jimtrue.com : school : BSC2011 : CH 28: Origins of Eukaryotic Diversity

Posted by Jim True on September 9, 2004 6:31 AM. Last Updated October 22, 2006 9:23 PM

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CH 28: Origins of Eukaryotic Diversity

Protistan Systematics

• For many years, systematists placed the simplest eukaryotic organisms into a highly diverse kingdom, the Protista ("first of all").
• It was always known that this kingdom was artificial and included simple eukaryotes.
• This was because they did not fit in the other three eukaryotic kingdoms even though theere were some similiar characteristics.
• However, other than designating protists as typically unicellular (excepting the multicellular algae), it was not possible until recently to actually break up the kingdom into different groupings.
• The "kingdom" is now in the process of being subdivided through the use of molecular and subcellular level characteristics.
• It has been suggested that there may be as many as TWENTY different kingdoms!
• The designations have not yet been worked out, so we will examine different clades.
• A clade is a phylogenetic grouping at some taxonomic level. The clades we will examine are suggestsed to be at the phylum or possibly kingdom level.
• Protists included the most diverse of all eukarotic types and the term will continue to be used informally to indicate eukaryotes not belonging to the animals, plants or fungi.
• The "simplicity" of protistans refers to the fact that most forms are unicellular.
• However, as individual cells, they are among the most complex of all eukaryotic cells since all life functions must be carried out within a single cell.

Protistan Energy

• Like bacteria, protists exhibit a wide array of means of obtaining and using energy.
• Photoautotrophy (via chloroplasts) and heterotrophy are common.
• Mixotrophy -- some protists are both autotrophic and heterotrophic, switching modes as conditions change. Euglena is an example; if they do not have light available, they will phaogocytize nutrients from external sources and act as a heterotroph.
• The means of acquiring and using energy are not taxonomic characters.
• However, biologists still refer to three ecological groupings of protists:
  1. Animal-like - heterotrophs that ingest energy. Commonly referred to as protozoa ("first animal").
  2. Plant-like - photoautotrophs using chloroplasts. Commonly referred to as algae.
  3. Fungus-like - saprobic heterotrophs. Includes the slime and water molds.

Protistan Movement

• Most protists are capable of movement.
• In some cases , movement may only during certain life stages.
• The ability to move is provided by one of three possible structures, which are also often used by the organisms to capture food.
  1. Flagellum ("whip") (Flagella) - long hairlike filaments on suface of cell, usually few (1 - <1-10 per cell), may be longer than the cell itself.
  • Internal structure of 9 + 2 microtubules.
  • Generate fluid flow for food capture and movement by whipping back and forth.
  • Protistans bearing flagella are usually known as flagellates. Photosynthetic flagellates are collectively referred to as phytoflagellates ("phyto" = plant), while heterotrophic flagellates are collectively called zooflagellates ("zoo" = animal).
  2. Cilium ("eyelash") (cilia) - Short, hairlike extensions on the surface of the cell, usually 100's to 1000's per cell.
  • Internal structure of 9 + 2 microtubules.
  • Cilia beat back and forth sequentially to create fluid movement for feeding and locomotion.
  • Protistans bearing cilia are referred to as ciliates, and there is only one known phylum (Ciliophora).
  3. Pseudopodium ("false foot") (pseudopodia) - A temporary extension of the cytoplasm used for crawling and for engulfing prey (prey ingested by phagocytosis).
  • Used primarily by amebas and ameba-like forms.

Protistan Reproduction

• There is also diversity in reproduction among protists.
• Many reproduce solely by asexual reproduction via mitosis.
• Sexual reproduction and the production of diploid zygote via meiosis and syngamy fusion of 2 haploid gamete cells also occurs in some groups.
• In protists that exhibit sexual reproduction, the life history will usually be divided into two stages:
  1. Haploid - the vegetative ("feeding") stage.
  2. Diploid - the reproductive stage, usually the zygote.
• Cyst - similar to bacterial endospores, this encases a protistan cell within an outer covering that is resistant to harsh environmental conditions, thus ensuring that some members of the population will survive.

Protistan Habitat

• Most are aquatic, living in a water environment.
• When present in terrestrial (land) environments, the habitat has a high moisture content.
• Many protists live on or in the seiments in both environments, but there are numerous groups that float or swim free in the water. These are referred to as plankton.
• Many protists are included in the phytoplankton, photosynthetic plankton.
• Several of these groups are responsible for the greatest output of oxygen into the atmosphere, far exceeding that from terrestrial plants.
• Many protists are symbiotic forms, and there are examples of all forms of symbionts in this group.
• A number of protists are pathogenic.

Eukaryote Evolution

• It has long been thought that eukaryotes evolved from a composite of prokarotic cell structures and types.
• The presence of mitochondria and chloroplasts in modern eukaryotic cells is thought to have resulted from the incorporation of separate, independently functioning bacteria cells into a larger bacterial cell resulting in an endosymbiotic relationship (one symbiont lives within the other).
• Molecular biology is now beginning to show that this conclusion is true, based on molecular structures common to all cells.
• Much attention has focused on nucleotide sequencing of small ribosomal subunit RNA, known as SSU-rRNA.
• Mitochondria SSU-rRNA most closely resembles a primitive group of bacteria known as alpha proteobacteria (Fig 27.13, p.536).
• Chloroplasts are one of a group of similar plant organelles called plastids.
• Chloroplast SSU-rRNA most closely resembles that of cyanobacteria.
• It is hypothesized that in the process of becoming endosymbionts within the host bacterial cell, some of the endosymbiont genome was transferred to the genome of the host cell and integrated.
• While modern mitochondria and chloroplasts still retain some of their own DNA, they still require the cellular genome in order to function.
• Photosynthetic eukaryotes, including both protists as well as modern green plants all possess plastids, however, plastid structure varies among different groups, especially in the nature of the membrane surrounding the plastid.
• Some groups have two membranes, while others have three or four.
• It has been hypothesized that all groups possessing plastids ORIGINALLY had plastids with TWO membranes.
• One membrane came from the original independently living bacterium, the second was acquired from a vacuole membrane of the host cell when the plastid bacterium was incorporated during primary endosymbiosis.
• Secondary endosymbiosis - hypothesis related to those protists possessing three or four membranes around the plastids.
• Cells already containing endosymbiotic plastids were themselves engulfed by one or more new host cells, with each new event adding an additional layer to the plastid membrane.
• In each of the cases of secondary endosymbiosis, the endosymbiont lost all structures (which were duplicates of those in the host) except the plastid.
• These findings are quite interesting when you consider that the commonly accepted phylogenetic tree has a single ancestral form giving rise to the Domains Bacteria, Archaea and Eukarya.
• In this arrangement, the Archaea have more in common with Eukarya than do the Bacteria, no peptidoglycan in their cell wall, similiar protieins involved in transcription.
• However, the plastids and mitochondria in eukaryotes are derived from Bacteria. How would this be possible?
• The original explanation, known as LUCA, or last universal common ancestor.
• According to this phylogenetic view, the only bacterial DNA in the domain Eukarya should be that associated with the bacterial plastids and mitochondria incorporated via endosymbiosis.
• Also, Archaea should exhibit no bacterial genes since they do not appear to have acquired any structures from bacteria.
• In recent years, it has been found that both modern Archaea and Eukarya possess genes of bacterial origin, and in the Eukarya, these additional genes are not associated with either plastids or mitochondria!
• So, what does all of this mean?
• It challenges the long held concept that all of evolution is based on divergence off a single common branch (the LUCA scenario).
• While this appears to be so for some groupings (for examples, the Kingdoms Plantae and Animalia both appear to arise from common ancestral forms), it does not appear to be so for the evolution of the three domains.
• It appears that domains arose from an array of different ancestral primordial cells that exchanged genetic material and structures widely.
• This means that our modern phylogenetic tree should possess a network of "roots" or interwoven "trunks" instead of the single form traditionally envisioned.
• Such exchanges appear to have occured much more often amongst the prokaryotic lineages than in the eukaryotes once they evolved.
• The evolution and diversity of modern Eukarya do exhibit more linear relationships.
• The more complex eukaryotic cell was first able to diversify metabolically because of the various organelles.
• Once multicellular forms evolved, even more evolutionary changes in form and function were made possible.

Current Protistan Clades

• Protistan systematics is rapidly undergoing changes.
• As a result, numerous clades have been identified, with many of the clades being suggested as kingdoms.
• However, some of these clades may be further subdivided and some groups, such as the amebas and their relatives have not yet been fit into the classification scheme.
• As a result, except where noted, the following group names represent as yet unverified clades (most likely kingdom of phylum level taxa):
1. Clades Diplomonadida ("two nuclei") and Parabasa -- They represent the most primitive eukaryotes.
• Both groups lack mitochondria, although genetic information suggests that they possessed them at one time.
• Both groups are flagellates.
• Diplomonads possess two separate nuclei, while parabasalids possess an undulating cell membrane that assists in a sliding type of locomotion.
• Both groups include human pathogens:
• Giardia lamblia (diplomonad) - small intestinal parasite that cause abdominal cramping and severe diarrhea. Usually infection is from drinking contaminated water.
• Trichomonas vaginalis (parabasalid) - A sexually transmitted pathogen that infects the vaginal canal of women and urethra of men.
2. Clade Euglenozoa ("eugena" "animal") - includes several protistan groups:
   • Euglenoids, including the Phylum Euglenophyta, in which the common genus Euglena is found.
   • Possess one long and one short flagellum.
   • Most are photoautotrophs or mixotrophs
   • Contain a unique carbohydrate storage molecule, paramylon.
   • They typically move using the one long flagellum, but can also "crawl" by flexing the cell wall (euglenoid motion).
   • Phylum Kinetoplastida -- all are flagellated symbionts.
     • Possess one large mitochondrion, plus a special organelle, the kinetoplast, which holds additional DNA.
     • Most common are the trypanosomes, which are blood parasites of mammals. Diseases caused in humans include sleeping sickness, Chaga's disease and leishmaniasis.
     • Trypanosoma brucei causes sleeping sickness in Africa. The parasite enters and gradually destroys the central nervous system.
3. Clade Alveolata - All members of this clade share the common characteristic of alveoli, small membrane bound cavities under the main cell membrane, which are thought to assist in regulating water and ion content.
   • Apart from this the group is currently very diverse and will probably be further subdivided.
   • Current phyla include:
     • Phylum Dinoflagellata ("whirling whip" or "Terrible") -- These are photoautotrophs and most are phytoflagellates.
     • Includes both armored and naked forms. The "armor" is formed by cellulose plates.
     • Armored forms possess two flagella, many naked forms typically lack flagella and are usually endosymbionts living within another organisms.
     • The best known symbionts are referred to as zooxanthellae and are mutualists inside jellyfishes, certain mollusks and especially hard corals. Produce food by photosynthesis. Very sensitive to the environment.
     • The pigments in dinoflagellates are predominantly reddish-brown.
     • All are marine forms. Some species can increase population sizes so rapidly that their pigments stain the water. (The 'Red tide')
     • when this occurs, we call the phenomenon a red tide. Red tide organisms may secrete neurotoxins that poison marine animals. Local species of significance is Karenia brevis.
     • Another toxic dinoflagellate is Pfiesteria only discovered in 1991. It secretes toxins that eat into fish flesh, causing large ulcers.
     • Both of these can harm humans.
     • Phylum Apicomplexa ("a complex structure at the apex") -
       • All are unicellular parasitic forms (pathogens).
       • Movement is limited to cellular flexion.
       • All possess a specialized structure at one end of the infective cell (the apical complex) that allows them to penetrate host cells.
       • Includes the genus Plasmodium, which causes malaria. It is calculated that 300-500 million people have malaria, and an estimated 2 MILLION people die per year from this disease alone!
     • Phylum Ciliophora - ciliates, the most structurally complex of the protozoans, e.g. Paramecium, Spirostomum.
       • Includes both free swimming and sessile (attached) forms. Swimming forms are usually covered with cilia, while sessile forms have cilia in specific locations.
       • Ciliates typically possess a cytostome ("cell mouth") and a funnel-like cytopharynx ("cell throat") for bringing food into cell. Ciliary beat draws food into the cytostome and down the cytopharynx.
       • Food is then enclosed within vacuoles and moved to the cell interior for digestion.
       • Many ciliates have trichocyts ("hair bladder") which are small bubble-like organelles that can release filaments to hold prey.
       • Ciliates possess two types of nuclei, a macronucleus, which controls most cell metabolic functions, and one or more micronuclei, which function in the exchange of genetic information during conjugation.
4. Clade Stramenopila - ("stramen" - straw, "pilos" - hair).
   • This is also a diverse clade that has one common characteristic, the presence of flagella with fine hairlike projections.
   • These "hairy flagella" are usually only observed on the reproductive cells and are always paired with one "nonhairy" flagellum."
   • Phylum Oomycota ("oo" - egg, "myco" - fungus). Known as the water molds.
   • Oomycetes are very similar in structure, shape and feeding to true Fungi.
   • Forms are unicellular, and are either small or are composed of long branching multinucleate filaments called hyphae.
   • Differ from Fungi in that the cell walls are formed of cellulose (fungal walls are chitin).
   • Also, reproductive cells are biflagellate. True fungi never exhibit any motile stage.
   • Although superficiailly similar to Fungi, oomycetes are genetically distant.
   • The presence of the hyphae appears to be a case of convergent evolution (development of similar characteristics among unrelated forms).
   • Oomycetes can undergo both asexual and sexual reproduction.
   • When environmental conditions are good, reproduction is asexual. Tips of hyphae swell, are walled off, and haploid spores (flagellated) develop and are released into new hyphae.
   • Sexual reproduction occurs during bad environmental conditions.
   • Some hyphae produce sperm nuclei, others produce ovum nuclei and these fuse to form a diploid zygote which then develops into diploid oospores.
   • When favorable conditions return, oospores undergo meiosis and release haploid spores which form new hyphae.
   • All other stramenopile phyla are photosynthetic algae referred to as heterokonts, in reference to the two difference flagella types.
   • Phylum Bacillariophyta (" a little stick plant") - These are known as the diatoms.
     • These are unicellular aquatic forms in which the cells are sandwiched between two overlapping shells of SiO₂.
     • Main pigments are yellows and browns.
     • These are some of the most abundant protistan plant organisms in the oceans and are a major producer of atmospheric O₂.
     • Their abundance is so high that when they die, their shells form sediments.
   • Phylum Chrysophyta ("chryso" - golden).
     • These are the golden brown algaes, so called because of their primary pigments.
     • These are biflagellated.
     • Most are photoautotrophs, but some are mixotrophic.
     • The group includes both unicellular and colonia (group of cells living together) forms.
   • Phylum Phaeophyta ("phaeo" - brown, "phyta" - plant).
     • These are the brown algaes, and are one of several multicellular protistan groups called the "seaweeds".
     • Many have structures that look like leaves, stems and roots, but are not.
     • All are multicellular and marine.
     • This phylum includes the largest of all protists, the kelp (Laminaria) which may grow to 75 m or more!
     • Also in this phylum is Sargassum, which is so abundant in the North Atlantic Ocean that it forms mats 2-3m thick and miles in diameter.
     • Because of their abundance in some areas, Laminaria and Sargassum form unique ecosystems in the ocean, with organisms living there that are found nowhere else on Earth.
     • Many brown algae are of commercial importance to humans.
     • An extract from their cell walls, algin, is used to thicken toothpaste, ice cream, shaving cream, etc.
     • Cells are a rich source of elemental Iodine
     • Also eaten directly as food in some Asian countries.
5. Clade Rhodophyta ("rhod" - red, "phyta" - plant).
   • The red algaes, so named because their primary pigments make them appear red to maroon or deep purple (almost black).
   • Most rhodophytes are multicellular.
   • Main aquatic forms, a few in soil.
   • These occur the deepest among photosynthetic organisms in the ocean because of their pigment makeup.
   • Some species incorporate calcium carbonate (CaCo₃) into their cell walls ("coralline algae") and are important in reef formation.
   • Red algae also are important commercially.
   • Extracts include agar, used as a culture nutrient for bacteria and other organisms, and carrageenan, a stabilizer for chocolate milk, ice cream, paints and makeup.
   • Also eaten directly and provide a good source of vitamins A and C.
6. Clade Chlorophyta -- ("chlor" - green, "phyta" - plant).
• The green algae, which are the most closely related to plants, and which may ultimately be combined with the kingdom Plantae into a single kingdom.
• Includes unicellular, colonial (e.g. Volvox) and multicellular forms.
• Inhabit aquatic and damp terrestrial environments.
• Many of the simpler forms are symbionts, e.g. some lichens, a green algae and a fungus.
• Alternation of Generations - First exhibited amongst the algaes.
• Refers to the alternation between haploid (asexual) and diploid (sexual) multicellular generations.
• In the three algae groups plus the kingdom Plantae, the haploid generation is referred to as the gametophyte generation because the cells asexually produce haploid gametes by mitosis.
• The diploid generation is referred to as the sporophyte generation because the cells sexually produce spores which will grow into the new gametophytes.
• The algae spores are referred to as zoospores because they are flagellated.
• They will produce both male and female gametophytes, with the male gametes also being flagellated.
• In the kingdom Plantae, there is an evolutionary progression away from motile spores and gametes.
7. Clade (Amebas.. unnamed at present) --
   • The protists that use pseudopodia for movement and feeding into the newly developing taxonomy.
   • These include the three currently recognized phyla:
     • Phylum Rhizopoda ("rhizo" - root, "pod" - foot), - the naked amoebas, which lack hard shells around the cell, e.g. Chaos, Amoeba.
       • Lacking cell walls, the amoebas (also spelled ameba as a common name) are amorphous, lacking any distinctive shape.
       • They are found in both freshwater and moist soil enivonrments.
       • One of a number of amebic parasites, Entameba histolytica causes amebic dysentery, which affects many people, especially in the poorer nations.
     • Phylum Actinopoda - ("actin" - branch). "Armored" amebas, the radiolarians and heliozoans.
     • Possess a secreted shell of silicon dioxide (SiO₂); some heliozoans have a chitin shell.
     • Have very thin stalked axopods along which the pseudopodia stream for trapping food.
     • All are marine and float in H₂O.
     • Phylum Foraminifera -- ("foramen" - hole; "fera" - bearer).
     • Also armored amebas.
     • Possess porous shells of CaCO3, through which pseudopodia extend to trap food and move the organism.
     • Almost all are marine forms, floating in H2O or crawling on bottom.
     • The shells of both actinopods and foraminiferans form ocean seiments (THINK of numbers!!!).
8. Clade Mycetozoa ("myc" - fungus)
• although these superficially resemble Fungi, and may use pseudopodia similiar to amebas, it appears that this group forms a distinct and probably separate kingdom from these other organisms.
• Molecular analysis places them closest to the Fungi and Animalia.
• Phylum Myxogastrida -- ("myxo" - slime; "gastro" - stomach).
• Commonly known as the plasmodial "slime molds".
• These form large unicellular organisms that exhibit two distinct stages during life:
1. Vegetative (feeding) stage - Formed by a multi-nucleate unicellular mass called a plasmodium. The nuclei are diploid (2n).
• The plasmodium is often brightly colored and is typically found in moist, terrestrial habitats.
• The plasmodium feeds on bacteria and decomposing organic matter by phagocytosis.
2. Reproductive Stage - This typically develops when there is insufficient food and/or moisture. The plasmodium produces a stalked structure called a sporangium (sporangia).
• Numerous sporangia are produced and each produces haploid spores by meiosis.
• The thick spore wall protects the spore until favorable conditions return.
• spore releases either an ameboid or flagellated gamete. Two gametes fuse to produce a diploid zygote
• New plasmodium forms by mitosis without cytokinesis (being multinucleate).
• Interestingly, the nuclei are all synchronized to go through the same stages of mitosis at exactly the same time!
• Phylum Dictyostelida - ("dictyo" - a net; "stel" - a pillar)
• These are the cellular slime molds
• Like the myxogastrids, these have two distinct life stages:
1. Vegatative stage - in this stage, the feeding cells are tiny, individual ameba-like cells, each possessing a haploid nucleus.
2. Reproductive stage -- also develops during adverse environmental conditions.
• In this case, however, hundreds of individual cells join together to form a larger multicellular mass known as a pseudoplasmodium (also known as a "slug").
• The entire slug, forms a stalked sporangium, in which cells form haploid spores by mitosis.

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