Protists: Discussion 3 PDF

BIOL227 Discussion 3: Protists Protists: Diversity Protist diversity 2 Protists: Diversity Protists are eukaryotes Eukaryotic cells have organelles and are more complex than prokaryotic cells It is important to bear in mind that: Most protists are u...

BIOL227 Discussion 3: Protists Protists: Diversity Protist diversity 2 Protists: Diversity Protists are eukaryotes Eukaryotic cells have organelles and are more complex than prokaryotic cells It is important to bear in mind that: Most protists are unicellular, though there are some colonial and multicellular species Single-celled protists can be very complex, as all biological functions are carried out by organelles in each individual cell Protists exhibit more structural and functional diversity than any other group of eukaryotes 3 Protists: Structural and Functional Diversity Protists, the most nutritionally diverse of all eukaryotes, include Photoautotrophs, contain chloroplasts Heterotrophs, which absorb organic molecules or ingest larger food particles Mixotrophs, combine photosynthesis and heterotrophic nutrition Some protists reproduce asexually Others reproduce sexually, or at least employ sexual processes of meiosis and fertilization 4 Protists: Reproductive Diversity (Chlamydomonas) …and someprotists All three basic types of sexual reproduction are represented in protists, along with variations that fit none of the above (e.g. conjugation) 4 Protists can also reproduce asexually Protists: Ecological Diversity Protists are found in diverse aquatic environments Protists play two key roles in their habitats: as symbiont and as producer Some protist symbionts benefit their hosts Dinoflagellates nourish coral polyps that build reefs Wood-digesting protists digest cellulose in the gut of termites Figure 28.31 A symbiotic protist (parabasalid) Many protists important producers that obtain energy from the sun In aquatic environments, photosynthetic protists and prokaryotes are the main 6 producers Protists: Endosymbiosis Considerable evidence indicates that much protist diversity originates from endosymbiosis The process in which a unicellular organism engulfs another cell, which becomes an endosymbiont and then an organelle in the host cell Mitochondria evolved by endosymbiosis of an alpha proteobacterium that was engulfed by a cell from an archaeal lineage Plastids evolved later by endosymbiosis of a photosynthetic cyanobacterium that was engulfed by a heterotrophic eukaryote The plastid-bearing lineage of protists evolved into photosynthetic protists, red and green algae 7 Protists: Plastids derived from Endosymbiosis Plastid genes in red algae and green algae closely resemble those of cyanobacteria On several occasions during eukaryotic evolution, red and green algae underwent secondary endosymbiosis, in which they were ingested by a heterotrophic eukaryote Figure 28.3 Diversity of plastids produced by endosymbiosis. 8 Protists: Endosymbionts become Organelles It would have been advantageous to the host to maintain the cyanobacterial endosymbiont, as a source of sugar from photosynthesis Gene transfer from endosymbiont to the host nucleus made it dependent on the host Transferred genes need to be expressed Proteins must be targeted back into the organelle, where they function linear chromosomes that are very densely packed with little repetitive DNA 9 Protists: Four Supergroups Current hypothesis divides all eukaryotes (including protists) into four supergroups There are two groups of protists on this tree that are not affiliated with any of the four main supergroups the haptophytes and cryptomonads The evolutionary relationship of these groups of organisms to other eukaryotes is uncertain 10 Protists: Excavates The clade Excavata is characterized by its cytoskeleton Some members have an “excavated” feeding groove are mostly heterotrophic This group includes diplomonads, parabasalids, and euglenozoans 11 Excavates: Diplomonads These groups lack plastids, have modified mitochondria, and most live in anaerobic environments Diplomonads Have modified mitochondria (mitosomes- no ETC) Derive energy from anaerobic biochemical pathways Have two equal-sized nuclei and multiple flagella Are often parasites, for example, Giardia intestinalis 12 Excavates: Parabasalids These groups lack plastids, have modified mitochondria, and most live in anaerobic environments Parabasalids Have reduced mitochondria called hydrogenosomes that generate some energy anaerobically (H2 is realeased as by-product) Include Trichomonas vaginalis, the pathogen that causes yeast infections in 13 human females Excavates: Euglenozoans Euglenozoa is a diverse clade (group) that includes predatory heterotrophs photosynthetic autotrophs parasites The main feature distinguishing them as a clade is a spiral or crystalline rod of unknown function inside their flagella Clade includes kinetoplastids and euglenids Figure 28.6 Euglenozoan flagellum. 14 Excavates: Euglenozoans Kinetoplastids have a single mitochondrion with an organized mass of DNA called a kinetoplast They include free-living species that are consumers of prokaryotes in freshwater, marine, and moist terrestrial ecosystems Some are parasitic, eg Trypanosoma, which causes sleeping sickness in humans Trypanosomes evade immune responses by switching surface proteins These frequent changes prevent the host from developing immunity 15 Excavates: Euglenozoans Euglenids have one or two flagella that emerge from a pocket/excavated groove at one end of cell Some species can be both autotrophic (have chloroplast) and heterotrophic (mixotrophs) Eye spot: a light shield which allows light from specific direction to strike…. Light detector: detects the light that is not blocked by eye spot. The pellicle is protein bands beneath the plasma membrane that provide strength and flexibility (Euglena lacks a cell wall). 16 Protists: SAR Super group SAR: The SAR Clade is a Highly Diverse Group of Protists Defined by DNA Similarities The “SAR” clade is a diverse monophyletic supergroup named for the first letters of its three major clades: stramenopiles, alveolates, and rhizarians 17 Figure 28.un02 SAR: Stamenopiles The stramenopiles clade includes some of the most important photosynthetic organisms on Earth Stramenopiles include diatoms, golden algae, and brown algae Most have a “hairy” flagellum paired with a “smooth” flagellum Members include the diatoms, brown algae and oomycetes. Figure 28.9 Stramenopile flagella. 16 SAR: Stamenopiles (diatoms) Diatoms are unicellular, autotrophic algae with a unique two-part, glass-like wall. The walls are called frustules and made of of silicon dioxide Diatoms are a major component of phytoplankton and are highly diverse After a diatom population has bloomed, many dead individuals fall to ocean floor undecomposed. Figure 28.10 The diatom Triceratium This removes carbon dioxide from the atmosphere and morlandii “pumps” it to ocean floor Diatoms are mostly non-motile, but in some species a mucilage is secreted from the raphe. This mucilage allows gliding motion. 19 SAR: Stamenopiles (Brown Algae) Brown algae are the largest and most complex algae, commonly known as ”seaweed” All are multicellular, and most are marine Brown algae have most complex multicellular anatomy of all algae 20 SAR: Stamenopiles (Brown Algae) The diploid sporophyte produces haploid flagellated spores called zoospores Zoospores develop into haploid male and female gametophytes, which produce gametes Fertilization of gametes results in a diploid zygote, which grows into new sporophyte Figure 28.12 The life cycle of the brown alga Laminaria: An example of alternation of generations. 21 SAR: Stamenopiles (Oomycetes) Include water moulds, the white rusts, and the downy mildews. These organisms were previously classified as fungi but there are key differences between oomycetes and fungi cell walls are made of cellulose, whereas the walls of fungi consist chitin. molecular systematics confirm that oomycetes are within the stramenopiles and not closely related to fungi. Figure 28.14 Phytopthtora infestans. SAR: Stamenopiles (Oomycetes) Life cycle of Saprolegnia Zoospore discharge : https://www.youtube.com/watch?v=DIH9qNsIfJU Sexual reproduction https://av.tib.eu/media/22975 Oospores discharge https://www.youtube.com/watch?v=7IMMHsYtDtg SAR: Alveolata Members of the Alveolata clade have membrane-enclosed sacs (alveoli) just under the plasma membrane The function of alveoli unknown The alveolates include Dinoflagellates Apicomplexans Ciliates 20 Figure 28.un02 SAR: Alveolata (Dinoflaggelates) Dinoflagellates have two flagella in the groove: longitudinal and transverse. They are abundant components of both marine and freshwater phytoplankton They are a diverse group of aquatic phototrophs, mixotrophs, and heterotrophs Toxic “red tides” are caused by dinoflagellate blooms Figure 28.16a 21 Dinoflagellates. SAR: Alveolata (Apicomplexans) Apicomplexans are parasites of animals, and some cause serious human diseases They spread as infectious cells called sporozoites One end, the apex, contains a complex of organelles specialized for penetrating host cells and tissues Most have sexual and asexual stages that require two or more different host species for completion For example: The apicomplexan Plasmodium is the parasite that causes malaria and requires both mosquitoes and humans to complete its life cycle 26 SAR: Alveolata (Apicomplexans) 27 SAR: Alveolata (Ciliates) Ciliates, a large varied group of protists, use cilia to move and feed They have large macronuclei and small micronuclei Genetic variation results from conjugation, in which two individuals exchange haploid micronuclei Conjugation sexual process, and is separate from reproduction, which generally occurs by binary fission 28 SAR: Alveolata (Ciliates) Reproduction (see previous slide) 1. Two cells of compatible mating strains align side by side and partially fuse. The micronuclei are diploid. 2. Meiosis of micronuclei produces four haploid micronuclei in each cell. 3. Three micronuclei in each cell disintegrate. The remaining micronucleus in each cell divides by mitosis. 4. The cells swap one micronucleus. Asexual reproduction: 5. The cells separate. 6. Following micronuclear fusion, the two haploid micronuclei in the cell fuse into a diploid micronucleus. 7. Three rounds of mitosis produce 8 micronuclei inside the cell. 8. Four micronuclei become macronuclei and the original macronucleus disintegrates. 9. Two rounds of binary fission yield four daughter cells. Cells that are compatible mates can now continue at step 1. 29 SAR: Rhizarians Many species in the rhizarian clade are amoebas Amoebas are protists that move and feed by pseudopodia, extensions of the cell surface Rhizarian amoebas differ from amoebas in other clades by having threadlike pseudopodia Rhizarians include radiolarians, forams, and cercozoans 25 Figure 28. SAR: Rhizarians (Forams) Foraminiferans, or forams, are named for porous, generally multichambered shells, called tests Pseudopodia extend through pores in the test Many forams have endosymbiotic algae Foram tests in marine sediments form an extensive fossil record The magnesium content in fossilized forams can be used to estimate changes in ocean temperature over time 31 Figure 28.21 Fossil forams. SAR: Rhizarians (Radiolarians) Super group: SAR Marine radiolarians have delicate, symmetrical internal skeletons made of silica Radiolarians use pseudopodia to engulf microorganisms through phagocytosis Pseudopodia of radiolarians radiate from the central body 32 SAR: Rhizarians (Cercozoans) Cercozoans include most amoeboid and flagellated protists with threadlike pseudopodia They are common in marine, freshwater, and soil ecosystems Most are heterotrophs, including parasites and predators Chromatophore is photosynthetic and surrounded by peptidoglycan 33 Protists: Archaeplastida Supergroup Archaeplastida: Red algae and green algae are the closest relatives of land plants Red algae, green algae and land plants make the fourth eukaryotic subgroup Arachaeplastida 34 Archaeplastida: red algae Red algae are reddish in colour due to accessory pigment called phycoerythrin, which masks the green of chlorophyll The colour varies from greenish-red in shallow water to dark red or almost black in deep water Red algae are usually multicellular; largest are seaweeds Red algae are the most abundant large algae in coastal waters of the tropics Figure 28.23 Red algae. 35 Archaeplastida green algae Green algae are named for their grass-green chloroplasts Plants are descended from green algae Green algae are a paraphyletic group The two main groups are chlorophytes and charophytes Charophytes are most closely related to land plants Most chlorophytes live in fresh water, although many are marine Other chlorophytes live in damp soil, as symbionts in lichens, or in environments exposed to intense visible and ultraviolet radiation 36 Archaeplastida: green algae (charophytes) Charophytes share many distinct traits with land plants including the cellulose-synthesizing protein rings embedded in the plasma membrane of cells and the phragmoplast that helps to establish a new cell wall after cell division. https://www.researchgate.net/publication/351327763/figure/fig3/AS:10197526841999 37@1620139394076/Light-microscopic- images-of-Spirogyra-morphotypes-from-the- Lake-Baikal-region-A.jpg Eg: Spirogyra 32 Archaeplastida: green algae (chlorophytes) The chlorophytes range from unicellular (e.g. Chlamydomonas) to colonial (e.g. Volvox) Volvox (colonial algae) 38 SAR: Unikonta Super group Unikonts: include protists that are closely related to fungi and animals The supergroup Unikonta includes animals, fungi, and some protists This group includes two clades: the amoebozoans and the opisthokonts (animals, fungi, and related protists) The root of the eukaryotic tree remains controversial It is unclear whether unikonts separated from other eukaryotes relatively early or late 39 Unikonta: (slime mold) At one stage of the life cycle of a plasmodium slime mold, the diploid nucleus contained within a single cell divides mitotically to form a large multinucleated cell. This mass extends pseudopodia through decomposing material engulfing food by phagocytosis. Eventually, this mass becomes weblike producing fruiting bodies called sporangia within which meiosis occurs and haploid spores are discharged. The haploid spores produce amoeboid or flagellated cells that fuse together to restore the diploid state. Eg: Physarum (a plasmodial slime mold) 40 Unikonta: (slime mold) The cellular slime molds are composed of haploid solitary cells with amoeboid movements. When food supplies are depleted, these solitary cells aggregate together to form what appears like a multicellular body. Fruiting bodies (long slender stalks composed of individual cells stacked upwards) emerge up from the cell aggregate and spores are released in hopes of finding new sources of food. Figure 28.27 The life cycle of Dictyostelium, a cellular slime mould. 41 Unikonta: Tubulinids Tubulinids are a diverse group of amoebozoans with lobe- or tube-shaped pseudopodia They are common unicellular protists in soil as well as freshwater and marine environments Most tubulinids are heterotrophic and actively seek and consume bacteria and other protists Amoeba proteus, a favorite in introductory biology courses. 42 Unikonta: Entamoebas Entamoebas are parasites of vertebrates and some invertebrates Entamoeba histolytica causes amebic dysentery, third- leading cause of human death due to eukaryotic parasites These parasites secrete enzymes that degrade human epithelial cells on which they feed. Unikonta: Opisthokonts Opisthokonts include animals, fungi, and several groups of protists Fungi and animals: next discussions…