Animal-Like Protists: The Protozoa Chapter 8 Animal-Like Protists: The Protozoa

Evolutionary Perspective of the Protists The protists are a polyphyletic group that arose about 1.5 billion years ago when the Archaea and Eukarya diverged. Our understanding of the evolutionary relationships among protists is currently in flux (figure 8.1).

Figure 8.1 The Challenge of Protists Classification.

Evolutionary Perspective of the Protists Some protists are plant-like because they are primarily autotrophic (they produce their own food). Others are animal-like because they are primarily heterotrophic (they feed on other organisms).

Life Within a Single Plasma Membrane Protozoa occur as both single cells and entire organisms. A protozoan is a complete organism in which all of life activities are carried on within a single plasma membrane. Organelles specialized for the unicellular lifestyle carry out many protozoan functions.

Figure 8.2 A Protozoan.

Life Within a Single Plasma Membrane (continued) Most protozoa absorb dissolved nutrients through endocytosis. Some protozoa ingest food in a specialized region called a cytopharynx. Digestion and transport of food occurs in food vacuoles that form during endocytosis. After digestion, the vacuoles are called egestion vacuoles.

Life Within a Single Plasma Membrane (continued) Both sexual and asexual reproduction occur among the protozoa. Protozoa can reproduce by binary fission, budding, multiple fission or schizogony, and by sexual methods (figures 8.3 and 8.4).

Figure 8.3 Asexual Reproduction in Protozoa.

Figure 8.4 Binary Fission of a Ciliate.

Symbiotic Lifestyles Many protozoa live in symbiotic relationships with other organisms, often in a host parasite relationship. Symbiosis is an intimate association between two organisms. In parasitism, one organisms lives in or on a second organism called the host. The definitive host harbors the sexual stages of the parasite.

Symbiotic Lifestyles (continued) Off spring from the sexual stage may enter another host called the intermediate host where they reproduce asexually. Commensalism is a symbiotic relationship in which one member benefits and the other member is neither benefited or harmed. Mutualism is a symbiotic relationship in which both species benefit.

Protists and Protozoan Taxonomy The super group Excavata includes the oldest eukaryotes. Classification of Animal-Like Protists (Protozoans) is shown in Table 8.1. Members of the Fornicata have flagella, a feeding grove, and are uninucleate. They feed by a flagella-generated current. An example is the protozoan Giardia intestinalsis.

Table 8.1

Figure 8.5 The protozoan Giardia intestinalsis.

Protists and Protozoan Taxonomy (continued) The Amoebozoa have amoeboid motility with lobopodia, are naked or testate, have mitochondria with tubular cristae, are uninucleate or multinucleate, and many form cysts. Amoeba proteus is the classical example of an Amoebozoan (figure 8.11). Other examples of protozoan's are shown in figures 8.6-8.8.

Figure 8.11b Amoeba proteus.

Figure 8.6 The protozoan Trichomonas vaginalis.

Figure 8.7 The structure of Euglena.

Figure 8.8a The life cycle of Trypanosoma brucei.

Figure 8.8b Structure of a flagellate.

Protists and Protozoan Taxonomy (continued) Foraminiferans and radiolarians are common marine Rhizaria that posses pseudopodia (figure 8.12). The Alveolata is a large group that includes the dinoflagellates (figure 8.14), Apicomplexa (figure 8.15 which shows the different stages in the life cycle of the malaria parasite), and Ciliophora (figure 8.16).

Figure 8.12 A Forminiferan Test.

Figure 8.14 A Dinoflagellate.

Figure 8.15 Malaria life cycle.

Protists and Protozoan Taxonomy (continued) The ciliates represent some of the most complex protozoa. Ciliates possess cilia, a macronucleus, and one or more micronuclei (figure 8.16).

Figure 8.16b Ciliophora.

Protists and Protozoan Taxonomy (continued) Ciliates reproduce asexually by transverse binary fission, and occasionally by budding. Ciliates reproduce sexually by conjugation (figure 8.20)

Figure 8.20 Conjugation in Paramecium.

Further Phylogenetic Considerations Precise evolutionary relationships are difficult to determine for the protozoa. The fossil record is sparse. Ribosomal RNA sequence comparisons indicate that each of the four protists super groups probably had separate origins (figure 8.21).

Figure 8.21 Tentative Phylogeny of the Eukaryotic Tree of Life Based on 18S rRNA Sequence Comparisons