by Alastair Simpson
In the last ISoP newsletter, members were encouraged to answer the question “What is a protist?”, and to identify which of several diverse eukaryote taxa they considered to be protists. The answers revealed a (surprisingly even) split between two basic concepts. The first is a phylogeny-based definition; that protists are all eukaryotes that are not animals, fungi, or land plants (Embryophyta). The second concept includes various definitions based on biological organisation; that protists are unicellular (or at least non-multicellular) eukaryotes.
The first definition agrees with (what has become) the composition of the “Kingdom Protista” as part of the influential five-kingdom system, modified substantially from that of Whittaker, 1969. This Kingdom Protista was explicitly paraphyletic, and most biologists today do not consider it a formal taxon. Nonetheless, the underlying concept has proven useful from a communication and summary perspective – this utility helps explain the longevity of the five-kingdom system in pedagogy – and “protist” is the natural informal name to use in place of “Protista”.
The second type of definition identifies protists as eukaryotes that fall below some level of organismal complexity. Most respondents indicated simply that protists were ‘unicellular’. At first glance, this is a very restrictive definition, but it seems that (almost) all of these respondents actually also admit colonial forms, including colonies with at least some cell differentiation (since all respondents except one considered Volvox to be a protist). A couple of respondents were more inclusive, specifying that protists were eukaryotes that did not show complex multicellularity, or tissues. These definitions always exclude animals and land plants, then show a sliding scale of inclusion/exclusion; kelp and fungi (except yeasts) were almost always excluded, red algae were generally excluded, while the large-but-simple filamentous alga Spirogyra was often included. Yeasts and myxozoa were often included. All respondents considered Dictyostelium and Acrasis to be protists; with hindsight, it would have been instructive to see how myxomycetes and oomycetes were considered.
These concepts have different properties. The first is a simple definition with relatively little ambiguity in practice, especially as almost all interested researchers and students deal with living forms, and the relevant fossil record is sparse. The greatest ambiguity concerns the boundary between protists and fungi. However, it does not represent a group with any particular biological commonalities, and leads to a few cases where organisms that were formerly considered protists (and studied as such) are now not (certainly myxozoa, potentially microsporidia). The second category of definitions does try to identify some element of biological commonality for protists, but runs into the perennial problem of scientists seeking to place simple definitions over a diverse, messy biological reality. There is also a range of complexity levels to choose from when drawing a line. Clearly, there is a considerable variety among individual’s definitions of protists under this second concept; it is possible that no two survey respondents would agree completely on the composition of the assemblage.
For what it is worth, most members of the ISoP Nomenclature, Taxonomy and Systematics committee prefer the first definition. That being said, we also advocate an open-eyed pluralism for reasonable definitions of what a protist is, and especially what a protistologist is. If a researcher working on myxozoa, microsporidia, or, yes, even yeasts, finds value in ISoP, its meetings, or the journal, then nothing about a preferred definition of ‘protists’ should exclude them or make them feel unwelcome in the society.
