Evolution of Protists
Chapter 17
171
Protists are a diverse group
refers to mostly unicellular eukaryotes
that are not animals, fungi or plants
 Usually aquatic but can live in moist
locations in land
 Size ranges from microscopic to 200
meters
 Asexual reproduction by mitosis is
common among protists, but there are
exceptions

172
Protists are a diverse group

Different modes of nutrition
◦ Protozoans – heterotrophic protists


Some protists are of medical importance
Protists are of great ecological
importance
◦ Major component of plankton

Designation kingdom Protista…..
taxonomically acceptable???
◦ Divided into many kingdoms
◦ Number under debate
The Eukaryotic Big Bang

Eukaryotic big-bang hypothesis
 States that all the different groups of protists
evolved so quickly from the original eukaryote
that there is no known root to their tree


fossil record is very poor
Much remains to be decided about
eukaryotic classification
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Chlorophytes
Charophytes
Red algae
Water molds
Land plants
Dinoflagellates
Brown algae
Diatoms
Foraminiferans
Apicomplexans
Radiolarians
Amoeboids
Ciliates
Slime molds
Fungi
Diplomonads
2
Parabasalids
1
3 4
5
Animals
Choanoflagellates
Euglenoids
Bacteria
Archaea
This representation of the protist evolutionary tree has a more treelike
appearance than does Figure 17.2B
Protozoans called flagellates
move by flagella

flagellates
 have a type of flagellum described as
whiplike

Euglenoids
◦
◦
◦
◦
◦
◦
Include about 1,000 species
Freshwater unicellular organisms
1/3 have chloroplasts; rest do not
2 flagella
Eyespot – photoreceptor
Contractile vacuole
176
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Euglena, a
flagellate
long flagellum
short flagellum
eyespot
photoreceptor
carbohydrate
granule
contractile
vacuole
nucleolus
nucleus
pellicle band
pyrenoid
chloroplast
eyespot
contractile
vacuole
long flagellum
nucleus
LM 200×
(Bottom): © Michael Abbey/Visuals Unlimited
177
Protozoans called flagellates
move by flagella

Parasitic flagellates
◦ Trypanosomes
 Trypanosoma brucei, transmitted by the bite of the tsetse
fly, is the cause of African sleeping sickness in humans
 Trypanosoma cruzi, causes Chagas disease in humans in
Central and South America
 Leishmaniasis, characterized by skin sores and in some
cases damage to the internal organs, is caused by a
trypanosome transmitted by sand flies
 Giardia lamblia cysts transmitted by contaminated water
causing severe diarrhea
 Trichomonas vaginalis is a sexually transmitted flagellate
– most common cause of vaginitis in the US
Protozoans called amoeboids
move by pseudopods
Pseudopods – extensions that form when
cytoplasm streams in a particular direction
 Amoeboids are in a group called
amoebozoans

◦ Part of zooplankton
◦ Use pseudopods to move and to engulf their food




Phagocytize their prey
Digestion occurs in food vacuole
Reproduce asexually
Entamoeba histolytica causes amoebic dysentery in
humans
Amoeba proteus, an amoeboid
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food vacuole
nucleolus
nucleus
contractile
vacuole
mitochondrion
plasma membrane
pseudopod
cytoplasm
Protozoans called
ciliates move by cilia

Ciliates
◦ Approximately 8,000 species of unicellular protists
◦ Move by means of cilia
◦ Most structurally complex and specialized of all
protozoans
◦ Majority are free-living
 Several parasitic, sessile, and colonial forms exist
◦ Paramecium – classic example
 Trichocysts used in defense or prey capture
 Both asexual and sexual reproduction
 Sexual reproduction involves conjugation
Paramecium, a ciliate
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trichocyst
contractile
vacuole
(partially full)
cilia
food vacuole
macronucleus
oral
groove
micronucleus
anal pore
gullet
contractile
vacuole (full)
pellicle
© CABISCO/Phototake
1712
Some protozoans are not motile

Apicomplexans
 Have an apical complex of microtubules
 3900 species of nonmotile, parasitic, sporeforming
protozoans
 Pneumocystis carinii causes the type of pneumonia seen
primarily in AIDS patients
 Malaria kills 1 million people each year
 4 parasites in genus Plasmodium
 Toxoplasma gondii causes toxoplasmosis, particularly in
cats, but also in people
 In pregnant women, the parasite can infect the fetus and cause
birth defects
Algae
1714
The diatoms and dinoflagellates
are significant algae in the oceans

Diatoms
◦ Approximately 11,000 species
◦ Free-living photosynthetic cells in aquatic and
marine environments
◦ Most numerous unicellular algae in the oceans
and freshwater environments
◦ Significant part of the phytoplankton
 Photosynthetic organisms suspended in the water
 Serve as an important source of food and oxygen for
heterotrophs
◦ Cell wall has two halves, or valves, with the larger
valve acting as a “lid” that fits over the smaller
valve
Cyclotella, a diatom. Diatoms live in “glass houses” because
the outer visible valve, which fits over the smaller inner valve,
contains silica
1716
The diatoms and dinoflagellates
are significant algae in the oceans

Dinoflagellates
◦ About 4,000 species
◦ Usually bounded by protective cellulose
plates impregnated with silicates
◦ Typically, the organism has two flagella
◦ Important source of food for small animals in
the ocean
◦ Some are symbionts in the bodies of
invertebrates
 Corals usually contain large numbers of
zooxanthellae
◦ Some undergo a population explosion and
cause “red tides”
Gonyaulax, a dinoflagellate. This dinoflagellate is responsible for
the poisonous “red tide” that sometimes occurs along the
coasts
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cellulose
plate
transverse
flagellum
longitudinal
flagellum
2 µm
© Biophoto Associates/Photo Researchers, Inc.
17-18
Red algae and brown
algae are multicellular

Red algae
◦ >5,000 multicellular species living primarily in
warm seawater
◦ Some grow attached to rocks in the intertidal zone
◦ Others can grow at depths exceeding 200 m
◦ Variety of body forms – filaments, complex
branches
◦ Produce agar, a gelatin-like product used
commercially and in the laboratory
◦ Carrageenan is an emulsifying agent (causes fat
to disperse in water) for the production of
chocolate and cosmetics
Chondrus crispus, a red alga
1720
Red algae and brown
algae are multicellular

Brown algae
◦ >1,500 species of seaweeds
◦ Range from small forms with simple filaments
to large, multicellular forms that may reach
100 m in length
◦ Majority of brown algae, like Fucus, live in
cold ocean waters

Multicellular forms of green, red, and
brown algae are called seaweeds, a
common term for any large, complex
alga
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blade
air bladder
stipe
holdfast
© Walter Hodge/Peter Arnold/Photolibrary
Fucus, or rockweed, a
brown alga
Green algae are ancestral to
plants

Green algae
◦ Approximately 7,500 species
◦ Not plants – do not develop from a protected
embryo
◦ Not always green
 Some have an orange, red, or rust color
◦ Inhabit a variety of environments
 Oceans, freshwater, snowbanks, bark of trees, backs of
turtles
◦ Lichen-symbiotic algal relationship with fungi

Sexual reproduction
◦ Spirogyra undergoes conjugation, temporary
union, during which cells exchange genetic
material
Cell anatomy and conjugation in Spirogyra, a filamentous green alga
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
Spirogyra
◦ Filamentaous
green alga
◦ Filament –
end-to-end
chain of cells
◦ During sexual
reproduction
undergoes
conjugation
cell wall
chloroplast
vacuole
nucleus
zygote
cytoplasm
pyrenoid
Conjugation
© M.I. Walker/Science Source/Photo Researchers, Inc.
20 mm
1724
Volvox, a colonial green alga
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
Volvox
 Colony
 Loose association
of independent
cells
 Cells cooperate
in beating of
flagella
 Some cells
specialized for
reproduction
 Daughter
colonies
40 mm
15 mm
daughter colony Vegetative cells
(lower right): © Cabisco/Visuals Unlimited
Ulva, a multicellular alga
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Ulva
◦ Multicellular
green alga
◦ Sea lettuce
◦ Thallus (body)
is two cells
thick
◦ Alternation of
generations
life cycle
Ulva,several individuals
One individuals
© William E. Ferguson

Chara, a stonewort
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branch
main axis
node
Chara,several individuals
© Dr. John D. Cunningham/Visuals Unlimited
One individual
Stonewort
◦ Encrusted
with calcium
carbonate
deposits
◦ Main axis a
single file of
cells
◦ Haploid life
cycle
◦ DNA
suggests
these are
most closely
to plants