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Putting Protozoa in Their Place

This massive, beautifully produced, two-volume work is a compendium of information on the structure and diversity of groups of unicellular eukaryotic organisms that have traditionally been included among the protozoa. It will be of value to researchers working on protists, to teachers of courses on protists or eukaryote diversity, and to libraries. This book, the most extensive compendium of its kind, greatly expands the first edition’s treatment of many groups. The concept of protozoa has little meaning at present other than as a general category for unicellular eukaryotes that are either nonphotosynthetic or both photosynthetic and motile. The traditional taxonomy of protozoa, based on the categories amoebae, flagellates, ciliates, and sporozoa, has almost completely broken down, and a new phylogenetic taxonomy is struggling to be born. These volumes reflect the uncomfortable state of the art. The rather bewildering array of 44 chapters on different groups of organisms is presented in alphabetical order within broad categories (alveolates, stramenopiles, amoebae, and flagellates), with no attempt to place related groups adjacent to one another or to fit them into a higher versus lower evolutionary sequence. As editor John Lee notes on the first page, “While this order of chapters may disappoint some readers, there are sufficient gaps in our knowledge to leave the relatedness of many groups in doubt. Others will respect our decision to defer judgment until a clearer overall picture emerges from unfolding studies.” This unsettled state of affairs is the consequence of the 40-year revolution— extensive but incomplete—in the small world of evolutionary and systematic studies on protists (unicellular eukaryotes that encompass the traditional categories of protozoa) and unicellular algae and fungi. These organisms, from which all multicellular plants, animals, and fungi have arisen, comprise the basal radiation of eukaryotic life and encompass the vast majority of eukaryotic diversity at the cellular level. This revolution began in the 1960s with electron microscopic studies on protists, which revealed a wealth of hitherto unrealized structural detail and diversity. These studies revealed the remarkable consistency of basic cellular architecture within most protist groups (for example, in the structures of mitochondrial cristae, of flagella and their associated microtubular bundles and striated fibres, and of chloroplast envelopes, thylakoids, and photosynthetic pigments in photosynthetic forms). On the basis of these similarities, a number of researchers argued that the traditional classification of these organisms was both inadequate and artificial, and that an integrated approach to the study of unicellular eukaryotes was needed, independent of their formal plant, animal, or fungal taxonomic affinities. The second major idea propelling the change in thinking about protists arose from the realization that mitochondria and chloroplasts represent symbiotic prokaryotes that have become integrated into their host organisms. This finding was important, because it made biologists consider the relationship between photosynthetic and nonphotosynthetic organisms in a new way. If chloroplasts could be acquired either as primary symbionts from cyanobacteria, or through secondary symbiotic events from other chloroplast-containing protists, then the evolutionary relationships between photosynthetic and nonphotosynthetic groups could be much more intimate and varied than biologists had previously supposed. While these factors led to a breakdown of the traditional view of protozoa and algae in the 1970s, there was no clear scheme to take the place of the traditional system, and so most texts and monographs continued to repeat the traditional systematic structures, with only minor changes. For example, the first edition of the Illustrated Guide to the Protozoa, published in 1985, reaffirmed the traditional division of protozoa into the four major groups—amoebae, flagellates, ciliates, and sporozoa—in use since the 1880s, despite the widespread feeling of many, if not most, workers in the field that this view was both artificial and inadequate. Although structural studies made it possible to establish substantial numbers of structurally consistent eukaryotic groups, working out the relationships among these groups was problematic. Some early linkages between groups (e.g., euglenoids and kinetoplastids) were suggested in the early 1970s solely on the basis of structural features of the organisms, but deducing the relationships between most of the major assemblages of organisms requires a combination of gene sequence information and structural features. A lot of this information has come from ribosomal DNA sequences, and in the last few years an increasing number of genes (such as actin, EF-1, EF-2, tubulins, HSPs) have been used to provide phyletic information about relationships between protist groups. Consequently, although much work remains to be done to fully illuminate the relationships among all of the diverse protist groups, the phyletic structure of large clusters of eukaryotic groups— alveolates, euglenozoa, opisthokonts, and heterokonts—has been established. These groupings will undoubtedly form the basis of a new taxonomy of protists when one emerges. In the meantime, D. J. Patterson, in his excellent essay in the introductory section of the Illustrated Guide to the Putting Protozoa in Their Place