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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

what is deglutition
Nalumansi Reply
what is biology
Sierrah Reply
biology is a type of science that studies living things
xjuicy_editzz
viruses are composed of what
Daniel Reply
what are some disease caused by virus
Daniel
what are some disease caused by virus
Daniel
what are some disease caused by virus
Daniel
Ebola
Alhassan
Ebola,rabies,polio,hepatitis and small pox
Magoba
Corona (viral disease)
Musoke
viruses kinda in a sense not "living" what mean that a virus molecule that attaches to living organisms cells to destroy/kill them. so to answer your question they are composed/made up of molecules (please note that I am not 100% sure, please feel free to correct me :] )
xjuicy_editzz
what is the use of biology
Tarkaa Reply
To widen our reasoning abilities about the past and the present events in biology and life at large
Musoke
To understand our biography
Musoke
to study living things
Narh
Among all these which one is the major difference between platyhelminthes and coelenterates? 1. are multicellular 2. have developed a mesoderm 3. reproduce sexually 4. reproduce asexually
Habeeb Reply
what is evolution?
Emmanuelfray Reply
how can you define biology in terms of a burning fire in the bush?
Emmanuelfray
what is a cell
Kagelelo Reply
cell is a functional and unit of life
Sarita
it is the building block n basic unit of life
Narh
describe the process of testing the presence of starch in a leaf
Estel Reply
give me somes of examples.
Manu Reply
give example of what.
Ante
example of what
Elizabeth
what are chromosomes?
Ante
what is Biology
Darkoa
Biology is the study of animals and plants on their characters and death
Okol
No biology is the study of life (living thing's)
Achibiri
Chromosomes refers to one of the small parts like threads in the NUCLEI (central part )of plants and animals cell that carry the GENES
Estel
Yeah okol....Bio-means life and -logy means study/knowledge
Estel
Ecology is the study of
Abdul Reply
Ecology is the study of the interaction of organism in each other and their environment.
Ante
Ecology is the study of the interaction of organism in each other and their environment
Everlove
oh yes!!! exactly
Great
thank much
Ante
what is biome?
Ante
a geographical area with dinstic species
Francis
thanks much it very helpful
Ante
my pleasure
Francis
characteristics of living things
Jane Reply
excretion, movement, sensitivity, reproduction,growth, respiration,nutrition
Estel
what are there function
Ante
what are their function
Ante
what is the two type of reproduction?
Ante
sexual reproduction and asexual reproduction
Abdul
what are their function
Manu
Hi
Olufunmilokun
What is light independent reaction
Benjamin Reply
correct word or phrase. ______A____ uses carbon dioxide and _____B______ to make sugar during______C______. _____D______ traps light energy during this process. *
Andrea Reply
A_plant ,B_water,C_photosynthesis,D_chlorophyll
Abdul
Plant,sunlight,photosynthesis
Olufunmilokun
by which of the following feature c3 and c4 plant similar A .enzyme used to fix carbon dioxide B.cell where calvin cycle takes place C.cell where carbon dioxide fixation takes place
Kaleab Reply
correct word or phrase. ______A____ uses carbon dioxide and _____B______ to make sugar during______C______. _____D______ traps light energy during this process. *
Andrea
what is biology ?
Bizone Reply
study of organisms
Aurelia
is the study of living organisms
Emmanuelfray
what is reproductive system
Asiya
Is the giving birth of offspring of the same one
YAYAH
it is the study of living organism
Narh

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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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