The genome of the commonest type has fragmented into minicircles and only two genes have been identified in the mitochondrial genome. Their morphology is astonishingly diverse, ranging from unicellular to multicellular and even coenocytic. The heterotrophic forms have various, complex feeding mechanisms and even the photosynthetic ones may engulf prey (mixotrophy). They show remarkable examples of form mimicking multicellular organisms, such as resemblances to tapeworms, jellyfish, nematocysts and even eyes. They can be beneficial symbionts or deadly parasites. More than 70 species can produce deadly toxins which can kill marine life or, if concentrated by shellfish or fish, can kill humans.