Chapter Summary

20.1

  • Opisthokonta includes true fungi (Eumycota) and multicellular animals (Metazoa), as well as certain kinds of protists.
  • Viridiplantae includes green plants and primary endosymbiont algae.
  • Protist groups include Amoebozoa, the unshelled amebas; Rhizaria, shelled amebas; Alveolata, ciliates and flagellates with complex cortical structure; Heterokonta, the kelps, diatoms, and flagellates with nonequivalent paired flagella; and Euglenozoa and Excavata, primarily parasites.
  • Many protists are phototrophs as well as heterotrophs, based on secondary or tertiary endosymbiosis derived from engulfed algae.

20.2

  • Fungi form hyphae with cell walls of chitin. Hyphae absorb nutrients from decaying organisms or from infected hosts. Some fungi remain unicellular; these are called yeasts, or mitosporic fungi.
  • Chytridiomycete fungi have motile zoospores. Motile reproductive forms are a trait shared with animals. Flagellar motility has been lost by other fungi through reductive evolution.
  • Zygomycete fungi form haploid mycelia. Hyphal tips differentiate into gametes and grow toward each other to undergo sexual reproduction. Some zygomycetes form mycorrhizae that connect the roots of plants.
  • Ascomycete fungal mycelia form paired nuclei. Within the hyphal cells, the paired nuclei fuse, followed by meiosis and development of ascospores. Some ascomycetes form fruiting bodies called conidia.
  • Basidiomycete fungi form mushrooms. Cells with paired nuclei (secondary mycelium) form large fruiting bodies called mushrooms. The paired nuclei fuse to form the diploid basidium, which generates haploid basidiospores. The basidiospores develop underground hyphae or mycorrhizae that interconnect plant roots.
  • Microsporidia are fungus-related parasites of animal hosts.

20.3

  • Chlorophyta (green algae) absorb red and blue light and grow near the top of the water column. Green algae include unicellular, filamentous, and sheet forms.
  • Rhodophyta (red algae) have the accessory photopigment phycoerythrin, which absorbs green and longer-wavelength blue light, enabling growth at greater depths. Red algae include species of diverse forms, many of which are edible for humans.
  • Secondary endosymbiotic algae are derived from protists that had engulfed primary symbiotic algae. They are mixotrophs, combining phototrophy and heterotrophy.
  • Diatoms are heterokonts with silicate shells called frustules. Diatoms replicate by an unusual division cycle generating successively smaller frustules.
  • Kelps are heterokonts that grow in long sheetlike fronds. Kelps play an important role in the ecology of the open ocean, as well as the ecology of marine beaches.

20.4

  • Amebas move using pseudopods. In different species, pseudopods are lobe-shaped, lamellar, or filamentous (filopodia).
  • Cytoplasmic streaming through cycles of actin polymerization and depolymerization drives the extension and retraction of pseudopods.
  • Slime molds show an asexual reproductive cycle in which individual amebas aggregate to form a fruiting body that produces spores.
  • Radiolarians have silicate shells penetrated by filamentous pseudopods.
  • Foraminiferans have calcium carbonate shells with helical arrangement of chambers, the most recent of which opens to extend filamentous pseudopods.

20.5

  • Ciliates are covered with numerous cilia. Cilia provide motility and help capture prey. Ciliates undergo complex reproductive cycles involving exchange of micronuclei through conjugation.
  • Dinoflagellates are phototrophic predators. Dinoflagellates are tertiary endosymbiotic algae. Their alveoli contain calcified plates; they have paired flagella, one of which is used for propulsion. Predation occurs by extension of a lamellar pseudopod.
  • Apicomplexans are parasites that penetrate host cells. The apicoplast is a specialized organ for cell penetration. Apicomplexans such as Plasmodium falciparum conduct complex life cycles within mammalian and arthropod hosts.