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
Chapter 28
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The Origins of
Eukaryotic
Diversity
General characteristics

Classification criteria
– eukaryotes
– not animal, plant or fungi
Protist Diversity
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The full spectrum of modes of life
–
–
–
–
–
from unicellular to multicellular
autotrophic to heterotrophic
asexual to sexual reproduction
pathogenic to beneficial
sessile to mobile
Mobility
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How Protists move
– flagellum
– cilia
– pseudopod
Protists
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Ingestive
(animal-like); ameoba,
paramecium, stentor

Absorptive
(fungus-like)
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Photosynthetic
(plant-like);
alga, kelp
The Endosymbionic Theory
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Mitochondria and chloroplasts were formerly
from small prokaryotes living within larger
cells (Margulis)
Endosymbiosis

Evolution of eukaryotes
– origin of mitochondria
– engulfed aerobic bacteria, but
did not digest them
– mutually beneficial relationship
• natural selection!
internal membrane
system
aerobic bacterium
mitochondrion
Endosymbiosis
Ancestral
eukaryotic cell
Eukaryotic cell
with mitochondrion
Endosymbiosis

Eukaryotic
cell with
mitochondrion
Evolution of eukaryotes
– origin of chloroplasts
– engulfed photosynthetic bacteria,
but did not digest them
– mutually beneficial relationship
• natural selection!
photosynthetic
bacterium
chloroplast
Endosymbiosis
Eukaryotic cell with
chloroplast & mitochondrion
mitochondrion
Theory of
Endosymbiosis

Evidence
– structural
• mitochondria & chloroplasts
resemble bacterial structure
– genetic
Lynn Margulis
• mitochondria & chloroplasts
have their own circular DNA, like bacteria
– functional
• mitochondria & chloroplasts
move freely within the cell
• mitochondria & chloroplasts
reproduce independently
from the cell
Protist Systematics & Phylogeny, I

1- Groups lacking mitochondria;
early eukaryotic link; Giardia
(human intestinal parasite; severe
diarrhea); Trichomonas (human
vaginal infection)

2- Euglenoids; autotrophic &
heterotrophic flagellates;
Trypanosoma (African sleeping
sickness; tsetse fly)
Protist Systematics & Phylogeny, II

Alveolata: membrane-bound
cavities (alveoli) under cell
surfaces; dinoflagellates
(phytoplankton);
Plasmodium (malaria);
ciliates (Paramecium)
Protist Systematics & Phylogeny, III

Stamenophila: water molds/mildews
and heterokont (2 types of flagella)
algae; numerous hair-like projections on
the flagella; most molds are
decomposers and mildews are parasites;
algae include diatoms, golden, and
brown forms
Protist Systematics & Phylogeny, IV

Rhodophyta: red
algae; no flagellated
stages; phycobilin
(red) pigment

Chlorophyta: green
algae; chloroplasts;
gave rise to land
plants; volvox, ulva
Protist Systematics & Phylogeny, V



Affinity uncertain:
Rhizopods: unicellular with
pseudopodia; amoebas
Actinopods: ‘ray foot’ (slender
pseudopodia; heliozoans,
radiolarians
Protist Systematics & Phylogeny, VI

Mycetozoa: slime
molds (not true fungi);
use pseudopodia for
locomotion and
feeding; plasmodial
and cellular slime
molds
Protist Diversity

Beneficial & necessary Protists
– phytoplankton
• small algae + diatoms
• much of the world’s photosynthesis
• produces ~90% of atmospheric oxygen
– zooplankton
• heterotrophic protists + animals
• key ecological role at base of
marine food web