28.1 Eukaryotes probably arose by endosymbiosis.
Endosymbiosis
• Single-celled, nonphotosynthetic Pelomyxa palustris appears to represent an early stage in eukaryotic evolution. (p. 562)
• Pelomyxa lacks mitochondria, but contains two kinds of bacteria that may play a similar role. (p. 562)
• Most biologists believe mitochondria originated as symbiotic, aerobic bacteria; this theory is called endosymbiosis. (p. 562)
• Aerobic bacteria are believed to have evolved into mitochondria when they were engulfed by ancestral eukaryotic cells. (p. 562)
• Although most mitochondrial genes have been transferred to the host cell's chromosomes, each mitochondrion still has its own circular, closed molecule of DNA. (p. 562)
• Many eukaryotic cells contain other endosymbiotic bacteria in addition to mitochondria. (p. 563)
• Three biochemically distinct classes of chloroplasts exist, with each resembling a different bacterial ancestor. (p. 563)
• Recent investigations of chloroplast DNA suggest a single origin of chloroplasts. (p. 563)
28.2 The kingdom Protista is by far the most diverse of any eukaryotic kingdom.
The Challenge of Classifying the Protists
• Protists are the most diverse of the four kingdoms in the domain Eukarya. (p. 564)
• The classification of Protista is artificial and not representative of any evolutionary relationships. (p. 564)
General Biology of the Protists
• Protists are united on the basis that they are eukaryotes that are not fungi, plants, or animals. (p. 565)
• Protists exhibit a varied array of cell surfaces and diverse mechanisms for locomotion. (p. 565)
• Many protists form cysts with resistant coverings; they lack cell metabolism. (p. 565)
• Protists are known to be phototrophs, phagotrophs, and osmotrophs. They typically reproduce asexually through binary fission. (p. 565)
28.3 Protists can be categorized into six groups.
Euglenozoa
• Euglenoids were among the earliest free-living eukaryotes to possess mitochondria. (p. 566)
• It seems likely that chloroplasts ultimately evolved from a symbiotic relationship through ingestion of green algae. (p. 566)
• Kinetoplastids have a unique, single mitochondrion in each cell. (p. 567)
• Trypanosomes cause multiple serious diseases in humans, including trypanosomiasis (African sleeping sickness), East Coast fever, and Chagas disease. (p. 567)
• Rearrangement of genes during the asexual cycle allows for the expression of a large number of varieties of different antigen genes. (p. 567)
Alveolata
• Most dinoflagellates are photosynthetic unicells possessing two flagella. (p. 568)
• Protists cannot be grouped together based solely on the presence of flagella. (p. 568)
• At least 20 dinoflagellate species are known to produce powerful toxins that provoke respiratory failure in vertebrates. (p. 568)
• Apicomplexes are spore-forming animal parasites, such as the malarial parasite Plasmodium, that have a unique arrangement of organelles at one end of the cell. (pp. 568-569)
• Ciliates are extremely complex organisms with cilia usually arranged in longitudinal rows or in spirals around the cell. (pp. 569-570)
• All known ciliates have both a micro- and a macronucleus. (pp. 569-570)
Stramenopila and Rhodophyta
• Stramenopiles include brown algae, diatoms, and oomycetes. (pp. 571-572)
• Rhodophyta is made up of over 7000 species of red algae. (p. 572)
• Evidence exists supporting both early eukaryotic origins of Stramenopila and Rhodophyta and a common ancestry with green algae. (p. 572)
• Tentatively, Rhodophyta and Chlorophyta will be treated as sister clades. (p. 572)
Chlorophyta
• Chlorophytes make up one distinct lineage of green algae, while the other lineage, the streptophytes, gave rise to land plants. (p. 573)
• Chlamydomonas most likely represents a primitive state for green algae. (p. 573)
• Nonmotile, unicellular green algae and motile, colonial organisms are two lines of evolutionary specialization derived from early green algae. (p. 573)
Choanoflagellida and Protists That Are Difficult to Categorize
• Choanoflagellates are most similar to the sponges and to all animals. (p. 574)
• Amoebas and slime molds are very difficult to categorize because amoebas have similar cell morphology but are not monophyletic, and because slime molds originated at least three distinct times and the three lineages are only distantly related. (pp. 574-576)