Chapter 17
Evolution of
Protists
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Protists May Represent the
Oldest Eukaryotic Cells
17-2
17.1 Eukaryotic organelles
probably arose by endosymbiosis
 Protists (kingdom Protista) are
eukaryotes
 Endosymbiotic theory - at least
mitochondria and chloroplasts are derived
from independent prokaryotic cells
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Figure 17.1 Origin of mitochondria (above) and chloroplasts
(below)
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17.2 Protists are a diverse group
 Protists vary in size from microscopic to
macroscopic exceeding 200 m in length
 Most protists are unicellular, but they have attained a
high level of complexity
 Asexual reproduction by mitosis is the norm in
protists
 Sexual reproduction generally occurs only in a hostile
environment
 They are of enormous ecological importance
 They are a major component of plankton
 Organisms suspended in the water and are food for animals
 Protists have symbiotic relationships from parasitism to
mutualism
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Figure 17.2 Protist diversity
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APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES
17.3 How can the
protists be classified?
 Lumping all the single-celled eukaryotes
(protists) into a single kingdom is artificial
 Does not represent evolutionary history
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Figure 17.3 Proposed
evolutionary tree of
protists (blue branches)
based on DNA and RNA
sequencing
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Protozoans Are
Heterotrophic Protists
with Various Means
of Locomotion
17-9
17.4 Protozoans called
flagellates move by flagella
 Zooflagellates - thousands of species of mostly
unicellular, heterotrophic protozoans that move with
a flagellum
 Many zooflagellates are symbiotic and some are parasitic
 Euglenoids include about 1,000 species of small
(10–500 μm) freshwater unicellular organisms
 One-third of all genera have chloroplasts; the rest do not
 Those that lack chloroplasts ingest or absorb their food
 Some do both
 Euglena deces, an inhabitant of freshwater ditches and ponds
can undergo photosynthesis as well as to ingest food
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Figure 17.4
Euglena, a
flagellate
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17.5 Protozoans called
amoeboids move by pseudopods
 Pseudopods - extensions that form when
cytoplasm streams in a particular direction
 May be zooplankton, microscopic suspended organisms
that feed on other organisms
 Feed by phagocytosis, surrounding prey with
pseudopods and digesting it in a food vacuole
 Foraminiferans and Radiolarians have shells
called tests
 Intriguing and beautiful
 In foraminiferans the test is often multichambered
 Deposits of foraminiferans formed the White Cliffs of
Dover
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Figure 17.5A Amoeba proteus, an amoeboid
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Figure 17.5B Foraminiferans, such as Globigerina, built the White
Cliffs of Dover, England
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Figure 17.5C Radiolarian tests
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17.6 Protozoans called
ciliates move by cilia
 Ciliates - approximately 8,000 species of unicellular
protists
 Range from 10 to 3,000 μm in size
 The most structurally complex and specialized of all
protozoans
 The majority are free-living
 Several parasitic, sessile, and colonial forms exist
 When a paramecium feeds, food particles are swept
down a gullet into food vacuoles
 Asexual reproduction
 Ciliates divide by transverse binary fission
 Sexual reproduction involves conjugation
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Figure 17.6A Paramecium, a ciliate
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Figure 17.6B During conjugation, two paramecia first unite at oral
areas
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Figure 17.6C Stentor, a ciliate
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17.7 Protozoans called
sporozoans are not motile
 Sporozoans - nearly 3,900 species
 nonmotile, parasitic, spore-forming
 Many sporozoans have multiple hosts
 One million people die each year from malaria
 Widespread disease caused by four types of
sporozoan parasites in the genus Plasmodium
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Figure 17.7 Life cycle of Plasmodium vivax, the cause of one type
of malaria
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Some Protists Have
Moldlike Characteristics
17-22
17.8 The diversity of protists
includes slime
molds and water molds
 The Plasmodial Slime Molds
 Exist as a plasmodium, a diploid, multinucleated,
cytoplasmic mass
 Enveloped by a slimy sheath creeping along, phagocytizing
decaying plant material
 During droughts, plasmodium develops many
sporangia, spore producing reproductive structures
 An aggregate of sporangia is called a fruiting body
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Cellular Slime Molds
 Exist as individual amoeboid cells and are too
small to be seen
 Common in soil, feeding on bacteria and yeasts
 As the food supply runs out cells release a
chemical that causes them to aggregate into a
pseudoplasmodium
 Eventually gives rise to a fruiting body
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Figure 17.8 Life
cycle of plasmodial
slime molds
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Water Molds
 Water Molds
 Usually live in water, where they form furry growths
when they parasitize fishes or insects and
decompose remains
 Water molds have a filamentous body as do fungi, but
their cell walls are largely composed of cellulose
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17-27
Algae Are Photosynthetic Protists
of Environmental Importance
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17.9 The diatoms and dinoflagellates
are significant algae in the oceans
 Diatoms (approximately 11,000 species) are
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
 Diatom Structure
 Often compared to a hat box
 Cell wall has two halves, or valves, with the larger
valve acting as a “lid” that fits over the smaller valve
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Figure 17.9A Cyclotella, a diatom. Diatoms live in “glass houses”
because the outer visible valve, which fits over the smaller inner
valve, contains silica
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Dinoflagellates
 Dinoflagellates (about 4,000 species) are usually
bounded by protective cellulose plates impregnated
with silicates
 Typically, the organism has two flagella:
 One in a longitudinal groove with its distal end free
 One in a transverse groove that encircles the organism
 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”
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Figure 17.9B Gonyaulax, a dinoflagellate. This dinoflagellate is
responsible for the poisonous “red tide” that sometimes occurs
along the coasts
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17.10 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
 economically important
 Produce agar, a gelatin-like product made primarily from the algae
Gelidium and Gracilaria, used commercially and in the laboratory
 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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Figure 17.10A Chondrus crispus, a red alga
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Figure 17.10B
Rockweed, Fucus, a
brown alga
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17.11 Green algae are
ancestral to plants
 Green algae (Approximately 7,500 species)
 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
 Filaments - end-to-end chains of cells that form after cell
division in only one plane
 In some algae, the filaments are branched, and in others the
filaments are unbranched
 Asexual Reproduction
 Chlamydomonas produces 16 daughter cells still within the
parent cell
 Sexual reproduction
 Spirogyra undergoes conjugation, temporary union, during
which cells exchange genetic material
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Figure 17.11A Reproduction in Chlamydomonas, a motile green alga
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Figure 17.11B Cell anatomy and conjugation in Spirogyra, a
filamentous green alga
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Figure 17.11C Volvox, a colonial green alga
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Figure 17.11D Ulva, a multicellular alga
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Figure 17.11E Chara, a stonewort
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APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES
17.12 Life cycles among the
algae have many variations

Asexual Reproduction
 When environment is favorable to growth, asexual reproduction is a frequent
mode of reproduction among protists
 Offspring are identical to parent

Sexual Reproduction
 More likely to occur among protists when the environment is changing and is
unfavorable to growth
 May produce individuals more likely to survive extreme environments

Haploid life cycle
 The zygote divides by meiosis to form haploid spores that develop into haploid
individuals

Alternation of generations
 Diploid sporophyte produces haploid spores
 Spore develops into a haploid gametophyte that produces gametes
 Gametes fuse to form a diploid zygote that develops into sporophyte

Diploid life cycle
 Diploid individual produces haploid gametes by meiosis
 Gametes fuse to form a diploid zygote
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Figure 17.12A Haploid life cycle
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Figure 17.12B Alternation of generations
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Figure 17.12C Diploid life cycle
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Connecting the Concepts:
Chapter 17
 Protists we study today are not expected to include the
direct ancestors to fungi, plants, and animals
 They may be related to the other eukaryotic groups by way of
common ancestors that have not been discovered in the fossil
record
 May represent an adaptive radiation experienced by the first
eukaryotic cell
 Mutualism is a powerful force that shaped the eukaryotic
cell and also shapes all sorts of relationships in the living
world
 All possible forms of reproduction and nutrition are present
among the protists
 Each of the other eukaryotic groups specializes in a particular type
of reproduction and a particular method of acquiring needed
nutrients
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