The taxonomic category Protoctista was first coined by an English biologist, John Hogg , in an article entitled On the distinctions between a plant and an animal, and on a fourth kingdom of nature In this article, Hogg argued that the term Protoctist should be used to include "both the Protophyta Therefore, he said, there should be a "fourth kingdom of nature" in addition to the then-traditional kingdoms of plants, animals and minerals.
For nearly a century, however, his ideas were eclipsed by those of Haeckel, the reputed founder of protistology. Herbert F. Copeland resurrected Hogg's label almost a century later in his article, Progress report on basic classification Arguing that "Protoctista" literally meant "first established beings", Copeland complained that Haeckel's term included anucleated microbes such as bacteria.
Copeland's use of the term did not. In contrast, Copeland's term included nucleated eukaryotes such as brown and red algae -- but not the green algae , which he placed with the other green plants.
McKhann, and Lorraine Olendzenski The taxonomy of protists is still changing. Newer classifications [1] attempt to present monophyletic groups based on ultrastructure , biochemistry , and genetics. Because the protists as a whole are paraphyletic, such systems often split up or abandon the kingdom, instead treating the protist groups as separate lines of eukaryotes. The recent scheme by Adl et al.
Some of the main groups of protists, which may be treated as phyla, are listed in the taxobox at right. Most have been established as monophyletic, though for some this is still uncertain; for instance the metamonads, which may be paraphyletic to other excavates , and the Chromista , which may be paraphyletic to the alveolates see chromalveolates.
Various smaller groups of protists also existed; these are listed under the traditional categories, linked to above. Marguilis, L. Editors Handbook of Protoctista. Jones and Bartlett , Boston. ISBN Protists Fossil range: Neoproterozoic - Recent. Journal of Eukaryotic Microbiology. Protista make up all or part of each group, but one of the groups below also include plants and another the animals and fungi.
This re-organization recognizes that the kingdom Protista is not monophyletic. Group Characteristics Examples Excavata many with "excavated groove on cell body Giardia Chromalveolate with plastids or pastid genes from endosymbiosis with red algae Diatoms Rhizaria often amoeboid forms with threadlike pseudopodia; united mainly based on molecular systematics radiolarians Archaeplastida with chlorplasts from endosymbiosis with cyanobacteria also includes plants green alga Unikonta extremely diverse group that includes amoeboid forms, animal-like protists, and fungi-like protists that are united mainly based on molecular systematics also includes fungi and animals slime molds.
Does molecular phylogeny sometimes complicate our attempt to describe taxonomic groupings? Protists have a great variety of form and life style , and include photosynthetic alga, seaweeds , and protozoans the animal-like unicellular eukaryotes.
However, in terms of metabolic energy pathways, they are much less diverse than the prokaryotes. Many protists make-up the phytoplankton community along with the cyanobacteria in lakes and oceans. Why are they important? Euglena has a tough pellicle composed of bands of protein attached to the cytoskeleton.
The bands spiral around the cell and give Euglena its exceptional flexibility. The human parasite, Trypanosoma brucei , belongs to a different subgroup of Euglenozoa, the kinetoplastids. The kinetoplastid subgroup is named after the kinetoplast, a large modified mitochondrion carrying multiple circular DNAs. This subgroup includes several parasites, collectively called trypanosomes, which cause devastating human diseases and infect an insect species during a portion of their life cycle.
The parasite then travels to the insect salivary glands to be transmitted to another human or other mammal when the infected tsetse fly consumes another blood meal. Link to Learning Watch this video to see T.
The process of classifying protists into meaningful groups is ongoing, but genetic data in the past 20 years have clarified many relationships that were previously unclear or mistaken.
The majority view at present is to order all eukaryotes into six supergroups: Archaeplastida, Amoebozoa, Opisthokonta, Rhizaria, Chromalveolata, and Excavata. The goal of this classification scheme is to create clusters of species that all are derived from a common ancestor. At present, the monophyly of some of the supergroups are better supported by genetic data than others. Although tremendous variation exists within the supergroups, commonalities at the morphological, physiological, and ecological levels can be identified.
Figure Which of the following statements about Paramecium sexual reproduction is false? Figure Which of the following statements about the Laminaria life cycle is false? Which protist group exhibits mitochondrial remnants with reduced functionality? What genus of protists appears to contradict the statement that unicellularity restricts cell size? A marine biologist analyzing water samples notices a protist with a calcium carbonate shell that moves by pseudopodia extension.
The protist is likely to be closely related to which species? The chlorophyte green algae genera Ulva and Caulerpa both have macroscopic leaf-like and stem-like structures, but only Ulva species are considered truly multicellular. Explain why. Unlike Ulva , protists in the genus Caulerpa actually are large, multinucleate, single cells.
Because these organisms undergo mitosis without cytokinesis and lack cytoplasmic divisions, they cannot be considered truly multicellular. Why might a light-sensing eyespot be ineffective for an obligate saprobe?
Suggest an alternative organ for a saprobic protist. By definition, an obligate saprobe lacks the ability to perform photosynthesis, so it cannot directly obtain nutrition by searching for light. Instead, a chemotactic mechanism that senses the odors released during decay might be a more effective sensing organ for a saprobe.
Opisthokonta includes animals and fungi, as well as protists. Describe the key feature of this phylum, and an example of how an organism in each kingdom uses this feature. Describe two ways in which paramecium differs from the projected traits of the last eukaryotic common ancestor. Skip to content Protists. Learning Objectives By the end of this section, you will be able to do the following: 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 Identify defining features of protists in each of the six supergroups of eukaryotes.
Eukaryotic supergroups. This diagram shows a proposed classification of the domain Eukarya. Currently, the domain Eukarya is divided into six supergroups. Within each supergroup are multiple kingdoms. Although each supergroup is believed to be monophyletic, the dotted lines suggest evolutionary relationships among the supergroups that continue to be debated.
Archaeplastida Molecular evidence supports the hypothesis that all Archaeplastida are descendents of an endosymbiotic relationship between a heterotrophic protist and a cyanobacterium. Glaucophytes Glaucophytes are a small group of Archaeplastida interesting because their chloroplasts retain remnants of the peptidoglycan cell wall of the ancestral cyanobacterial endosymbiont Figure. Green Algae: Chlorophytes and Charophytes The most abundant group of algae is the green algae. Volvox aureus is a green alga in the supergroup Archaeplastida.
This species exists as a colony, consisting of cells immersed in a gel-like matrix and intertwined with each other via hair-like cytoplasmic extensions. Ralf Wagner. A multinucleate alga. Caulerpa taxifolia is a chlorophyte consisting of a single cell containing potentially thousands of nuclei.
An interesting question is how a single cell can produce such complex shapes. Link to Learning. Amoebozoa Like the Archaeplastida, the Amoebozoa include species with single cells, species with large multinucleated cells, and species that have multicellular phases.
Gymnomoebae The Gymnamoeba or lobose amoebae include both naked amoebae like the familiar Amoeba proteus and shelled amoebae, whose bodies protrude like snails from their protective tests. Amoebae with tubular and lobe-shaped pseudopodia are seen under a microscope. These isolates would be morphologically classified as amoebozoans.
Plasmodial slime molds. The life cycle of the plasmodial slime mold is shown. The brightly colored plasmodium in the inset photo is a single-celled, multinucleate mass. Cellular Slime Mold. The image shows several stages in the life cycle of Dictyostelium discoideum , including aggregated cells, mobile slugs and their transformation into fruiting bodies with a cluster of spores supported by a stalk. Opisthokonta The Opisthokonts are named for the single posterior flagellum seen in flagellated cells of the group.
A Colonial Choanoflagellate. Rhizaria The Rhizaria supergroup includes many of the amoebas with thin threadlike, needle-like or root-like pseudopodia Figure , rather than the broader lobed pseudopodia of the Amoebozoa. Ammonia tepida , a Rhizaria species viewed here using phase contrast light microscopy, exhibits many threadlike pseudopodia.
It also has a chambered calcium carbonate shell or test. Foraminiferan Tests. These shells from foraminifera sank to the sea floor. Radiolarian shell. This fossilized radiolarian shell was imaged using a scanning electron microscope.
A Chlorarachniophyte. This rhizarian is mixotrophic, and can obtain nutrients both by photosynthesis and by trapping various microorganisms with its network of pseudopodia. Alveolates: Dinoflagellates, Apicomplexians, and Ciliates A large body of data supports that the alveolates are derived from a shared common ancestor.
The dinoflagellates exhibit great diversity in shape. Many are encased in cellulose armor and have two flagella that fit in grooves between the plates. Movement of these two perpendicular flagella causes a spinning motion.
Dinoflagellate bioluminescence. Bioluminescence is emitted from dinoflagellates in a breaking wave, as seen from the New Jersey coast. They have a characteristic apical complex that enables them to infect host cells.
Paramecium has a primitive mouth called an oral groove to ingest food, and an anal pore to eliminate waste. Contractile vacuoles allow the organism to excrete excess water. Cilia enable the organism to move. Visual Connection. Conjugation in Paramecium. The complex process of sexual reproduction in Paramecium creates eight daughter cells from two original cells. Each cell has a macronucleus and a micronucleus. During sexual reproduction, the macronucleus dissolves and is replaced by a micronucleus.
Stramenopiles: Diatoms, Brown Algae, Golden Algae and Oomycetes The other subgroup of chromalveolates, the stramenopiles, includes photosynthetic marine algae and heterotrophic protists. Stramenopile flagella. This stramenopile cell has a single hairy flagellum and a secondary smooth flagellum.
Assorted diatoms, visualized here using light microscopy, live among annual sea ice in McMurdo Sound, Antarctica. Gordon T. Alternation of generations in a brown alga. Several species of brown algae, such as the Laminaria shown here, have evolved life cycles in which both the haploid gametophyte and diploid sporophyte forms are multicellular.
The gametophyte is different in structure than the sporophyte. A saprobic oomycete engulfs a dead insect. Diplomonads Among the Excavata are the diplomonads, which include the intestinal parasite, Giardia lamblia Figure. The mammalian intestinal parasite Giardia lamblia , visualized here using scanning electron microscopy, is a waterborne protist that causes severe diarrhea when ingested.
Sleeping sickness. Trypanosoma brucei , the causative agent of sleeping sickness, spends part of its life cycle in the tsetse fly and part in humans. Section Summary The process of classifying protists into meaningful groups is ongoing, but genetic data in the past 20 years have clarified many relationships that were previously unclear or mistaken. Visual Connection Questions Figure Which of the following statements about Paramecium sexual reproduction is false?
The macronuclei are derived from micronuclei. Both mitosis and meiosis occur during sexual reproduction. The conjugate pair swaps macronuclei. Each parent produces four daughter cells. Figure C. The sporophyte is the 2n plant. The gametophyte is diploid. Both the gametophyte and sporophyte stages are multicellular. Review Questions Which protist group exhibits mitochondrial remnants with reduced functionality?
What is the function of the raphe in diatoms? Dictyostelium Ulva Plasmodium Caulerpa. Critical Thinking Questions The chlorophyte green algae genera Ulva and Caulerpa both have macroscopic leaf-like and stem-like structures, but only Ulva species are considered truly multicellular.
The key feature of Opisthokonts is the flagellum on the posterior end of cells. Example organisms: Choanoflagellates use the flagellum for filter feeding. Sponges animals use the flagellum for filter feeding.
0コメント