Protistan diversity and origins of multicellular/multitissued organisms

Abstract

Some background information is first presented concerning the historically and evolutionarily significant prokaryote‐eukaryote split of the biotic world, widely accepted some 25 years ago, and the (re)emergence of the field of neoHaeckelian protistology in still more recent times. Two questions are then posed: how did eukar‐yogenesis come about, and what protistan groups may have served as progenitors of the plant, fungal, and animal kingdoms? Before either of these complex inquiries can be discussed properly, some data need to be given concerning the protists themselves, and progress made to date in research on these evolutionarily important forms of life requires brief review.The Protista are here tentatively considered to be a kingdom of ≪lower≫ eukaryotes embracing the algae, protozoa, and certain ≪lower≫ fungi. They are distinguished first of all by the fact that, while eukaryotic, they possess no more than one tissue. Most of the included 200 000 species (estimated) are unicellular in basic organization and microscopic in size, with some important exceptions. Additional characters are listed, revealing the great diversity and evolutionary plasticity within the (alleged) kingdom. Although fossil forms are known, their records throw little light on intraking‐dom relationships at the higher taxonomic levels.Do protists form a natural kingdom, or do they represent merely an evolutionary stage or structural grade between prokaryotes and the ≪higher≫ eukaryotes, or are they better distributed throughout several separately established eukaryotic kingdoms? If a single kingdom, are there clearcut evolutionary discontinuities between the Protista and the other eukaryotic kingdoms? That protists may be biological/taxonomic chimeras is a fact not widely appreciated. This viewpoint requires acceptance, of course, of at least parts of the celebrated Serial Endosymbiotic Theory (SET), an hypothesis championed in modern times by Lynn Margulis. Perhaps the single greatest obstacle to progress in learning more about the evolution and phylogeny of protists is represented by a combination of deficiencies: an overall dearth of data of uncompromising value to the questions we must ask; and the small number of different kinds of protistan species being subjected to rigorous study by the modern, often sophisticated, methodologies of the cell and molecular biologists.With regard to the two questions originally posed, we may draw several conclusions, not all positive in nature. In the case of eukaryogenesis, the protists are clearly the organisms of choice to study. There is widespread — although not universal — support among research biologists for the exogenous or xenogenous, rather than the endogenous or autogenous, origin of the mitochondria and the chloroplasts. The problems of nuclei, centrioles, and flagella/cilia remain unresolved and speculative.With respect to the origins of plants, fungi and animals from protistan progenitors, there is considerable evidence for the ancestry of the first two groups to have been from chlorobiont and chytrid protists, respectively. In the case of the kingdom Animalia, we still lack noncontroversial data in strong support of any of the standardly proposed evolutionary pathways allegedly bridging the gap between metazoa and protists. Complicating this last relationship overall is the continuing uncertainty over the exact taxonomic boundaries of the Protista, on the one hand, and the identification of the most primitive phylum or phyla in the possibly polyphyletic Animalia, on the other. © 1989 Taylor & Francis Group, LLC.