Download Origin of Eukaryotes

Origin of Eukaryotes
Prokaryotes
Eukaryotes
• No true nucleus
• No plastids
• True nucleus
• Specialized plastids
• Internal membrane
systems are folds of
plasma membrane
• Internal membrane
systems independent
of plasma membrane
Trends in Increased Prokaryote
Complexity
• Multicellular
prokaryotes with
specialized cells
• Complex bacterial
communities
• Compartmentalization
of different functions
within single cells
Heterocyst of Anabaena
Trends in Increased Prokaryote Complexity
• Multicellular prokaryotes with
specialized cells
• Complex bacterial communities
• Compartmentalization of different
functions within single cells
• These trends are important because
eukaryotes had to evolve from
prokaryotes
Origins of Eukaryotes
• Earliest evidence 1.5 billion years
• Acritarchs resemble
cysts produced by
living autotrophic
protists
• Development of
oxygen atmosphere
Electron micrograph of an
acritarch.
Models Proposed for the Evolution of
Eukaryotes
Autogenous Model - eukaryotic cells evolved
from specialization of internal membranes
derived from plasma membrane of
prokaryotes
Autogenous Model
• Single-membranes
organelles formed by
folding of inner
membrane only
• Double-walled
organelles by
complete
invagination
Autogenous Model
Endosymbiotic Model
Predecessors of eukaryotes where symbionts,
with small specialized species (endosymbionts)
living within larger prokaryotes
Fig. 22.12b
Endosymbiotic Model
Model uses chloroplasts and mitochondria
as examples
• Chloroplasts were
photosynthesizing
prokaryotes
• Mitochondria
evolved from aerobic
heterotrophs
(emphasis on role of
Krebs cycle)
Chloroplast
Mitochondrion
The Model was Controversial
The endosymbiotic model differs from
evolution as we discussed earlier
It is a merger of evolutionary lineages
giving rise to a new form of life
But,
Supporting evidence has strengthened
validity
e.g., mitochondrial and chloroplast DNA
Also, symbiosis is a common phenomenon
in nature
Kingdom Protista
• First eukaryotic organisms
• Typically thought of as the unicellular
eukaryotes
• Some colonial and multicellular species
• Addition of multicellular forms justified
by similarities in cell structure and life
cycles
Unicellular, but Complex
• Genesis of protists
reveals rise of
–
–
–
–
–
true nucleus
specialized organelles
9 + 2 flagella and cilia
mitosis
meiosis
• Share common
ancestry with
multicellular
eukaryotes
Protistan Systematics
• As would be expected, it is difficult to
develop phylogenetic relationships among
the protistans
– Poor fossils (except those with external
covering
– Some features (e.g., flagella and autotrophy)
arose and have been lost more than once
over their evolution
• Phyla are placed into supergroups (Table
28.1)
Protistan Systematics
• Evolutionary Relationships
– Cellular structure
– Gene sequences
• Evolutionary relationships constantly
changing
• Systematics
• Relationships into supergroups
Informal Classification – Ecological Roles
Protozoa – animal-like heterotrophic Protista
Ciliate consuming diatoms
Algae – autotrophic protists
Informal Classification – Ecological Roles
Fungus-like protists
Informal Classification – Motility
Ciliates
Flagellates
Informal Classification – Motility
Amoeboid
Life Processes??
Although unicellular, protistans can
carry out all life processes
Osmoregulation – Water Balance
Vacuoles increase effective surface area
in large cells.
Contractile vacuoles in freshwater
microbial eukaryotes such as
Paramecium are used to excrete excess
water.
Figure 27.10 Contractile Vacuoles Bail Out Excess Water
Nutrition
•
•
•
•
Phagotrophy
Osmotrophy
Autotrophy
Mixotrophy
Defense
•
•
•
•
Mucilage
Trichocysts
Bioluminescence
Toxins
27.3 How Did the Microbial Eukaryotes Diversify?
Food vacuoles are formed by protists
when solid food particles are ingested
by endocytosis.
The food is digested in the vacuole.
Smaller vesicles pinch off—increasing
surface area for products of digestion to
be absorbed by the rest of the cell.
Cell surfaces
Many microbial
eukaryotes have
diverse means of
strengthening their
surfaces.
Cytoskeleton
Internal structures that
provide support and
rigidity
27.3 How Did the Microbial Eukaryotes Diversify?
Some amoebas make a “shell” or test
from bits of sand beneath the plasma
membrane.
Diatoms form glassy cell walls of silica.
These walls are exceptionally strong,
and perhaps enhanced defense against
predators. Frustule
Figure 27.12 Cell Surfaces in the Microbial Eukaryotes
Asexual Reproduction
• All protists can
reproduce asexually
• Many produce cysts
with thick, protective
walls that remain
dormant in bad
conditions
• Many protozoan
pathogens spread
from one host to
another via cysts
34
Sexual Reproduction
• Eukaryotic sexual reproduction with
gametes and zygotes arose among
the protists
• Generally adaptive because it
produces diverse genotypes
• Zygotic and sporic life cycles
35
• Zygotic life cycles
Most unicellular sexually
reproducing protists
 Haploid cells transform into
gametes
 + and – mating strains
 Thick-walled diploid zygotes
 Survive like cysts
36
37
• Sporic life cycle
 Many multicellular green and brown
seaweeds
 Also known as alternation of
generations
 2 types of multicellular organisms
 Haploid gametophyte produces
gametes
 Diploid sporophyte produces spores
by meiosis
 Red seaweed variation involves 3
distinct multicellular generations
38
39
40
• Gametic life cycle
All cells except the gametes are
diploid
 Gametes produced by meiosis
Diatoms
 Asexual reproduction reduces
the size of the daughter cells
 Sexual reproduction restores
maximal size
41
42
• Ciliate sexual reproduction – Conjugation
 Most complex sexual process in protists
 Have 2 types of nuclei (single
macronucleus and one or more
micronuclei)
 Macronuclei are the source of the
information for cell function
 2 cells pair and fuse – conjugation
 Micronuclei undergo meiosis,
exchange, fusion and mitosis
43
44
Phylum Chlorophyta
• Green algae, diverse group
• unicellular, aggregates, colonial,
multicellular
• Believed to be the group that gave rise to
plants
– multicellular and colonial forms
– alternation of generation
Origins of Multicellularity
• Probably arose from colonial protistan
– something resembling Volvox
– (Volvox is only an example!!!!!!)
• Coordination and cooperation between
cells
• Specialized reproductive cells
– Volvox - locomotion and reproduction
• Ancestor probably flagellated
Alternation of Generation
• Life cycles that show an alternation
between a multicellular haploid form and
a multicellular diploid form
• Sporophyte - Diploid; produces
reproductive cells (haploid) called spores
• Gametophyte - Haploid; produces
haploid gametes. Fusion of gametes
produces diploid form
Fig. 28.20
Thus, the presence of alternation of
generation and other similarities suggests a
linkage between the Chlorophyta and the
Plant Kingdom