Chapter 28: Protists and evolution of eukaryotes

1. Systematists have split protists into many kingdoms. Explain why the kingdom Protista is no longer considered a legitimate taxonomic group.

2. Protists are the most diverse of all eukaryotes

• Describe the three ecological categories of protists. Explain why the terms protozoa and algae are not useful as taxonomic categories.

• Protists are eukaryotes and thus are much more complex than the prokaryotes.

• The first eukaryotes were unicellular.

• Not only were they the predecessor to the great variety of modern protists, but also to all other eukaryotes - plants, fungi, and animals.

• The origin of the eukaryotic cell and the emergence of multicellularity unfolded during the evolution of protists.

• Describe the evidence that supports the theory that mitochondria and plastids evolved by serial endosymbiosis. Explain which living organisms are likely relatives of the prokaryotes that gave rise to mitochondria and plastids.

• Describe the evidence that suggests that mitochondria were acquired before plastids in eukaryotic evolution.

• Explain the role of secondary endosymbiosis in the evolution of photosynthetic protists.

• Eukaryotic fossils date back 2.1 billion years and “chemical signatures” of eukaryotes date back 2.7 billion years.

• For about 2 billion years, eukaryotes consisted of mostly microscopic organisms known by the informal name “protists.”

• Protists are so diverse that few general characteristics can be cited without exceptions.

• Most of the 60,000 known protists are unicellular, but some are colonial and others multicellular.

• Describe three mechanisms by which large size and complexity have evolved in chlorophytes.

• Explain the basis for the proposal for a new “plant” kingdom, Viridiplantae.

• Explain the problem faced by Dictyostelium aggregates of constraining “cheaters” that never contribute to the stalk of the fruiting body. Discuss how research on this topic may lead to insights into the evolution of multicellularity.

• While unicellular protists would seem to be the simplest eukaryotic organisms, at the cellular level they are the most elaborate of all cells.

• A single cell must perform all the basic functions performed by the collective of specialized cells in plants and animals.

• Protists are the most nutritionally diverse of all eukaryotes,

• Most protists are aerobic, with mitochondria for cellular respiration.

• Some protists are photoautotrophs with chloroplasts.

• Still others are heterotrophs that absorb organic molecules or ingest larger food particles.

• A few are mixotrophs, combining photosynthesis and heterotrophic nutrition.

• Protists are found almost anywhere there is water.

• This includes oceans, ponds, and lakes, but also damp soil, leaf litter, and other moist terrestrial habitats.

• In aquatic habitats, protists may be bottom-dwellers attached to rocks and other anchorages or creeping through sand and silt.

• Protists are also important parts of the plankton, communities of organisms that drift passively or swim weakly in the water.

• Phytoplankton (including planktonic eukaryotic algae and prokaryotic cyanobacteria) are the bases of most marine and freshwater food chains.

• Describe the different nutritional strategies of protists. While nutrition is not a reliable taxonomic characteristic, it is useful in understanding the adaptations of protists and the roles that they play in biological communities.

• Protists can be divided into three ecological categories:

• protozoa - ingestive, animal-like protists

• absorptive, fungus-like protists

• algae - photosynthetic, plant-like protists.

Lecture Notes:

1) Time line for history of eukaryotes

2) Complexity of a unicellular protist (cell complexity) vs. complexity at the organism level (less complex cells, but cell organization at organism level very complex)

3) General definition of a eukaryote

4) Mitochondria and chloroplasts from Bacteria (not Archea). And, many independent transfers (i.e., different times and different lineages)

5) Example of convergent evolution between protists and fungi

6) Protists responsible for ½ of global carbon fixation (i.e., photosynthesis)