Protozoan Taxonomy and Secondary article Systematics Article Contents . Introduction John O Corliss, University of Maryland, College Park, Maryland, USA . Historical Considerations . Major Groups of Protozoa Taxonomy and systematics of major groups of the Protozoa, an assemblage of so-called . The Old ‘Phylum Protozoa’ lower eukaryotes, refer basically to the classification of such groups, that is, their . Modern Options Concerning the Place of Protozoa in arrangement into a hierarchy of evolutionary interrelated groups (taxa) of scientifically the Biotic World . named phyla, classes, orders, etc. The New ‘Kingdom Protozoa’ Introduction ently, a goodly number of chlorophyll-possessing algal The classification of protozoa and other microorganisms groups were included in the phylum), phagotrophic, and above the organizational level of the bacteria has always capable of independent locomotion. It is now abundantly been dependent on microscopy because the body sizes clear that this classical definition of protozoa is at best involved generally range from only one micrometre to one misleading and incomplete, and that it requires consider- or two millimetres in length. Any structures these species able refinement. possess, useful in comparative studies of their morphology In former times, zoologists were the principal investiga- and thus their taxonomy and systematics, are at the cellular tors – and namers and claimers – of such microorganisms; and subcellular levels, and invisible to the naked eye. The and they often worked with taxonomic disregard for physiological properties of protozoa (and the neighbour- studies of what might actually be the same group, ing algae)have also played a role in the classification of sometimes even the same species, by the botanists these ubiquitous eukaryotic microorganisms; and of (phycologists and mycologists). Thus, unfortunately, growing significance are the findings made possible by territoriality and authoritarianism also played major roles research using molecular biological approaches. in determining the early systematic status of the protozoa, Because we are continuing to learn more and more about algae and lower fungi (Corliss, 1986). such minute organisms, protozoan systematics – that is, Advances in our knowledge of the protozoa in general the taxonomy (classification)and the evolutionary inter- have followed progress in microscopy, as mentioned relationships of major groups of protozoa – remains a above. As early as a century ago, novel improvements in topic of debate and change, still today. Some of the rather methods of light microscopy and related techniques of large and unwieldy taxonomic groupings of past years are fixing and staining were already beginning to make particularly subject to revision with expansion of and possible the revelation of morphological cellular charac- refinement in our knowledge about the members of those – teristics that would have remained totally unrecognized and related – assemblages. Paradoxically, the protozoa before the appearance of such technological advances. themselves are becoming more difficult to define with Then, at mid-twentieth century, the use of electron precision as our information about them and other microscopical approaches in cell biology opened up a microbial assemblages increases. Thus, presenting a single whole new epoch of exploration in protozoology and satisfactory circumscribed definition for them is not an related fields. During the ‘Age of Ultrastructure’ (as it has easy task. It is attempted towards the end of this article; been called), a myriad of previously unknown subcellular but, for sake of clarity, further background information is structures were revealed that became of immense value in first supplied. the comparative taxonomy of the lower eukaryotes (especially protozoa and algae). Even more recently, the exciting development of molecular biological approaches, particularly in study of genealogical and phylogenetic Historical Considerations relationships within all groups of organisms, large and small, plant or animal, has offered a refinement in Until well beyond the middle of the twentieth century, taxonomic investigations that is without parallel in past protozoa were widely treated taxonomically as a (mere) decades. subset of the kingdom Animalia, retaining their ‘first- Here an attempt is made to review and understand the animal’ definition dating from 150 years earlier. As a systematics of protozoa both from the more traditional phylum of unicellular animals, they were thought to exhibit point of view, keeping in mind that many biologists are still major characteristics typical of that kingdom: colourless familiar mostly with older, conventional classifications of (i.e. no photosynthetic pigments present; yet, incongru- these minute organisms, and from more modern ap- ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 1 Protozoan Taxonomy and Systematics proaches. The latter are complicated by the relatively protozoa (Table 1). Although this popular arrangement – in recent advent of the ‘protistological perspective’ – lumping comparison with the long-followed classical one of protozoa, algae and some other eukaryotic microorgan- Bu¨ tschli of 80 years earlier – contained nearly four times isms together as Protista – and by their usage of more the number of taxonomic units above the level of family, it sophisticated data. For more detailed information con- showed few novelties of great significance. For an out- cerning protozoan–protistan overlapping relationships, standing example of its conservatism, the classification still the reader is referred particularly to comprehensive papers retained several major groups of algae, all treated as by Cavalier-Smith (1993, 1998)and Corliss (1994, 1998) comprising a single class, the Phytomastigophorea, of and to the insightful textbooks by Sleigh (1989)and flagellated protozoa (the Mastigophora). A subsequent Hausmann and Hu¨ lsmann (1996). revision supported by the international Society of Proto- zoologists, the ‘Levine Report’ of 1980, recognized seven separate phyla (in a subkingdom Protozoa)and increased the number of taxa above the familial level to 229 (Table 2). Major Groups of Protozoa But, fundamentally, it followed the arrangement of its predecessor (Table 1). Five years later, the well-known and For many decades (commencing with the perceptive widely used Illustrated Guide to the Protozoa (Lee et al., schemes of Otto Bu¨ tschli, published in the 1880s), it has 1985)appeared, with its classification scheme mainly that been convenient to assign species known as protozoa to just of the Levine Report. By then, the ‘protist revolution’ had four major categories or assemblages, as has been done in already been going strong for a full decade (see accounts in numerous textbooks of biology and zoology as well as Corliss, 1986, 1998)and many findings were indicating the protozoology. Despite advances and improvements in our pressing need for a fresh look at the old persisting problems knowledge of eukaryotic microorganisms in general, it is of how to treat the systematics of the conventional/ still often rather helpful, for sake of discussion under diverse traditional protozoan, algal and fungal assemblages of circumstances, to arrange protozoa taxonomically in such a microorganisms. way. The categories generally recognized are: (1)the One reason for including the above discussion here, amoeboid forms (the Sarcodina, in a broad sense); (2) the besides the fact that those classifications of 20–40 years ago flagellated forms (the Mastigophora, including groups of are still accepted by many biologists, is to help bridge the autotrophic – or photosynthetic – as well as heterotrophic gap between such neoclassical systems and the suggested species); (3) the ciliated forms (the Ciliophora, the most recent arrangements of the 1990s (see below). The stable and perhaps most circumscribed of all protozoan organisms involved, and often their common group names assemblages); and (4) the various totally symbiotic or as well, have not changed over the decades, but our ideas parasitic forms (primarily spore-forming species that are concerning their most likely interrelationships at the higher typically endoparasites, some highly pathogenic to their taxonomic levels have done so, primarily by fresh analyses hosts, once assigned to a very broad group called the of the continuing accumulation of data of high phyloge- Sporozoa, a high-level taxon that subsequently became netic and evolutionary significance from precise ultra- divided into the Sporozoa and the Cnidosporidia). structural and molecular biological investigations One of the pedagogically oft-regretted but inevitable (Coombs et al., 1998). changes, especially during the busy second half of the twentieth century (even before the ‘protist revolution’: see Corliss, 1986), was the tremendous expansion in the total numbers of high-level taxonomic groups (subphyla, Modern Options Concerning the Place classes, orders)of protozoa, while generally still recogniz- ing the four major top divisions mentioned above. The of Protozoa in the Biotic World discovery of new and unique differentiating characteristics There are a number of ways in which modern biologists are useful in classification – and in evolutionary and phyloge- viewing the overall placement of the protozoa with respect netic studies as well – required such a multiplication and to other major groups of organisms. Several of these are fragmentation of taxa even though it