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Chapter 17Evolution of

Protists

Protists May Represent theOldest Eukaryotic Cells

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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 17-5

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

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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 conjugation17-16

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

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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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Algae Are Photosynthetic Protistsof 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 valve17-29

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 17-36

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 17-42

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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