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By the end of this section, you will be able to:
  • Describe representative protist organisms from each of the six presently recognized supergroups of eukaryotes
  • Identify the evolutionary relationships of plants, animals, and fungi within the six presently recognized supergroups of eukaryotes

In the span of several decades, the Kingdom Protista has been disassembled because sequence analyses have revealed new genetic (and therefore evolutionary) relationships among these eukaryotes. Moreover, protists that exhibit similar morphological features may have evolved analogous structures because of similar selective pressures—rather than because of recent common ancestry. This phenomenon, called convergent evolution, is one reason why protist classification is so challenging. The emerging classification scheme groups the entire domain Eukaryota into six “supergroups” that contain all of the protists as well as animals, plants, and fungi that evolved from a common ancestor ( [link] ). The supergroups are believed to be monophyletic, meaning that all organisms within each supergroup are believed to have evolved from a single common ancestor, and thus all members are most closely related to each other than to organisms outside that group. There is still evidence lacking for the monophyly of some groups.

The chart shows the relationship of eukaryotic supergroups, which all arose from a common eukaryotic ancestor. The six groups are Excavata, Chromalveolata, Rhizaria, Archaeplastida, Amoebozoa, and Opisthokonta. Excavata includes the kingdoms diplomonads, parabasalids, and euglenozoans. Chromalveolata includes the kingdoms dinoflagellates, apicomplexans, and ciliates, all within the alveolate lineage, and the diatoms, golden algae, brown algae, and oomycetes, all within the stramenopile lineage. Rhizaria includes cercozoans, forams, and radiolarians. Archaeplastida includes red algae and two kingdoms of green algae, chlorophytes and charophytes, and land plants. Amoebozoa includes slime molds, gymnamoebas, and entamoebas. Opisthokonta includes nucleariids, fungi, choanoflagellates, and animals.
This diagram shows a proposed classification of the domain Eukara. Currently, the domain Eukarya is divided into six supergroups. Within each supergroup are multiple kingdoms. Dotted lines indicate suggested evolutionary relationships that remain under debate.

The classification of eukaryotes is still in flux, and the six supergroups may be modified or replaced by a more appropriate hierarchy as genetic, morphological, and ecological data accumulate. Keep in mind that the classification scheme presented here is just one of several hypotheses, and the true evolutionary relationships are still to be determined. When learning about protists, it is helpful to focus less on the nomenclature and more on the commonalities and differences that define the groups themselves.

Excavata

Many of the protist species classified into the supergroup Excavata are asymmetrical, single-celled organisms with a feeding groove “excavated” from one side. This supergroup includes heterotrophic predators, photosynthetic species, and parasites. Its subgroups are the diplomonads, parabasalids, and euglenozoans.

Diplomonads

Among the Excavata are the diplomonads, which include the intestinal parasite, Giardia lamblia ( [link] ). Until recently, these protists were believed to lack mitochondria. Mitochondrial remnant organelles, called mitosomes , have since been identified in diplomonads, but these mitosomes are essentially nonfunctional. Diplomonads exist in anaerobic environments and use alternative pathways, such as glycolysis, to generate energy. Each diplomonad cell has two identical nuclei and uses several flagella for locomotion.

The micrograph shows Giardia, which is shaped like a corn kernel and about 12 to 15 microns in length. Three whip-like flagella protrude from the middle of the parasite, and a whip-like tail protrudes from the narrow back end.
The mammalian intestinal parasite Giardia lamblia , visualized here using scanning electron microscopy, is a waterborne protist that causes severe diarrhea when ingested. (credit: modification of work by Janice Carr, CDC; scale-bar data from Matt Russell)

Questions & Answers

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The scientific study of life.
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the virus that causes mumps in humans is composed of a protein outer Shell containing a core of DNA
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Basic science and applied science question about cancer
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You go for a long walk on a hot day. Give an example of a way in which homeostasis keeps your body healthy.
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You sweat.
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Biology is the study of Life
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Describe the steps and results of reintroducing wolves to Yellowstone National Park.
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tag and release wolves into Yellowstone. wolves eventually reproduce and the pack grows. as wolves hunt they cull the sick and weak prey. the carcass that is left provides food for other species (scavengers and insect.. etc). this heals the circle of life and contributes to the biodiversity...
Will
before you know it species that are critical to the eco system return. having apex predators is crucial to an ecosystem... it helps run the deer and elk , etc around.
Will
example: there was a species of shrub/ plant that grows along river banks that moose love to eat.. the moose have no predator so they decimate that food source which also helps prevent erosion. when the wolves were reintroduced this changed. oddly enough this plant species started to repopulate in
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the areas where wolf feces sat and decayed
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which of the following statements about the parts of an egg are false?
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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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