recap

26.2 recap

The major lineages of eukaryotes began to diverge about 1.5 billion years ago. Major groups of eukaryotes are highly diverse in their habitat, nutrition, locomotion, and body form. Many protists are photosynthetic autotrophs, but heterotrophic lineages have evolved repeatedly. Although most protists are unicellular, multicellularity has arisen independently many times.

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learning outcomes

You should be able to:

  • Identify stramenophiles on the basis of phenotypic characteristics.

  • Distinguish among the major groups of eukaryotes.

  • Justify the position that multicellularity is relatively easy to evolve.

  • Explain why protists are important to our medical and economic concerns.

Question 1

For each pair of groups below, describe how you could recognize members of the two groups and differentiate them from one another. Then describe features that the two groups in each pair share.

  1. Foraminiferans and radiolarians

  2. Ciliates and dinoflagellates

  3. Diatoms and brown algae

  4. Plasmodial slime molds and cellular slime molds

  1. Foraminiferans have external shells of calcium carbonate, whereas radiolarians have long, stiff pseudopods and radial symmetry. The external shells of foraminiferans and the internal skeletons of radiolarians are both important components of ocean sediments and sedimentary rocks.
  2. Ciliates are covered with numerous hairlike cilia, whereas dinoflagellates generally have two flagella (one in an equatorial groove and the other in a longitudinal groove). Both ciliates and dinoflagellates have sacs, called alveoli, just beneath their cell membranes, which identify them as alveolates.
  3. Diatoms are unicellular and are typically composed of two nested plates (like a petri dish). In contrast, brown algae are large, multicellular organisms composed of branched elements or leaflike growths. Both diatoms and brown algae are photosynthetic.
  4. The vegetative unit of a plasmodial slime mold is a plasmodium: a wall-less mass of cytoplasm containing numerous diploid nuclei. In contrast, the vegetative unit of cellular slime molds consists of separate, single amoeboid cells. In both groups, when environmental conditions become unfavorable, the vegetative units form fruiting structures.

Question 2

The fossil record of eukaryotes from the Precambrian is poor compared with that from the Cambrian and later geological periods, even though eukaryotes were diversifying for the last billion years of the Precambrian. Can you think of some reasons why the eukaryotic fossil record became more extensive in the Cambrian?

Most eukaryotes were unicellular until the beginning of the Cambrian. The rapid diversification and increased size of multicellular eukaryotes (especially animals) near the beginning of the Cambrian led to greatly increased chances of fossilization.

Question 3

Give examples of alveolates, stramenopiles, and excavates that are important for medical or culinary reasons.


(Examples; other answers are possible.)

  1. Alveolates: Dinoflagellates can be toxic (e.g., they can poison mollusks during red tides); the apicomplexan Plasmodium is the causative agent of malaria.
  2. Stramenopiles: Some brown algae are harvested and used by humans as an emulsifier in ice cream, cosmetics, and other products.
  3. Excavates: The parasitic Giardia lamblia, a diplomonad, causes the intestinal disease giardiasis (an infection common among hikers and campers who drink spring or stream water in recreational areas). Kinetoplastids include several medically important species of pathogenic trypanosomes that cause diseases such as sleeping sickness, Chagas disease, and leishmaniasis.

The ancient origins of the major eukaryote lineages and the adaptation of these lineages to a wide variety of lifestyles and environments resulted in enormous protist diversity. It is not surprising, then, that reproductive modes among protists are also highly diverse.