Lecture III.2
Protists.
Protists are a paraphyletic group of eukaryotes that are neither plants, nor animals nor fungi. Most, but not all, are unicellular.
A Paraphyletic Group.
• Eukaryotes.
1. Nucleus; organelles;
2. Chromosomes;
3. Cytoskeleton;
4. Vesicles;
5. Sex, etc.
• Include the ancestors of
fungi, animals and plants.
• Abundant.
• At or near the base of the
food chain.
• Characteristics.
1. Most live in aqueous environments.
Brown algae (multicellular)
can be as large as a man and
have differentiated tissues.
2. Most small, some large.
3. Multicellularity evolved multiple times.
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4. Metabolically less diverse than prokaryotes; morphologically, more so.
5. Reproduce sexually or asexually.
6. Some have complex life cycles life cycles involving haploid and diploid stages.
7. Some cause diseases, e.g., malaria and toxoplasmosis.
8. Key primary producers in aquatic (marine and freshwater) habitats.
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Symbiogenesis and Organelles.
• Mitocchondria.
1. Double membranes of mitochondria remnants of engulfing – primary endocytosis – “way back when.”
2. Mitochondrial DNA groups them with Proteobacteria.
• Chloroplasts.
1. DNA groups chloroplasts with cyanobacteria.
2. In some species, structure reveals a history of multiple engulfings.
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Primary endosymbiosis involved the engulfing (phagocytosis) of a
cyanobacterium by a primitive eukaryote. The result was a chloroplast with two membranes. Secondary and tertiary endosymbiosis
involved the engulfing of a photosynthetic eukaryote by another eukaryote. The result was chloroplasts with three or four membranes.
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Sterols.
• Important components of eukaryotic
cell membranes.
1. Constitute ~ 50% of outer layer.
2. Modulate flexibility, permeability.
Cholesterol.
• Biosynthesis
1. Requires molecular oxygen – lots of it.
2. Oxidant protection – an internal sink for oxygen.
3. Made possible / selected for by early ocean oxygenation ~ coincident with evolution of multicellular cyanobacteria and / or origin of eukaryotes.
• Sterol synthesis genes recently found in myxobacteria (proteobacteria).
1. Did they get there by LGT from eukaryotes?
2. Is eukaryotic cell membrane descended (symbiogenesis) or borrowed (LGT) from myxobacteria?
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Sexual Reproduction.
• Gametes produced by meiosis – chromosome number reduced from 2N to N.
• Recombination => new gene
combinations.
• Haploid gametes fuse (fertilization) => diploid zygote,
• Offspring genetically distinct
from parents and each other.
Crossing over and recombination
during meiosis.
• Contrast with mitosis.
1. Daughter cells genetically identical. No crossing over /
recombination.
2. Both haploid and diploid cells can reproduce by mitosis.
• Defining sex as the production of new gene combinations
=> one can have
1. Sex without reproduction (bacterial, ciliate conjugation).
2. Reproduction without sex (mitosis).
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Sex (continued).
• Proposed advantages include:
1. Production of genotypic variability – recombination.
2. Inherent vigor of hybrids.
3. Facilitation of DNA repair when homologous chromosomes pair.
• Inherent Cost: A population of parthenogenetic females will
increase twice as fast as a comparable population of males
and females – “cost of males.”
• Many species alternate between sexual and asexual reproduction. In these cases,
1. Asexual reproduction typically associated with “good
times.”
2. Sexual reproduction often an inducible response to
stress;
3. Result is often the production of temperature-, desiccation-, etc., resistant spores that also constitute a dispersal stage.
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Questions.
1. (2 pts) The chloroplasts of Euglena have three membranes. What do you conclude?
2. (2 pts) What are myxobacteria?
3. (4 pts) In species with complex life cycles, why should
asexual reproduction be associated with “good times” and
sexual reproduction with times of stress?
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Protist Diversity.
• Diplomonads and Parabasalids.
• Euglenozoans – Euglenoids, Kinetoplastids.
• Alveolates – Apicomplexans, Ciliates, Dinoflagelates.
• Stramenopiles – Brown algae, Diatoms, Oomycetes.
• Red algae.
• Chlorophytes – Green algae; include ancestors of
plants).
• Choanoflagellates – include ancestors of animals and
fungi.
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Protist Diversity (Continued).
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Protist Diversity (Continued).
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Major Clades.
Principal protists. Diplomonads, Parabasalids, and Eugelozoans sometimes lumped into the phylum Excavata, the name of which refers to the
presence of an “excavated feeding groove” observable in some, e.g., Euglena, but not all members of the group. “PLANTS” here taken to include
only multicellular land species.
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Major Clades (continued).
• Diplomonads and Parabasalids.
1. Flagellates.
2. No mitochondria – contain
degenerate remnants of
mitochondria.
3. Some lack meiosis. But
have most meiotic proteins
=> derivation by loss.
4. Diplomonads.
a. Two nuclei, four flagella.
b. Giardia lamblia – infectious diarrhea from “bad
water”.
5. Parabasalids.
a. Undulating
membrane
Giardia (diplomonad) and
aids locomotion.
Trichomonas vaginalis (pab. Some in termite / cockroach rabasalid)
guts.
c. Some pathogenic – e.g., Trichomonas vaginalis causes
vaginal / urethral infections – 1.6X106 new cases /y.
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Major Clades (continued).
• Euglenazoans
1. Flagellates – two principal
groups.
2. Euglenoids
a. Most have two flagella.
b. Some ingest bacteria via a
mouth-like structure, the
“cyclostome.”
c. Many photosythetic –
chloroplasts surrounded
by three membranes.
d. Some mixtotrophic –
photosynthesize by day,
hunt by night.
Euglena is a photosynthetic
Euglenazoan. Depending on
cell’s orientation, the pigment
shield filters light reaching
light-sensitive structure at the
base of the flagellum, thereby
Contains multiple copies allowing phototaxis (attraction
of mit-DNA.
to light).
3. Kinetoplastids.
a. Named for the presence
of kinetoplast within the
single mitochondrion.
b.
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Euglenozoans (continued).
c.
Site of post-transcriptional RNA editing. May
facilitate host swtiching.
d.
Some parasitic – trypanosomes infect insects; some
have secondary hosts including humans. Diseases
include sleeping sickness and Chagas’ disease.
Life cycle of a trypanosome involves alteration between insect and
vertebrate hosts.
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Major Clades (continued).
• Alveolates.
1. Name refers to sacks (alveoli) packed into a flexible
structure (pellicle) that supports cell membrane.
2. Unicellular:
3. Ciliates, dinoflagellates and apicomplexans.
4. Ciliates.
a. Cilia for movement, feeding.
b. Macro- / micronuclei.
c.
Macronucleus is polyploid
i. Controls somatic cell functions.
ii. Micronucleus controls reproduction.
iii. Macronucleus periodically regenerated from micronuclei following conjugation – sex without
reproduction.
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Alveolates (continued).
d. Paramecium.
e. Trichonympha – Lives in termite guts.
i.
Endosymbiotic bacteria
cellulose digestion.
ii.
Ectosymbiotic
spirochetes movement.
Trichonympha lives in
termite guts. Endosymbiiii. Cellulose digestion in ter- otic bacteria produce celmites facilitated by two lulase, an enzyme that dilevels of symbiosis.
gests cellulose.
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Aloveolates (continued).
5. Dinoflagellates.
a. Two flagella.
b. Some have a theca composed
of cellulose plates.
c. Aberrant nuclear structure.
i.
No histones / nucleosomes
ii.
Chromosomes condensed
during interphase.
iii. Extra-nuclear mitotic spindle.
Dinoflagellates. Top. Anatomy. The two flagella lie
d. Impt ocean primary producers. in grooves called the sulcus and the cingulum and
e. Coral endosymbionts.
permit forward and rotary
motion. Bottom. Bioluminescence.
f. Others myxotrophic.
g. A few parasitic.
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h. Bioluminescence.
i.
Enzyme-catalyzed production of light.
ii.
Attracts secondary predators.
iii. Increases predation on dinoflagellate predators; selects for flashing avoidance in same, i.e., “police” on
the way – “burglar alarm” theory
Time required by squid to strike mosquito fish in the presence of luminous and nonluminous dinoflagellates Movement by dinoflagellate
predators stimulates bioluminescence, thereby increasing their own
susceptibility to predation as predicted by the burglar alarm hypothesis. From Fleisher and Case (1995).
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i.
Red tides
i.
Trillions of cells.
ii.
Induced by nutrient
upwelling and warm
temperatures.
iii. ⇒ massive fish kills
via oxygen depleRed tide off the California coast
tion and / or neuro- at La Jolla.
toxin production.
iv. Accumulation of dinoflagellate toxins in shellfish
causes paralytic shellfish poisoning if eaten by humans.
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Alveolates (continued).
6. Apicomplexans.
a. Parasitic – includes the organisms that cause malaria.
b. Name refers apical complex that facilitates penetration of
host cells.
c. Associated structure the apicoplast – non-photo-synthetic plastid surrounded by four membranes.
d. Killing the apicoplast does not kill the parasite, but prevents penetration of host cells.
e. Promising target for new anti-malarial drugs because of
differing plastid (prokaryote), mammalian cell sensitivities.
f. Malaria caused by various species of Falciparum.
g. Extra-cellular parasite of mosquitoes; Intra-cellular parasite of vertebrate (secondary) host.
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Two apicomplexans: Toxoplasma gondii (left) and Plasmodium
faciparum (right), The former causes toxoplasmosis; the latter,
malaria. Toxoplasmosis is spread by contact with cat feces; malaria by mosquitoes of the genus Anopheles.
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Malaria life cycle. Gametes fuse in the mosquito gut, and the resulting ookinete inserts in the intestinal wall forming an oocyst. Haploid
sporozoites form within the oocyst. When the latter ruptures, the sporozoites make their way to the salivary glands from which they are
injected into the human host along with anticoagulant that facilitates
feeding. In the human host, the parasite infects both the liver and red
blood cells. Eventually, male and female gametocytes are produced.
When another mosquito bites, she ingests gametocytes with the
blood meal, and the cycle repeats. Malaria is an extra-cellular parasite of mosquitoes and an Intra-cellular parasite of the secondary,
vertebrate host.
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Toxoplasmosis (Toxoplasma gondii).
• ~1/3 world’s population infected – by eating raw meat.
• Typically mild, flu-like illness
follows initial exposure; no
symptoms thereafter.
• Severe toxoplasmosis –
damage to brain, eyes and
other organs can develop in
1. Neonates (infants).
2. AIDS patients.
Toxoplasma life cycle.
• Cats the primary host – only
species in which parasite can reproduce sexually.
• Parasite appears capable of manipulating host behavior
via cyst formation in brain.
1. Infected lab mice lose fear of cats / cat urine.
2. Possible link to altered behavior (increased risk taking /
promiscuity), schizophrenia and autism in humans.
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Major Clades (continued).
• Stramenopiles.
1. Diatoms.
a. Important primary producers in the ocean.
b. Mostly unicellular.
c. Life Cycle.
i. Haploid gametes fuse to form diploid zygote.
ii.
Diploid cells can divide mitotically before undergoing meiosis and producing gametes.
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Stramenopiles (continued).
2. Brown algae.
a. Large, multicllular, marine
– giant kelp up to 60 m.
b. Independently (of plants)
evolved stem-like stipes,
root-like hold-fasts and
leaf-like blades.
c. Alternation of generations
i.
Haploid gametophyte
ii.
Diploid sporophyte.
d. Both multicellular.
i.
Compare w. diatoms
and apicomplexans.
ii.
Develop via mitosis.
iii. Sporophyte => haploid cells by meiosis.
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Top. Fucus serratus, a brown
alga with differentiated parts.
Bottom. Alternation of generations in brown algae. The
sporophyte and gametophyte
generations can be similar
(isomorphic) or dissimilar
(heteromorphic).
Major Clades (continued).
• Green Algae and Plants.
1. Chlorophytes.
a. Chlorophyll A (photosynthetic pigment) in “plants”.
b. Unicellular, colonial, and
truly multicellular.
c. Free-living and endosymbionts.
Multicellular green algae. Left. Volvox, a colonial form. Right. Multicellular Ulva lactuca (sea lettuce) attached to the substrate via a
holdfast. The main part of the “plant” is two cell layers thick.
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Green Algae and Plants (continued).
2. Embryophytes – (”green” / “land”) plants.1
a. Differentiation of cells / tissues, e.g., xylem, phloem.
b. Differentiation of organs – roots, stems, leaves, etc.
3. Chlorophyte Life Cycles.
a. Haploid (gametophyte) and diploid (sporophyte)
“generations.”
b. Isomorphic.
tions similar.
c.
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Genera-
Heteromorphic. Generations differ. Extremes:
i.
Haplontic – no multiThe life cycle of sea lettuce is
cellular sporophyte; isomorphic.
the zygote undergoes meiosis to immediately after fertilization.
ii.
Diplontic – gametes produced by sporophyte directly – no intervening multicellular gametophyte.
The name refers to retention of an embryo protected by parental tissue.
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Ulothrix, a filamentous green alga, has a haplontic life cycle. The
diploid zygote undergoes meiosis to produce haploid zoospores
that develop into the haploid “plant”, i.e., the gametophyte. Zoospores, and hence new gametophytes, can also be produced asexually.
4. Embryophyte (Green Plant) Life Cycles.
a. Heteromorphic.
b. Evolution marked by progressive gametophyte reduction and sporophyte enlargement.
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Major Clades (continued).
• Opisthokonts.
1. Word means “posterior
pole”, the “pole” being the
flagellum.
2. Include
a. Fungi (next lecture).
b. Choanoflagellates.
i.
Filter feeders – heterotrophs;
ii.
Solitary or colonial.
iii.
Similar to collar cells
of sponges.
c. Animalia – “Higher” animals.
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Colonial
late.
choanoflagel-
Amoebas.
• Polyphyletic – not a clade.
• Move, feed by pseudopods.
Generally lack flagella.
• Some pathogenic – e.g., Entamoeba histolytica causes
amoebic dysentery.
• Foraminifera – calcareous
shells with internal partitions.
1. Include giant, multinucleate
Xenophyophores – deep
water (up to six miles) benthic deposit feeders.
2. One theory holds that Ediacaran communities (LecTop. The humble amoeba.
ture II.4) dominated by
Bottom. Foram with protruding pseudopods.
a. Bacterial mats and
b. Giant protozoans [GPs] related to living xenophyophores living within them.
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3. This idea championed by
Adolf Seilacher2 who discussed strategies by which
protists can achieve large
size. Among them:
a. Multiple nuclei;
b.
c.
A xenophyophores on the sea
Isomorphic growth – bottom off the Galapagos Isincreases exchange of lands. Some species can grow
to over 20 cm. in diameter.
materials with env’t.
Division of cytosol into
semi-independent
compartments;
d. Endosymbionts.
4. Seilacher hypothesized that Eocene foraminiferan with isomorphic growth.
a. Many Ediacaran forms were GPs.
b. Coexisted with the ancestors of contemporary phyla
until the latter “learned” to eat them.
2
Seilacher, A. et al. 2003. Ediacaran biota: The dawn of animal life in the
shadow of giant protists. Paleont. Res. 7: 43-54.
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• Cellular slime molds – aggregating amoebae form multicellular fruiting bodies when bacteria on which they feed depleted.
Aggregating “social amoebae” first form a motile “slug” and then a
sessile fruiting body. The molecule inducing aggregation is cAMP
– an important signaling molecule in metazoans.
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Recurrent Evolutionary Trends in Protists.
• Evolution of complex life cycles.
• Evolution of multicellularity and cell differentiation.
Right. Phylogenetic relations among unicellular, colonial and multicellular eukaryotes. Some lineages
are strictly multicellular
(filled circle); some, unicellular or colonial (open circle). Still others have a mix
of unicellular / colonial and
multicellular forms (halffilled circle). From Abedin
and King (2010).
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Questions.
4. (8 pts) The organisms that produce malaria and sleeping
sickness alternate between vertebrate and invertebrate
hosts. What are the costs and benefits of such a life cycle
as compared to one in which the pathogen completes its
life cycle within a single host species? How might such a
life cycle have evolved?
5. (8 pts) In response to diminished food availability, the
amoebae of cellular slime
molds aggregate to form a
multicellular “slug” that develops into a fruiting body. Only
some of the original cells produce the spores that contribute to the next generation.
The rest become the stalk and its base. This is shown schematically in the figure at the right. The cells at the front and
back of the “slug” become the stalk, which dies, while the
cells in the slug’s middle become spores. Being the first or
the last cells to join a developing slug would thus appear to
be an evolutionary dead end. Yet every slug has to have a
front and a rear. Discuss.
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