INTRODUCTION TO PROTOZOA

Protozoa and other protists evolved from the first eukaryotes (Patterson 1999) and probably have been present at least twice as long as any of the major multicellular organisms (Patterson 1999).

More recently, based on the comple­ment and organization of organelles within the organisms, use of 16S ribosomal-RNA sequencing data, and protein and gene sequences, it was recognized that many groups previously classified among the pro­tozoa were polyphyletic and probably only distantly related (Patterson 2000). There has been a gradual move to replace the notion of “protozoa” with the broader concept of “protista,” recognizing that even the protista continue to contain some polyphyletic groups of eukaryotes. There still is no agreement on how various groups are related or how they should be classified. In one substantial revi­sion, about 71 monophyletic lineages, within a set of four major lineages, were identified among the Protista, based on common pat­terns of cell organization (ultrastructural identity) (Patterson 1999).

In a more recent revision, there has been a continued focus on emphasizing evolution­ary relationships; the numbers of paraphyletic groups are further decreased from earlier studies (Adl et al. 2005). In Adl et al.'s (2005) taxonomic scheme, both unicellular and mul­ticellular eukaryotic organisms are included, organized into six supergroups based on molec­ular phylogenies: Amoebozoa, Opisthokonta, Rhizaria, Archaeplastida, Chromalveolata, and Excavata (App.

For purposes of this classification, eukary­otes are defined as organisms having a distinct nucleus bounded by a double membrane, the outer being derived from the endomembrane network, with the nuclear pore complex tra­versing both membranes, and with one or more linear chromosomes typically packaged by histones and usually with a centromere and telomeres (Adl et al. 2005). Protozoa are predominantly non-filamentous heterotrophic species. The term protist is used to describe eukaryotes with a unicellular level of organi­zation and without cell differentiation into tis­sues (Adl et al. 2005). For this discussion, the more traditional term, “protozoa,” generally will be retained.

While some protozoa are simple, others are among the most complex cells known; all of the biological and biochemical mechanisms needed for a complex lifestyle are contained within a single cell (Sleigh 1991). Reproduction may be either asexual or sexual, and often is complex. Many species alternate sexual and asexual modes in their life cycles, or may vary the pattern in relation to environmental con­ditions (Roberts and Janovy 2000). Asexual reproduction usually is by binary fission, but also may involve several forms of budding or multiple fission; with multiple fission there is repeated division of the nucleus and other essential organelles before cytokinesis (Roberts and Janovy 2000). Sexual reproduction involves meiosis and the process of gametogony. Many protozoa can encyst by secreting a thick resin­ous case around themselves and becoming dor­mant (Keeton and Gould 1993). For life cycles, the term cyst is restricted to a vegetative quies­cent stage, while the term spore is restricted to a reproductive stage (Adl et al. 2005).