Protists

Chapter 29

Chapter 29

2

Eukaryotic Origins

• Eukaryotic cells differ from prokaryotes– Presence of a cytoskeleton– Compartmentalization (nucleus and organelles)

• Appearance of eukaryotes in microfossils occurred about 1.5 BYA

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50 µm© Andrew H. Knoll/Harvard University

• The nucleus and endoplasmic reticulum arose from infoldings of prokaryotic cell membrane

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DNAPlasma membrane

plasma membrane

Endoplasmic reticulum (ER) Nuclear

Nucleus

Plasma membrane

Eukaryotic cell

envelope

Infolding of the

of eukaryotic cellsProkaryotic ancestor

Prokaryotic cell

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• Many organelles evolved via endosymbiosis between an ancestral eukaryote and a bacterial cell– Mitochondria

• Aerobic bacteria engulfed by larger bacteria

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Chloroplast

Eukaryotic cell with chloroplastand mitochondrion

Endosymbiosis

Photosynthetic bacterium

Mitochondrion

Aerobicbacterium

Eukaryotic cell with mitochondrion

Endosymbiosis

Internal membrane system

eukaryotic cellAncestral

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• Chloroplasts– Larger bacteria

engulfed smaller photosynthetic bacteria

– Chloroplasts come from single line of cyanobacteria

– Hosts are not monophyletic

• Brown algae engulfed red algae that already had chloroplasts– Secondary

endosymbiosis

Organelle withfour membranes

Red algal nucleus lostBrown alga

Nucleus

Nucleus

SecondaryEndosymbiosis

Chloroplastwith twomembranes Eukaryotic cell

PrimaryEndosymbiosis

Eukaryotic cell

Nucleus

Cyanobacteria Red alga

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• Endosymbiosis supported by– DNA inside mitochondria and chloroplasts

• DNA similar to bacteria DNA in size and character

– Ribosomes inside mitochondria similar to bacterial ribosomes

– Chloroplasts and mitochondria replicate by binary fission – not mitosis

• Mitosis evolved in eukaryotes

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Defining Protists

• Most diverse of the four eukaryotic kingdoms

• United on the basis that they are not fungi, plants, or animals

• Vary considerably in every other aspect– Unicellular, colonial, and multicellular groups– Most are microscopic but some are huge– All symmetries– All types of nutrition

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8

Working model for protists classification

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Cil

iate

s

Ap

ico

mp

lex

ans

ArchaeaEubacteria Chromalveolata Rhizaria Archaeplastida Excavata Amoeboza Opisthokonta

Par

aba

sal

id

Din

ofl

ag

ella

tes

Alveolata Stramenopila Rhodophyta Chlorophytes Diplomonads Euglenozoa Choanoflagellates

An

ima

ls

Fu

ng

i

Am

oeb

ozo

a

La

nd

pla

nts

Ch

aro

ph

yte

s

Ce

rco

zoa

Fo

ram

inif

era

Ra

dio

lara

Bro

wn

alg

ae

Dia

tom

s

Oo

myc

ete

s

• Locomotion– Flagella

• One or more

– Cilia• Shorter and more numerous than flagella

– Pseudopodia (“false feet”)• Lobopods – large, blunt• Filopods – thin, branching• Axopods – thin, long

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• Asexual reproduction– Typical mode of reproduction– Some species have an unusual mitosis

• Mitosis – equal size daughter cells• Budding – one daughter cell smaller• Schizogony – cell division preceded by several nuclear

divisions; produces several individuals

• Sexual reproduction – May be obligate, or only under stress– Meiosis is a major eukaryote innovation– Union of haploid gametes which are produced by

meiosis– Advantage in allowing frequent genetic recombination

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• Multicellularity– From single cells to colonies to true

multicellularity– Arisen multiple times– Fosters specialization– Few innovations have had as great an

influence on the history of life

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• Euglena– Two anterior (and unequal) flagella

• Attached at reservoir

– Contractile vacuoles – collect excess water

– Stigma – movement towards light– Numerous small chloroplasts

• From ingestion of green algae

– Concept of a single Euglena genus is now being debated

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a. 6.5 µm© Andrew Syred/Photo Researchers, Inc.

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Second flagellumStigma

Contractile vacuole

Paramylon granule

Nucleus

ChloroplastFlagellum

Pellicle

Mitochondrion

Basal bodies

Reservoir

b.

Apicomplexans

• Spore-forming animal parasites• Apical complex is a unique arrangement of

organelles at one end of the cell– Enables the cell to invade its host

• Plasmodium causes malaria– Complex life cycle – sexual, asexual, different

hosts– Eradication focused on eliminating mosquito

vector, drug development, vaccines• DDT-resistant mosquitoes

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Inside Mosquito Inside Mammal

Sporozoite

Merozoite

Gametocyte

Fertilization

Host ’sliver cell

Host ’s redblood cell

1. While feeding, mosquito injects Plasmodium sporozoites into human.

2. Sporozoites enter the liver, reproduce asexually and release merozoites into the bloodstream.

6. The game-tocytesdevelop intogametes andreproducesexually,formingsporozoiteswithin themosquito.

5. Gametocytes are ingested by another, previously uninfected mosquito.

4. Certain merozoites develop into gametocytes.

3. MerozoitesmultiplyInside redblood cellsand arereleased.The cyclerepeats.

Oocyst

Sporozoite

Gametes

Brown algae

• Conspicuous seaweeds of northern regions

• Life cycle involves alternation of generations– Sporophyte – multicellular

and diploid– Gametophyte –

multicellular and haploid

• Not plants

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17

ME

IOS

ISn 2n

Sporophyte (2n)

Zygote (2n)Sperm

Egg

Zoospores (n)

Gametophytes (n)

Developingsporophyte

Germinatingzoospores

MIT

OS

IS

FE

RT

ILIZ

AT

ION

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Green Algae

• Land plants arose from an ancestral green alga only once during evolution

• Green alga consist of 2 monophyletic groups– Chlorophyta– Charophytes

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5 µm© Dr. Richard Kessel & Dr. Gene Shih/Visuals Unlimited

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Chlamydomonas life cycle.

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–

+

–

Gamete

Asexualreproduction

MITOSIS

MITOSISStrain

Strain FERTILIZATION

Zygospore(diploid)

+GametePairing of positiveand negativemating strains

n

2n

MEIO

SOS

• Cell specialization in colonial chlorophytes– Multicellularity arose many times in the

eukaryotes – Colonial chlorophytes are examples of cellular

specialization• Volvox - hollow sphere made up of a single layer of

500 to 60,000 individual cells each with 2 flagella

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Vegetative cells

Reproductive cells

© Wim van Egmond/Visuals Unlimited

20 nm

Haplodiplontic Life Cycle

• Multicellular diploid stage – sporophyte– Produces haploid spores by meiosis

– Diploid spore mother cells (sporocytes) undergo meiosis in sporangia

• Produce 4 haploid spores• First cells of gametophyte generation

• Multicellular haploid stage – gametophyte– Spores divide by mitosis

– Produces gametes by mitosis

– Gametes fuse to form diploid zygote• First cell of next sporophyte generation

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