Chapter 28: The
Origins of Eukaryotic Diversity
(including an
extensive survey of the protists)
a formal taxon such as kingdom, but serves to temporarily separate the species.
This is a fairly new approach, since for the past 30 years or so, all of the protists, including the algae, have been lumped together in one kingdom
For example, this group includes unicellular, colonial, and multicellular species. The group also includes photoautotrophic, heterotrophic, and mixotrophic species.
60,000 extant species, equal number extinct (from fossil record)
Most are aquatic, and have in common those characteristics common to all eukaryotes:
· true nucleus, membrane-bound organelles--some of most complex cells known
· much larger genome (1000X) than prokaryotes
· DNA in linear chromosomes with associated proteins
· true mitosis vs. binary fission seen in prokaryotes
· true meiosis and syngamy as means of genetic recombination (not all have sexual reproduction)
other general similarities:
· aquatic species include some marine plankton - drifting passively or swimming weakly near the water's surface, some in moist terrestrial environments
· mostly aerobic, using true mitochondria for respiration
· most have motile form at some stage of life cycle:
flagella or cilia--extensions of cytoplasm with bundles of microtubules covered by plasma membrane (not naked protein bundle as in monerans)
· many form cysts at a certain stage in their life cycle--survival under adverse conditions
Fossil acritarchs--ruptured coats of cysts dating back 2.1 bya.
By then, environmental pressures favored greater cellular complexity
1. Endomembrane model – infolding of the prokaryotic plasma membrane may have led to the formation of some organelles
2. Serial endosymbiosis theory--symbiotic consortiums of prokaryotic cells engulfed by larger cells (photosynthetic cells give rise to chloroplasts; aerobic heterotrophs give rise to mitochondria) (Mereschkovsky and Lynn Margulis)
Much circumstantial evidence to support the theory of serial endosymbiosis, with mitochondria arising first
· such symbiotic associations actually exist in nature now
· organelles right size, membrane structure, enzymes and transport systems
· reproduce by simple splitting (binary fission)
· circular chromosomes lacking histones or other proteins
· base sequences of rRNA and antibiotic sensitivity more similar to eubacterial than to cytoplasmic rRNA
SURVEY
OF THE PROTISTS
Protists are often grouped three informal categories (based on nutritional diversity, not based on evolutionary relationships). Though quite commonly used, these names have no basis in phylogeny and no significance in taxonomy:
1. Protozoa: animal-like protists, procure food via ingestion
2. Fungus-like Protists: absorptive decomposers (slime and water molds)
3. Algae: plant-like protists
· all photosynthetic autotrophs
·
account for 1/2 of total global photosynthetic
output!
· mostly aquatic
· mostly have sexual + asexual life stages, diverse life cycles
· almost all have cell walls, of differing composition
· photosynthetic pigments are the basis of their taxonomy
all have chlorophyll a and b; other accessory pigments lead to unique coloration
Molecular systematics now being used to separate protists into monophyletic groupings. See Figure 28.8 (p. 554) & Table 28.1 (p. 573) for a listing of the major clades:
1.
(Clade) Diplomonadida and Parabasala
May be most primitive eukaryotes--lack mitochondria – this is probably a secondary loss, i.e. once had them and later lost them
a. Diplomonads--have flagella, 2 separate nuclei, no mitochondria or plastids
ex. Giardia lamblia --intestinal parasite, causes severe cramps and diarrhea
b. Trichomonads, microsporidians--all parasitic, also lack mitochondria
ex. Trichomonas
2.
(Clade) Euglenozoa
All have flagella, (not the only flagellates). Includes photosynthetic, heterotrophic, and mixotrophic species.
a. Euglenoids
· same pigments as the green algae (chlorophyll a and b)
· only algal phylum that lacks cell walls, have flexible internal protein plates instead
· can be phototrophic or heterotrophic (mixotrophic)
· live in fresh water
· food storage molecule is paramylum (complex glucose polymer)
· movement by both tinseled flagellum and unique squirming or “euglenoid” movement
· eyespot at base of flagellum functions in phototaxis
ex. Euglena
b. Kinetoplastids
· all have whiplike flagellum
· single large mitochondrion assoc. with a kinetoplast which houses extra-nuclear DNA
· most unicellular; many form colonies
· most free-living; endosymbionts in termite gut, digest cellulose (Trichonympha)
· some parasitic eg. Trypanosoma spp. causes African sleeping sickness, (changes protein coat to evade host immunity) also Chagas disease in S. America
3.
(Clade) Alveolata
Possess alveoli (membrane-bound cavities under cell surface--purpose unknown)
a. Dinoflagellates (dino=whirling)
· unicellular or colonial
· internal cellulose plates give them form
· paired perpendicular flagella cause whirling movement
· as phytoplankton, are foundation for many marine and freshwater food chains
· also in coral reefs as symbionts of the coral polyps; parasitic, carnivorous spp as well eg. Pfiesteria piscicida
· dominant red pigments called xanthophylls
· cause “red tides” during blooms (toxic to fish and humans)
· mainly known for causing “paralytic shellfish poisoning” (toxin blocks sodium pumps in nervous and muscular tissue, amplified in food chain--shellfish concentrate the toxin during filter feeding)
examples in lab: Ceratium
b. Apicomplexans (sporozoans)
· all parasites of animals
·
disseminate as infectious cells called sporozoites
· all have complex life cycles; sexual and asexual stages; often two or more host species
eg. Plasmodium species--causes malaria worldwide; considerable world health problem
-both Plasmodium and host mosquito Anopheles developing resistance to drugs and pesticides used for control
-new strains, 300 million infections, 2 million deaths annually
-“hides” in human liver and blood cells for most of life cycle, evading immune system
-can also alter surface proteins to “outwit” antibody production
1. female Anopheles mosquito ingests blood from infected human, containing gametocytes within red blood cells
2. gametocytes form male and female gametes, fertilized in mosquito gut, forming shortlived 2n zygote
3. zygote encysts as oocyst in gut wall; sporozoites (1n) develop and rupture out, migrating to mosquito’s salivary glands
4. next human bite transfers sporozoites to new host
5. sporozoites migrate to liver cells; undergo several divisions over 2-3 days to form merozoites, which infect red blood cells
6. merozoites reproduce asexually to yield large #’s of new merozoites; cycles of cell lysis 48-72 hours yield characteristic chills and fever of disease
7. new red blood cells infected; some merozoites develop into gametocytes
c. Ciliates
· among most complex of all cells
· all ciliated (multiple short flagella which may beat synchronously)
· submembrane system of microtubules coordinate movement
· mostly solitary, fresh water
· unique feature: two types of nuclei:
· one large macronucleus with 50+ copies of genome in “packages” of several genes each (controls everyday cell functions, protein synthesis, asexual repro.)
· several to many micronuclei (up to 80 in Paramecium spp.)--involved only in sexual redistribution of genes during conjugation
·
eg. Stentor -- anterior membranelles finlike, beat causes a whirlpool to move food to mouth, posterior attaches to a rock, etc.
eg. Paramecium caudatum (see conjugation below)
conjugation
1. partial fusion of 2 individuals -all but one diploid micronucleus disintegrates in each cell
2. Remaining micronucleus undergoes meiosis, yielding 4 1n micronuc.
3. 1 divides, other 3 disintegrate
4. Mates swap 1 micronuc.
5. Syngamy yields fusion of 2 haploid micronuc. to reestablish diploidy; partners separate.
6. New 2n nucleus divides to form 8 diploid nuclei.
7. Original macronucleus disintegrates; 4 micronuc. become macronuclei through replication of DNA.
8. Two cycles of cell division yield eight new daughter cells, all with identical macro and micronuclei from combination of both mating partners. Parents are lost.
*Note that sexual conjugation is distinct from reproduction in this system.
4.
(Clade) Stramenopila
· Grouped together for the numerous hairlike projections on their flagella
· Includes both photosynthetic algae and heterotrophs
· Photosynthetic species have unusual chloroplasts
a. Oomycotes (water molds, white rusts, downy mildews)
some of most destructive funguslike organisms; devastating plant pathogens
eg. potato late blight (Phytophthora infestans)--Irish potato famine of 1845-7
eg. downy mildew of grapes (Plasmopara viticola)-French vineyard blight of 1870’s
eg. new (summer 2000) outbreak of oak blight in live oaks of central CA coast (also a Phytophthora sp.)
· mostly saprophytes (absorptive), important decomposers
· also some important parasites of fish
· named for resistant zygote, the oospore
· funguslike appearance of coenocytic hyphae
· cell walls made of cellulose, unlike fungi
· diploid stage predominates
· have biflagellated stage, the zoospore, lacking in true fungi
· sexual repro. involves a large egg cell fused with smaller sperm nucleus to yield a resistant zygote, the oospore--very resistant in soil
Motile zoospores plus resistant oospores contribute to
damaging effects of these organisms in plant pathogenic outbreaks.
b. Diatoms
· unique 2 sided cell wall composed of silica
· yellow and brown pigments (once grouped w/ Chrysophyta below)
· mostly reproduce asexually
· sexual repro. via egg and sperm (only flagellated form)
· store food reserves as oils, aid in buoyancy; also laminarin (glucose polymer)
· sediments form diatomaceous earth (filters, grinding and polishing)
c. Golden algae
· color due to yellow (carotenoid) and brown (xanthophyll) accessory pigments
· fresh water plankton
· mostly colonial, biflagellated (see Dinobryon p. 535)
· form cysts under dry conditions, found in Precambrian fossils
d. Brown algae
· brown coloration from carotenoids, xanthophylls
· most complex of all protists
· all multicellular, mostly marine, include the largest seaweeds and kelps
· most common in temperate coastal waters (Fucus in rocky intertidal zone; Laminaria just below low tide zone; sea kelp or Macrocystis along continental shelf, stipes up to 100 meters long)
Examples = Sargassum, Fucus, Laminaria (Japanese soups)
analogous structures found in: seaweeds vs. plants
(parallel or convergent evolution) thallus plant body
stipes stems
holdfast roots
blades leaves
tubular cells vascular tissue
Alternation of Generations a common theme in life cycles of plants and higher algae phyla (Chlorophyta, Phaeophyta, Rhodophyta)
Common features in all examples:
1. sporophyte generation is asexual diploid (2n), gives rise to asexual spores (n) via meiosis
2. spores germinate to give rise to sexual gametophyte generation (haploid, n)
3. gametophyte eventually forms 1n gametes which fuse during sexual reproduction to form a 2n zygote
4. zygote grows and reestablishes the sporophyte generation
eg. Chlamydomonas --isogamous (morphologically identical) gametes
· most of life cycle spent as haploid cells reproducing asexually
· stress leads to sexual reproduction; gametes fuse and form resistant coat; meiosis occurs only after breaking of dormancy
eg. Ulva also isogamous, isomorphic (morphologically identical) haploid and diploid generations
eg. Laminaria oogamous (union of egg and flagellated sperm); heteromorphic (morphologically distinct generations)--most similar to higher plants
· 2n sporangia form 1n zoospores which grow into multicellular male and female gametophytes
· gametophytes are free-living, morphologically distinct from sporophyte (heteromorphic)
· male gametophyte form flagellated sperm; female gametophyte forms ovum
· zygote grows into new sporophyte attached to female gametophyte parent
5. (Clade) Rhodophyta (red algae)
· red color from phycoerythrin (shared only with cyanobacteria)
· mostly marine; some freshwater and soil spp.
· Most abundant in warm coastal waters
· may have other accessory pigments to optimize light harvesting at different water depths (up to 260 meters)--pigments vary in same species depending on depth
· others lack pigments, parasitize other algae
· most multicellular, filamentous
· no flagellated stages in life cycle--gametes carried on water currents
· source of agar, carageenan
examples = Porphyra (sushi wrap = "nori"), Chondrus
6. Viridiplantae (includes the green algal group Chlorophyta = closely linked to the Plant Kingdom
· green color from dominant chlorophyll a, b
· likely the direct precursors of terrestrial plants (similarities in chloroplast structure and pigments; cellulose in cell walls; plant starch as food reserve molecule)
· 7000 spp., mostly freshwater, some marine, soil, endosymbionts, even ice and snow
· in mutualism with fungi as lichens
Examples = unicellular (Chlamydomonas); colonial (Volvox, Spirogyra); multicellular ( Ulva)
7.
(Clade) Mycetozoa
a. Plasmodial slime molds
· all heterotrophic
· feeding stage an amoeboid mass called a plasmodium (multinucleated coenocytic mass without cell divisions)
· 2n nuclei all divide synchronously (useful for studies of mitosis)
· cytoplasmic streaming through channels moves nutrients and wastes throughout plasmodium
· engulfs food through phagocytosis; moves via pseudopods
· undergoes sexual reproduction under stress, yields sporangia that produce 1n spores
· spores germinate and flagellated gametes fuse to 2n zygote under favorable conditions; nuclear replication without cell division yields multinucleate plasmodium
eg. in lab Physarum polycephalum
b. Cellular slime molds
example in text = Dictyostelium discoideum
· obligate parasites of bacteria (we feed ‘em E.coli in lab)
· feeding stage unicellular, haploid; phagocytize bacteria, amoeboid movement
· lack of food yields aggregation of membrane-bounded cells into “migrating slug”
· fruiting bodies formed for asexual reproduction only--some aspects of multicellular organization
· sexual repro. yields a diploid zygote, undergoes meiosis within resistant cell wall (cyst)
Amoeboid Organisms
This polyphyletic grouping doesn't fit neatly into any of the current candidate kingdoms. Formerly classified as protozoans and fungus-like protists, they are now grouped together due to amoeboid movement and phagocytotic feeding at some point in their life cycles.
a. Rhizopods (means “root-foot”)-- amoebas and their relatives
· all unicellular
· no flagellated stages in their life histories
·
move and feed by extensions of cytoplasm called pseudopods
(“false feet”)
· no meiosis or sexual reproduction; mitotic stages also absent
· most are free-living; endoparasite Entamoeba histolytica causes amoebic dysentery
example
in lab: Amoeba proteus
b. Actinopods (Heliozoans and Radiozoans)
· move and feed by use of axopodia or “ray feet” are pseudopods stiffened w/ microtubule bundle covered by cytoplasm--phagocytize food which sticks to axopods, cytoplasmic streaming carries it to cell body
a) heliozoans (“sun animals”)--components of fresh-water plankton (shells of silica or chitin)
b) radiolarians--all have silicaceous shell, components of marine plankton
c. Forams - "little hole bearers"
· all marine, with porous multichanneled shells of CaCO2 and organic matter (up to 1 inch +)
· small openings in shell allow cytoplasmic strands to extrude for swimming, feeding
· 90% of spp. are fossils, found in large deposits in sediments, sedimentary rocks (White Cliffs of Dover)
· mostly live attached to sand, rocks, algae; also some planktonic types
· may have endosymbiont algae within their shells