CHAPTER 17 PROTISTS: THE RISE OF THE EUKARYOTES. ORIGIN OF EUKARYOTIC CELLS Eukaryotic cells appeared first around 1.7 billion years ago. Eukaryotic cells

Chapter 17

Protists: the rise of the eukaryotes

Origin of Eukaryotic Cells

• Eukaryotic cells appeared first around 1.7 billion years ago.• Eukaryotic cells possess a nucleus.

• Some bacterial cells have infoldings of their outer membranes extending into the interior.• The endoplasmic reticulum (ER) and nuclear

envelope of eukaryotes is thought to have evolved from this.

Origin of the nucleus and endoplasmic reticulum

Infolding of theplasma membrane

Plasma membrane Endoplasmicreticulum (ER)

Nuclearenvelope

Eukaryotic cell

Plasma membrane

Nucleus

Prokaryotic ancestorof eukaryotic cellsProkaryotic cell

Cell wall

DNA

Origin of Eukaryotic Cells

• The endosymbiotic theory is a widely accepted explanation for the origin of mitochondria and chloroplasts in eukaryotes from bacteria.• Present-day mitochondria and chloroplasts still

contain their own DNA.• This DNA is remarkably similar in size and

character to the DNA of bacteria.

The theory of endosymbiosis

Internalmembranesystem

Aerobicbacterium

Endosymbiosis

Mitochondrion

Eukaryotic cellwith mitochondrion

Eukaryotic cell with chloroplast and mitochondrion

Chloroplast

Ancestral eukaryotic cell

Endosymbiosis

Photosyntheticbacterium

The Evolution of Sex

• A profound difference between prokaryotes and eukaryotes is that eukaryotes have the capacity for sexual reproduction.• Sexual reproduction involves two parents

contributing gametes to form the offspring.• Gametes are usually formed by meiosis and are

haploid.• The resulting offspring are diploid.

Asexual reproduction

• But sexual reproduction is not the only way that eukaryotes can reproduce.• Many eukaryotes reproduce by asexual

reproduction, which is reproduction without forming gametes.

• The offspring of asexual reproduction are genetically identical to the parents.

• Many eukaryotes reproduce mainly by asexual reproduction, switching to sexual reproduction only during environmental stress.

Reproduction among paramecia

Paramecium cells

(a)

Nuclei

(b)

Parameciumcells


• A different asexual strategy in eukaryotes is parthenogenesis, the development of an adult from an unfertilized egg.

• Many plants and marine fishes undergo a form of sexual reproduction that does not involve partners called self-fertilization and involves one individual providing both male and female gametes.


• The advantages of sexual reproduction are not immediately obvious.• The segregation of chromosomes that occurs in

meiosis tends to disrupt advantageous combinations of genes.

• Why should progeny not maintain a parent’s already successful gene combinations?


• Sexual reproduction may have evolved in protists because the diploid cell stage allows for chromosomal repair.• Double-strand breaks in the DNA are induced

by desiccation.• The environmental stress of drying out triggers

the diploid stage.• Perhaps chromosome-pairing in the early

stages of meiosis evolved originally as a mechanism for repairing double-strand damage.• The undamaged version could be used as a

template to guide fixing of the damaged one.


• Sexual reproduction is one of the most important evolutionary innovations of eukaryotes.• It provides a means of shuffling genes,

creating genetic diversity.• Genetic diversity is the raw material of

evolution.• The greater the genetic diversity, the

more rapid the evolutionary pace.


• The sexual life cycle involves the production of haploid gametes by meiosis, followed by the union of two gametes in sexual reproduction.

• Eukaryotes are characterized by three major types of sexual life cycles.• Zygotic meiosis• Gametic meiosis• Sporic meiosis


• In zygotic meiosis, the zygote formed by the fusion of gametes is the only diploid cell.• This type of sexual life cycle is common in

many algae.• The zygote is the only cell that undergoes

meiosis.• Haploid cells occupy the major portion of the

life cycle.

Three types of eukaryotic life cycles: Zygotic meiosis

n

Syngamy

n

+

–

+

+

–

–

(a) Zygoticmeiosis

DiploidHaploidKey:

Haploidcells

Meiosis

Gametes

Haploidindividuals

Zygote

2n


• In gametic meiosis, the gametes are the only haploid cells.• This sexual life cycle is common to most

animals.• Meiosis produces the gametes.• The diploid cells occupy the major portion of the

life cycle.

Three types of eukaryotic life cycles: Gametic meiosis

2n–

+

+––

+

MeiosisGametes

(b) Gameticmeiosis

2n

Reproductivecell

2n

Diploidindividual

SyngamyGametes

Zygote

DiploidHaploidKey:


• In sporic meiosis, the spore-forming cells undergo meiosis.• This life cycle is common to plants.• In plants, there is a regular alternation of

generations between a haploid phase and a diploid phase.

• The diploid phase produces spores that give rise to the haploid phase.

• The haploid phase produces gametes that fuse to give rise to the diploid phase.

Three types of eukaryotic life cycles: Sporic meiosis

n

Sporophyte (diploid)

n

2n

++––

–

+

(c) Sporicmeiosis

MeiosisSpores

SyngamyGametes

Gametophytes(haploid)

Spore-formingcell

2n

2n Zygote

DiploidHaploidKey:

General Biology of Protists, the Most Ancient Eukaryotes

• Protists are eukaryotes united on the basis of a single negative characteristic.• They are not fungi,

plants, or animals.• In all other respects,

they are highly variable with no uniting features.

Main body ofVorticella cell

Cilia

Substrate towhich thisVorticella isattached

Contractilestalk


• Cell surface varies among protists.• All protists have plasma membranes.• Some, like algae and molds, have cell walls.• Others, like diatoms and radiolarians,

secrete glassy shells of silica.


• Locomotor organelles also vary among protists.• Protists move by means of cilia, flagella,

pseudopods, or gliding mechanisms.

http://youtu.be/9duvzqvVflwhttp://youtu.be/4QtGdqy-CLA http://youtu.be/7pR7TNzJ_pA


• Cyst formation allows protists who have delicate surfaces to survive in harsh habitats.• Cysts are dormant forms of a cell with a

resistant outer covering.• In dormancy, the cell metabolism is more or

less completely shut down.


• Protists employ a variety of forms of nutritional acquisition.• Some protists are photosynthetic autotrophs,

called phototrophs.• Among the heterotrophic protists:

• Phagotrophs ingest visible particles of food.• Osmotrophs ingest food in soluble form.


• Protists typically reproduce asexually.• Sexual reproduction occurs usually only in

times of stress.

• Asexual reproduction involves an unusual form of mitosis.• The nuclear membrane usually persists

throughout mitosis and the spindle apparatus forms within the nucleus.


• Asexual reproduction in protists may involve spore formation or fission.• The most common type of fission is binary

fission, in which a cell simply splits into two nearly equal halves.

• Another type of fission is called budding.• In multiple fission, called schizogony, several

individuals are produced almost simultaneously.


• Being a single-celled organism presents certain problems.• Size is limited due to surface-to-volume ratio

problems.


• The evolution of multicellularity alleviates the size constraints.• A multicellular organism is composed of many

cells.• Having multiple cells allows for specialization.

• Distinct cell types can have different functions.

• This is a “division of labor”.


• Many protists form colonial assemblies consisting of many cells with little differentiation or integration.• A colonial organism is a

collection of cells that are permanently associated but in which little or no integration of cell activities occurs.


• An aggregation is a more transient collection of cells that come together for a period of time and then separate.• For example, individual amoeboid cells of

cellular slime molds come together to form an aggregate called a slug.• The slug allows the aggregate of slime mold

cells to move to a new feeding location as a unit.


• True multicellularity occurs only in eukaryotes.• It requires that the activities of individual cells

be coordinated and that the cells be in contact.


• Three groups of protists have independently evolved multicellularity.• Brown algae (Phylum Phaeophyta)• Green algae (Phylum Chlorophya)• Red algae (Phylum Rhodophyta)

• But not all types of algae are multicellular; there are also unicellular varieties.

Classifying the Protists

• Protists are the most diverse of the four kingdoms in the domain Eukarya.• As a matter of convenience, protists are

grouped into one kingdom.• Molecular data indicates that 12 of the 17 major

protist phyla can be assigned to 7 clades.• Five phyla cannot yet be placed on the protist

phylogenetic tree.

• The relationships among the protists will likely be revised as more information becomes available.

The major protist clades

?

Bacteria ArchaeaDiplomonads,Parabasalids

Stramenopila Alveolata Rhodophyta Chlorophyta PlantsUnknown

protistFungi Choanoflagellida AnimalsEuglenozoa

The Base of the Protist Tree

• Two groups of protists resemble early eukaryotes.• Both groups have flagella but lack mitochondria.


• Diplomonads are single-celled with two nuclei.• Include Giardia

intestinalis, which be transmitted in contaminated water.


• Parabasalids have undulating membranes.• Include Trichomonas

vaginalis, which causes a sexually-transmitted disease in humans.

• Other species play key roles in forest ecosystems.

A Diverse Kingdom

• The largest branch of the protist phylogenetic tree contains 7 phyla clustered into 3 clades:• Euglenozoa

• 1 phylum• Stramenopila

• 3 phyla• Alveolata

• 3 phyla

A Diverse Kingdom

• Euglenozoa are freshwater protists with the majority having two flagella.

• Include two groups in the phylum Euglenozoa:• Euglenoids • trypanosomes

Euglenozoa: Euglenoids

• Euglena is a representative euglenoid.• Two flagella.• Contractile vacuole

to help regulate the osmotic pressure within the organism.

• A light-sensitive stigma which helps this photosynthetic form find light.

• Asexual reproduction.

Flagellum

Secondflagellum

Reservoir

Stigma Basal body Nucleus Pyrenoid Chloroplast

Paramylongranule

PellicleContractilevacuole

(top): © Michael Abbey/Visuals Unlimited

Euglenozoa: Euglenoids

• About one-third of euglenoid species are photosynthetic and have chloroplasts.

• The remaining types lack chloroplasts and are heterotrophic.

• The photosynthetic forms can become heterotrophic when light levels are low.

Euglenozoa: Trypanosomes

• Have a unique single mitochondrion that contains two circles of DNA.

• Are important human pathogens, including those that cause sleeping sickness, and leishmaniasis.

Stramenopila

• Stramenopila are protists that possess fine hairs on their flagella.• These flagellated cells may appear only at

certain times in the life cycle, or they may have been lost completely (as in diatoms).

• Phylum Phaeophyta (brown algae)• Phylum Chrysophyta (diatoms)• Phylum Oomycota (water molds)

Stramenopila: Brown Algae

• Phylum Phaeophyta• Are the longest, fastest-growing, and

most photosynthetically productive living things.

• All are multicellular and most are marine.

• Their life cycle has alternating generations.

Stramenopila: Diatoms

• Phylum Chrysophyta• Photosynthetic• Encased by unique

double wall of silica.• Abundant in both

oceans and freshwater habitats.

• Fossilized deposits of diatom shells are mined as “diatomaceous earth”.

Stramenopila: Water Molds

• Phylum Oomycota• The downy mildews that are often seen in moist

environments.• All members of the group either parasitize living

organisms or feed on dead organic matter.• Many water molds are important plant

pathogens.• Include Phytophthora infestans, the cause of

the potato blight disease.

Alveolata

• Alveolata includes three phyla that have a layer of flattened vesicles (alveoli) beneath their plasma membrane.• phylum Ciliophora

(ciliates)• phylum Pyrrhophyta

(dinoflagellates)• phylum Apicomplexa

(sporozoans)

Alveolata: Ciliates (phylum Ciliophora)

• Complex and unicellular.• Possess large numbers of cilia.• Have a defined cell shape and two nuclei

per cell.• The pellicle is a proteinaceous scaffold,

found inside the plasma membrane, that confers flexible support.

• Asexual reproduction is by fission while sexual reproduction is by conjugation.

A ciliate

Anterior contractile vacuole

Micronucleus

Macronucleus

Pellicle

Posteriorcontractilevacuole

Cytoproct

Cilia

Gullet

Food vacuole

Alveolata: Dinoflagellates (phylum Pyrrhophyta)

• Photosynthetic unicellular protists.• Usually bear two flagella of unequal length.• About 1,000 species.

• Some occur in freshwater, but most are marine.• A stiff outer coating of cellulose and silica gives

them unique shapes.

Noctiluca Ptychodiscus Ceratium Gonyaulax

Alveolata: Dinoflagellates (phylum Pyrrhophyta)

• Some dinoflagellates produce bioluminescence or powerful toxins that cause “red tides”.

Alveolata: Sporozoans (phylum Apicomplexa)

• Are unicellular parasites.• Are spore-forming and nonmotile.• They cause many diseases in humans and

domestic animals.• They have complex life cycles that involve

both asexual and sexual phases, often involving an alternation between different hosts.

A sporozoan life cycle

1

54

3

2 Stagesin liver

Merozoites

Certainmerozoitesdevelop intogametocytes

Gametocytesingested bymosquito

Oocysts

Zygote

Mosquito injectssporozoites

Sporozoites

Stages inred bloodcellsGametocytes

6 Sporozoitesform withinmosquito

The Road to Plants

• The red and green algae are included on a lineage that led to the evolution of plants.

• Red and green algal chloroplasts are similar, indicating a common ancestry.

Eu

gle

no

zoa

Str

amen

op

ila

Alv

eola

ta

Rh

od

op

hyt

a

Ch

loro

ph

yta

Ch

oan

ofl

agel

lida

Dip

lom

on

ads,

Par

abas

alid

s

The Road to Plants

• Red algae comprise the phylum Rhodophyta and have red pigments.• Most are multicellular and marine.• They grow more deeply than other

photosynthetic organisms.

• The lab medium agar is made from red algae.

The Road to Plants

• Green algae are members of the phylum Chlorophyta.• The ancestor of plants belonged to this group.• Mostly mobile and aquatic but a few species occur

in moist soil or on tree trunks.• Most are microscopic and unicellular, but some are

intermediate or colonial, while others are large and multicellular.

• Some, like Ulva, exhibit an alternation of generations between a multicellular haploid gametophyte and a multicellular diploid saprophyte.

Green algae: (a) Chlamydomonas, a unicellular green algae; (b) Volvox, which forms colonies

Flagellum

20.42 μm

Chlamydomonascells

Volvoxcells

Volvoxcolony

New Volvox colonies forming

(a)

(b)

A green algae life cycle: Ulva

(left): © Bob Barnes/Alamy

nSporangia

2n

MEIOSIS

+Gametophyte(n)

Spores

+ –Gametes

– Gametangia

+ Gametangia

Germinatingzygote

Sporophyte(2n)

Zygote

SYNGAMY

Gametes fuse

– Gametophyte (n)

The Road to Animals

• Protists called choanoflagellates were likely the ancestors of animals.

Dip

lom

on

ads,

Par

abas

alid

s

Eu

gle

no

zoa

Str

amen

op

ila

Alv

eola

ta

Rh

od

op

hyt

a

Ch

loro

ph

yta

Ch

oan

ofl

agel

lid

a

Choanoflagellates (phylum Choanoflagellida)

• Unicellular, heterotrophic, with a long flagellum.• The whipping motion of the flagellum allows

choanoflagellates to draw water in and filter-feed on bacteria; this same type of filtering is found in sponges, primitive animals.

• Colonies can form that look very much like freshwater sponges.

• Molecular similarities with sponges.

“NOT YET LOCATED ON THE PROTIST PHYLOGENETIC TREE”

• Five phyla cannot yet be located on the protist phylogenetic tree.

• These five groups use their cytoplasm to aid movement.• Phylum Rhizopoda (amoebas)• Phylum Foraminifera (forams)• Phylum Actinopoda (radiolarians)• Phylum Acrasiomycota (cellular slime molds)• Phylum Myxomycota (plasmodial slime molds)

Amoebas (phylum Rhizopoda)

• Lack cell walls and flagella.• Move using

pseudopodia, flowing projections of cytoplasm.

• Amoebas are abundant in soil and many are parasitic in animals.

• Reproduction is entirely asexual.

(a)

(b)

Pseudopodia

Nucleus

Amoebacells

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Forams (phylum Foraminifera)

• Possess rigid cells and move by cytoplasmic streaming.

• Marine protists with pore-studded shells called tests.

• Long, thin, cytoplasmic projections called podia radiate through the test pores and are used for swimming and capturing prey.


Foram body

Podia

© Manfred Kage/Peter Arnold

Radiolarians (phylum Actinopoda)

• Are amoeboid cells with a glassy skeleton made of silica.

• Have shells that are radially or bilaterally symmetrical.

• Have needlelike pseudopods that radiate out from the body.

Shell ofradiolarian body Pseudopods

Cellular slime molds (phylum Acrasiomycota)

• Are closely related to amoebas and form complex associations called slugs when under stress.• Once in a new

habitat, the slug can differentiate to become a spore-forming structure. Life cycle of the cellular slime mold

Dictyostelium discoideum


2

3

5

46

1

Movingamoebamass iscalled aslug.

Slug beginsto right itself.

Spores

Free-livingamoeba isreleased.

Amoebas beginto congregate.

Amoeba massforms.

Slugis transformedinto spore-formingbody, the sorocarp.

Plasmodial slime molds (phylum Myxomycota)

• Stream along as a plasmodium, a nonwalled multinucleate mass of cytoplasm.• Spore-producing

structures are formed under stressful conditions.

• The spores germinate when favorable conditions return.

Documents

CHAPTER 17 PROTISTS: THE RISE OF THE EUKARYOTES. ORIGIN OF EUKARYOTIC CELLS Eukaryotic cells appeared first around 1.7 billion years ago. Eukaryotic cells