Protist Phylogeny

174 CHAPTER FIVE UNCORRECTED PAGE PROOFS short kinetosome at the base of each flagellum, these are not associated with the typical ciliate kinetidal system. Lipscomb and Corliss pointed out that Stephanopogon appears to be far from the main trunk on any phylogenetic tree that depicts the origin of ciliates, and they also demonstrated that the use of Stephanopogon to derive the Metazoa from a ciliate ancestry is no longer plausible. Protist Phylogeny We can do no more than touch upon the myriad questions and interesting points of view concerning the ori- Figure 5.49 Phylum Opalinida. Opalina. gin and evolution of the protists. Beyond the problems of relationships among the various protist groups them- selves, we are faced on one hand with questions about the very origin of eukaryotic life on Earth, and on the these rows; thus it is longitudinal (as it is in flagellates) other with interpreting the ancestral forms of the rest of rather than transverse (as it is in ciliates). Some opalin- the living world. The origin of eukaryotic cells probably ids are binucleate, others multinucleate, but all are took place 2 to 2.5 billion years ago, and this event homokaryotic (i.e., the nuclei are all identical). marked the origin of the protist grade of life. Although There are about 150 species of opalinids, in several there are over 30,000 known fossil species of protists, genera, almost all being endosymbiotic in the hindgut they are of little use in establishing the origin or subse- of anurans (frogs and toads), where they ingest food quent evolution of the various protist groups. Only anywhere on their body surface. Sexual reproduction is those with hard parts have left us much of a fossil by syngamy and asexual reproduction is by binary fis- record, and only the foraminiferans and radiolarians sion and plasmotomy, the latter involving cytoplasmic have well established records in Precambrian rocks (and divisions that produce multinucleate offspring. Opalina there is some debate even about this). The origin of the and Protopalina are two genera most commonly encoun- eukaryotic condition was, of course, a momentous tered (Figure 5.49). Opalinids are often found in routine event in the biological history of the Earth, for it enabled dissections of frogs in the classroom; their large size and life to escape from the limitations of the prokaryotic graceful movements through the frog’s rectum make bauplan by providing the various subcellular units that them a pleasant discovery for students. have formed the basis of specialization among the Protista and the Metazoa. Of the number of hypotheses explaining how eu- Stephanopogon karyotes might have evolved, the most popular is the We must briefly mention the enigmatic genus Steph- anopogon (Figure 5.50). These organisms have played an important role in phylogenetic speculations regarding protist evolution. Several theories have implicated Stephanopogon not only in the origin of the ciliates from a flagellate ancestor and in the origin of the ciliate binu- clear condition, but also in the origin of the Metazoa from a ciliate protist line. Until recently, Stephanopogon was classified in the phylum Ciliophora because they have a conspicuous cytostome and rows of cilia. In 1982, however, D. L. Lipscomb and J. O. Corliss provided evi- dence based on ultrastructural studies that these protists have little in common with ciliates and are probably more closely related to euglenids. Lipscomb and Corliss found that the two (or up to 16) nuclei of Stephanopogon are identical, rather than differentiated into macro- and micronuclei as they are in the ciliates (this long-ignored fact was actually first noticed in the 1920s). Nuclear di- vision is very much like that seen in euglenids and kine- toplastids. While Stephanopogon cells have an unusually Figure 5.50 The enigmatic protist, Stephanopogon. .

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