BIOA01H3 – Lecture 33rd
Chapter 22
22.1 The Evolution of Protists Involved Endosymbiosis
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Likely evolved 1.5-2bya
We don’t understand how exactly they’ve evolved but know that it involves
endosymbiosis
Protists contain mitochondria and many also contain chloroplasts
22.2 Characteristics of Protists
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Protists have membrane-bound nucleus, with multiple, linear chromosomes
In addition to cytoplasmic organelles, incl. mitochondria and chloroplasts,
also have microtubules and microfilaments, providing motility and
cytoskeletal support
Share characteristics of transcription and translation
All organisms in eukaryotic lineages consist of protists except for animals,
land plants, and fungi, which arose from protist ancestors
Although some protists have characteristics resembling those of fungi, plants,
or animals, several features are distinctive in only protists
 In contrast to fungi, most protists are motile or have motile stages in
life cycle and cell walls made of cellulose not chitin
 Unlike plants, may photoautotrophic protists can also live as
heterotrophs & some regularly combine both modes of nutrition
 Protists don’t retain developing embryos in parental tissue nor have
highly differentiated structures such as roots, stems, leaves
 Photosynthetic protists also sometimes referred to as algae; generally
aquatic & often unicellular and microscopic
o All animals are multicellular and have features i.e. internal digestive
tract & complex developmental stages
o Protists also lack nerve cells, highly differentiated structures i.e. limbs
o Also lack a heart, collagen, which is an extracellular support protein
Most protists don’t share a common ancestor and are no more closely related
to each other than they are to fungi, plants, or animals
22.3 Protists’ Diversity Is Reflected in Their Metabolism, Reproduction, Structure,
and Habitat
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Protists are highly diverse in metabolism, reproduction, structure, and
habitat
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Habitat
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Live in aqueous habitats, incl. aquatic or moist terrestrial locations
In bodies of water, small photosynthetic protists called phytoplankton, the
organisms that capture energy of sunlight in all aquatic habitats
Phototrophs provide organic substances and oxygen for heterotrophic
bacteria, other protists, and small crustaceans and animal larvae that are the
primary constituents of zooplankton
Protists living on terrestrial, play important roles among detritus feeders
that recycle matter from organic back to inorganic
Structure
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Most protists single cells, others live in colonies in which individual cells
show little or no differentiation
Within colonies, individuals use cell signaling to cooperate on tasks i.e.
feeding or movements
Some protists are larger multicellular organisms; i.e. giant kelp of coastal
waters
Have complex intracellular structures, some of which are found nowhere else
among living organisms
These unique structures reflect key aspects of habitats in which protists live
i.e. unicellular protists living in freshwater pond
o Its cytoplasm hypertonic to water surrounding it, meaning water
flows into cell by osmosis
o Has specialized cytoplasmic organelle, contractile vacuole, which
gradually fills w/ water
o When vacuole reaches maximum size, moves to plasma membrane
and forcibly contracts
Cells of some protists supported by external cell wall or by internal/external
shell build up from organic or mineral matter
Instead of cell wall, other protists have pellicle, layer of supportive protein
fibers located inside cell just under plasma membrane, providing strength
and flexibility
Sometime during live, almost all protists move
Some move by amoeboid motion, in which cells extends one or more lobes of
cytoplasm called pseudopodia
Rest of cytoplasm and nucleus flow into pseudopodium, completing
movement
Others move by flagella or cilia
Metabolism
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Almost all protists are aerobic organisms, live either as heterotrophs or
photoautotrophs
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Some heterotrophic protists obtain organic molecules by engulfing part or all
of other organisms (phagocytosis) and digesting internally
Others absorb small organic molecules from environment by diffusion
Some live as both heterotrophs and autotrophs
Reproduction
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May be asexual, by mitosis, or sexual, through meiotic cell division and
formation of gametes
Those that reproduce by both mitosis and meiosis, two modes of cell division
often combined into life cycle that is highly distinctive among different
protists group
22.4a Excavates Lack “Typical” Mitochondria
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Groups takes name from hollow (excavated) ventral feeding groove found in
most members
All are unicellular animal parasites lacking mitochondria and move by
flagella
b/c lacking mitochondria, limited production of ATP via glycolysis
lack of mitochondria led biologists to consider group as most ancient line of
protists; now appears ancestor of group had mitochondria
nuclei of Excavates contain genes derived from mitochondria and also have
organelles likely evolves from mitochondria
may’ve lost mitochondria as adaptation to parasitic way of life, in which
oxygen in short supply; consider two subgroups, Diplomonadida and
Parabasala
o Diplomonadida
 means double cell and organisms look like two cells together
w/ two identical, functional nuclei and multiple flagella
arranged symmetrically around cell’s longitudinal axis
 Best known diplomonad is Giardia lamblia
 Some are free living, many live in animal intestines; some don’t
cause harm to hose, others do
o Parabasala
 STD trichomoniasis caused by parabasalid
Trichomonasvaginalis
 Infection usually symptomless in men, but in women can cause
severe inflammation and irritation of vagina and vulva
 If untreated, can cause infection in uterus and fallopian tubes
resulting in infertility
 Parabasalids take names from cytoplasmic structures
associated w/ nucleus, parabasal bodies; some consider these
as Golgi apparatus of cells
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Also characterized by fin called undulating membrane 
formed by flagella buried in fold of cytoplasm, in addition to
freely beating flagella
Buried flagellum allows parabasalids to move through thick,
viscous fluids
22.4b Discicristates Include the Euglenoids and Kinetoplastids: Highly Motile
Protists
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Sometimes referred to as protozoa (proto = first; zoon = animal) b/c they’re
similar to animals in that they ingest food and move by themselves
Discicristates named for disk shaped mitochondrial cristae
Group includes about 1800 species, almost all unicellular and highly motile,
swimming by means of flagella
Although most are photosynthetic, some can also live as heterotrophs and
some even alternate between photosynthetic and heterotrophs
Euglenoids
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Important primary producers in freshwater ponds, streams, etc.
Most are autotrophs that carry out photosynthesis w/ same mechanisms like
plants
If light not available, many euglenoids also live as heterotrophs
Others lack chloroplasts and live entirely as heterotrophs
Name Euglena roughly translates as “eyeball organism”, reference to large
eyespot which is feature of photosynthetic euglenoids
o Contains pigment granules in association w/ light sensitive structure
and part of sensory mechanism stimulating cells to move toward
moderately bright light or away from intense ones so organisms find
optimal photosynthetic conditions
Also contains numerous organelles incl. contractile vacuole
Rather than external cell wall, have spirally shaped pellicle formed from
strips of transparent, protein-rich material underneath membrane
Some euglenoids, strips arranged in spiral allowing cell to change shape in
wriggling sort of motion that allows cell to change direction
Can also swim by flagella; has two; one rudimentary and short, other long
Kinetoplastids
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Sleeping sickness fatal disease, caused by various subspecies of Trypanosoma
brucei that are transmitted from one host to another by bites of tsetse fly
Early symptoms incl. fever, headaches, rashes, and anemia
Untreaed, disease damages central nervous system, leading to sleeplike coma
and eventual death
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Disease proven difficult to control b/c present in wild mammals, providing
inexhaustible reservoir for parasite
Other kinetoplastids are heterotrophs and are characterized by single
mitochondrion that contains large DNA-protein deposit called kinetoplast
Most kinetoplastids also have leading and trailing flagellum
22.4c Alveolates Have Complex Cytoplasmic Structures and Move via Flagella or
Cilia
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Groups named for small, membrane-bound vesicles called alveoli in layer just
under plasma membrane
Includes two motile, primarily free-living groups: Ciliophora and
Dinoflagellata, and a nonmotile, parasitic group, Apicomplexa
Ciliophora
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Ciliophora large group, w/ nearly 10k known species of primarily unicellular
but highly complex heterotrophic organisms that swim using cilia
Some live individually; some colonial
Some animal parasites; others live and reproduce in hosts as mutually
beneficial symbionts
Have many highly developed organelles, incl. mouthlike gullet lined w/ cilia,
structures that exude toxins and other defense materials from cell surface,
contractile vacuoles, and complex system of food vacuoles
Pellicle reinforces cell’s shape
Complex cytoskeleton anchors cilia just below pellicle and coordinates ciliary
beating
Cilia can stop, reverse beating in synchrony, allowing ciliates to stop, back up,
and turn if encounter negative stimuli
Ciliates are only eukaryotes that have two types of nuclei in each cell: one or
more small nuclei called micronuclei and single larger macronucleus
Micronucleus is diploid nucleus contains complete complement
genes.
o Fncs primarily in cellular reproduction, which can be
asexual or sexual
o Number present depends on species
Macronucleus develops from micronucleus but loses all genes except
those required for basic functions of cell and for synthesis of
ribosomal RNA.
o Contains numerous copies of these genes, allowing it to
synthesize larger quantities of proteins and rRNA
Ciliates abound in freshwater and marine habitats, where they feed on
bacteria, algae, and each other
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Dinoflagellata: The Dinoflagellates
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Red tides caused by certain dinoflagellates that make up large proportion of
marine phytoplankton
Typically have shell formed from cellulose plates
Beating of flagella, which fit into grooves in plates, makes dinoflagellates spin
like a top as they swim
Most are unicellular
Live as heterotrophs or auto
Some lives as symbionts in tissues of other marine organisms i.e. jellyfish
Apicomplexa
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Nonmotile parasites
Take name from apical complex, group of organelles at one end of cell which
helps cells attach to and invade host cells
Absorb nutrients through plasma membranes and lack food vacuoles
Can reproduce asexually and sexually forming gametes that fuse to produce
cysts
As in Giardia, when host organism engulfs cysts, they divide to produce
infective cells
22.4d Heterokonts Have Two Dissimilar Flagella at Some Stage of Their Life Cycles
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Named for two different flagella: one smooth and a second covered w/
bristles
in most heterokonts, flagella occur only on reproductive cells
heterokonts include Oomycota (water moulds), Bacillariophyta (diatoms),
and Chrysophyta (golden algae), and Phaeophyta (brown algae)
Oomycota
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organism causing blight was water mould
water moulds not fungi; they’re oomycetes but do share some features w/
fungi
like fungi, oomycetes grow as microscopic, nonmotile filaments called
hyphae forming network called mycelium
also like fungi, heterotrophs, secreting enzymes that digest complex
molecules of surrounding organic matter or living tissue into simpler
molecules that are small enough to be absorbed into cells
other features not similar to fungi are differences in nucleotide sequence,
clearly indicating close evolutionary relationships to other heterokonts than
fungi
live almost exclusively in freshwater lakes, streams, etc. where they’re key
decomposers
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Bacillariophyta: Diatoms
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unicellular organisms w/ glassy silica shell
substances move in and out of cell through patterned perforations in shell
photoautotrophs w/ photosynthetic pathways seen in plants
abundant in freshwater habitats
most are free-living, some are symbionts inside other marine protists
asexual reproduction in diatoms occurs by mitosis followed by form of
cytoplasmic division
Chrysophyta: Golden Algae
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nearly all are autotrophs and carry out photosynthesis similar to pathway
found in plants
colour due to brownish carotenoid pigment, fucoxanthin, which masks green
colour of chlorophylls
most chrysophytes also lives as heterotrophs if not sufficient light
important in freshwater habitats and in nanoplankton  community of
marine phytoplankton composed of huge numbers of extremely small cells
most are colonial forms, in which each cell of colony bears pair of flagella
have glassy shells but in form of plates or scales
Phaeophyta: Brown Algae
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kelp is a form of brown algae
o largest and most complex of all protists
o tissues differentiated into leaflike blades, stalklike stipes, and rootlike
holdfasts that anchor them to bottom
o hollow, gas-filled bladders give buoyancy to stipes and blades and
help keep them upright and oriented toward sunlit upper layers of
water
o stipes of some kelps contain tubelike vessels, similar to vascular
elements of plants, which rapidly distribute products of
photosynthesis throughout body of alga
all phaeophytes are photoautotrophs but not all are as larges as kelps
nearly all of species inhabit temperate or cool coastal marine waters
like golden algae, phaeophytes contain fucoxanthin, which gives them their
characteristic coour
cell walls contain cellulose and mucilaginous polysaccharide, alginic acid
alginic acid, called algin when extracted, s essentially tasteless substance
used to thicken such diverse products as ice cream, salad dressing, etc.
life cycles are typically complex and consist of alternating haploid and diploid
generations
large structures we recognize as kelp and other brown seaweeds are diploid
sporophytes  called that b/c give rise to haploid spores by meiosis
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o spores, which are flagellated swimming cells, germinate and divide by
mitosis to form independent haploid gametophyte generation
o gametophytes give rise to haploid gametes
o most gametophytes are multicellular
22.4e Cercozoa Are Amoebas with Filamentous Pseudopods
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amoeba (amoibe = change) is a descriptive term for unicellular protists that
moves by means of pseudopodia
several major groups of protists contain amoebas, which are similar form but
aren’t closely related
cercozoan amoebas produce stiff, filamentous pseudopodia, and many
produce hard outer shells called tests
two heterotrophic groups of cercozoan amoebas: Radiolaria and Forminifera,
and a photosynthetic group, the Chlorarachniophyta
Radiolaria
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marine organisms characterized by glassy internal skeleton and axopods 
slender, raylike strands of cytoplasm supported internally by long bundles of
microtubules
glassy skeleton is heavy, so how they keep afloat?
Axopods provide buoyancy as do numerous vacuoles and lipid droplets in
cytoplasm
o Involved in feeding as well: pretty stick to axopods and then engulfed,
brought into cell, and digested in food vacuoles
Skeletons that accumulate on ocean floor become part of sediment which
turns into sedimentary rock
Foraminifera: Forams
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Organisms take name from perforations in shells through which extend long,
slender strands of cytoplasm supported internally by network of needlelike
spines
Shells consists of organic matter reinforced by calcium carbonate
Most foram shells are chambered, spiral structures
Forams live in marine environments (like radiolarians)
Some species planktonic but they’re most abundant on sandy bottoms and
attached to rocks along coasts
Forams feed in manners similar to that of radiolarians
o Engulfs prey that adhere to strands, conduct them through
perforation in shell into central cytoplasm, where digested in food
vacuoles
Some have algal symbionts that carry out photosynthesis, allowing to live as
heterotrophs and autotrophs
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Chlorarachniophyta
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Are amoebas that contain chloroplasts and thus are photosynthetic
Combine this mode of nutrition w/ heterotrophy, engulfing food w/ many
filamentous pseudopodia
22.4f Amoebozoa includes Slime Moulds and Most Amoebas
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Includes most of amoebas (others are in Cercozoa) as well as cellular and
plasmodial slime moulds
Use pseudopods for locomotion and feeding
Amoebas
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Unicellular organisms that are abundant in marine and freshwater
environments and in soil
Some are parasitic
Most are heterotrophs that feed on bacteria, other protists, and bits of
organic matter
Unlike stiff, supported pseudopodia of cercozoans, pseudopods of amoebas
extend and retract at any point on body surface and are unsupported by any
internal cellular organization – amoebas are thus “shaep-shifters”
How can amoeba capture a fast-moving organism?
o As amoeba moves, its cytoplasm doesn’t just move but also changes
state, from more liquid state to more solid state and back again,
allowing amoeba to send out pseudopodia in different directions very
quickly
o These fast-moving pseudopods can capture even fast-swimming prey
such as ciliates
Reproduce only asexually via binary fission
Unfavourable environmental conditions, some amoebas can form cyst,
essentially by rolling up and secreting protective membrane
Survive as cysts until favourable conditions return
Slime Moulds
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Unusual heterotrophic protists
Exist for part of their lives as individuals that move by amoeboid motion but
come together in coordinated mass that ultimately differentiates into stalked
structure called fruiting body  in which spores are formed
Two major evolutionary lineages of slime moulds: cellular slime moulds and
plasmodial slime moulds, which differ in cellular organization
Both have ability to differentiate into fruiting bodies w/ stalks and sporebearing structures
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This differentiation much simpler than complex developmental pathways of
other eukaryotes, providing unique opportunity to study cell differentiation
at its most fundamental level
Respond to stimuli in environment, moving away from bright light and
toward food
Live on moist, rotting plant material
Cells engulf particles of dead organic matter, along w/ bacteria, yeasts, and
other microorganisms, and digest them internally
Can be range ofcolours: brown, yellow, green, red, and even violet or blue
Organisms exist primarily as individual cells, either separately or as
coordinated mass
Reproduces asexually and sexually
Plasmodial Slime Moulds
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Exists primarily as multinucleate plasmodium, in which individual nuclei
suspended in common cytoplasm surrounded by single plasma membrane
o Plasmodium flows and feeds by phagocytosis like single huge amoeba
– single cell that contains thousands to millions or even billions of
diploid nuclei surrounded by single plasma membrane
o Plasmodium, which may range in size, typically moves in thick,
branching strands connected by thin sheets
o Movements occur by cytoplasmic streaming, driven by actin
microfilaments and myosin
Often in response to unfavourable environmental conditions, fruiting bodies
form on plasmodium
o At tips of fruiting bodies, nuclei become enclosed in separate cells
o Cells undergo meiosis, forming haploid, resistant spores that are
released from fruiting bodies and carried by water o wind
If favourable conditions reached, continue to reproduce
22.4g Archaeplastida Include the Red and Green Algae and Land Plants
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Group consist of red and green algae, which are protists, and land plants,
which make up kingdom Plantae
The three groups of photoautotrophs share common evolutionary origin
Rhodophyta: The Red Algae
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Small marine seaweeds
Few species found in freshwater lakes and streams, or in soils
Cell walls contain cellulose and mucilaginous pectins that give red algae
slippery texture
o Pectin widely used in industry and science
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Extracted agar is used as culture medium in laboratory and as
setting agent for jellies and deserts
Some species secrete calcium carbonate into their cell walls; these coralline
algae are important in building coral reefs
Red algae typically multicellular w/ diverse morphologies; most have plantlike bodies composed of stalks bearing leaf-like blades
Most are free-living autotrophs, some are parasites that attach to other algae
or plants
Most are reddish in colour, some are greenish purple or black
Have complex reproductive cycles involving alternation between diploid
sporophytes and haploid gametophytes
No flagellated cells occur in red algae; instead, gametes released into water to
be brought together by random collisions in currents
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Chlorophyta: The Green Algae
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Carries out photosynthesis using same pigments as plants
o Shared pigment composition is one line of evidence that one lineage of
green algae was ancestor of land plants
Green algae shows more diversity than any other algal group
Also have diverse morphologies, incl. unicellular, colonial, and multicellular
species
Multicellular forms have range of morphologies, incl. filamentous, tubular,
and leaf-like forms
Most live in freshwater aquatic habitats, but some are marine, or live on
rocks, soil surfaces, tree bark, or snow
Other organisms rely on green algae to photosynthesize for them by forming
symbiotic relationships
Life cycles are diverse as body forms; many reproduce sexually or asexually
and some alternate between haploid and diploid generations
Green algae most closely related to land plants, based on molecular,
biochemical, and morphological data
Which green alga might’ve been ancestor of modern land plants?
o Evidence points to group known as charophytes as being most
similar to algal ancestors of land plants
22.4h Opisthokonts Include the Choanoflagellates, Which May Be the Ancestors of
Animals
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Named for single, posterior flagellum found at some stage in life cycle of
these organisms
 Diverse group includes choanoflagellates, protists thought to be ancestors of
fungi and animals
Choanoflagellata (choanos = collar) named for collar surrounding flagellum that
protist uses to feed and in, some species, to swim.
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o Collar resembles upside down lampshade and is made up of small,
fingerlike projections (microvilli) of plasma membrane
o As flagellum moves water through collar, projections engulf bacteria
and particles of organic matter in water
Live in either marine or freshwater habitats
Some are mobile but most are sessile
Number of species are colonial w/ cluster of cells on single stalk
22.5 Some Protist Groups Arose from Primary Endosymbiosis and Others from
Secondary Endosymbiosis
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Endosymbiosis involving photoautotrophs happened more than once,
resulting in formation of wide range of photosynthetic eukaryotes
First chloroplasts evolves from free-living photosynthetic prokaryotic
organisms (cyanobacteria) ingested by eukaryote cells that had already
acquired mitochondria
In some cells, cyanobacterium not digested but instead formed symbiotic
relationship w/ engulfing host cell – became an endosymbiont 
independent organism ling inside another organism
Over evolutionary time, prokaryotic organisms lost genes no longer required
for independent existence and transferred most of its genes to host’s nuclear
genome
In this process, endosymbiont became an organelle; moving genes from
enosymbiont to nucleus would’ve given host cell better control of cell
functioning
Chloroplasts of red and green algae and land plants result from evolutionary
divergence of photosynthetic eukaryotes formed from this primary
endosymbiotic event
Organisms originated from this event have chloroplasts w/ two membranes,
one from plasma membrane of engulfing eukaryote and other from plasma
membrane of cyanobacterium
Primary endosymbiosis followed by at least three secondary
endosymbiosis events, each time involving diff. heterotrophic eukaryotes
engulfing photosynthetic eukaryote, producing new lineages
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