2 mm
10 um
105 cells
Slime Molds:
The Social Amoebae
Dictyostelium
discoideum
Diploid amoebas
Fusion
Meiosis
Haploid amoebas
Dispersal
starvation
Life Cycle
The
Dictyostelium
Genome
• 6 chromosomes
• 33.6 Mb of DNA (77%[A+T])
• 12,500 ORFs
Movie
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Heterotrophic
Feeding by
Phagocytosis
Movie
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Actin Microfilament-Based Motility
Drives Cell Movements
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Actin::GFP
Myosin II::GFP
The Microtubule Cytoskeleton
Is Not Involved in Motility
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Tubulin::GFP
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Bacteria in phagosomes
Mitosis and
Cytokinesis
Tubulin and Actin
Actin and Myosin II
Myosin I and Myosin II
Chemotaxis to cAMP
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Chemotaxis
Triggers a
Signal
Transduction
Cascade
cAMP
cAR
+
PLC
High
Ca++
 GTP
Heterotrimeric G-protein
Ca++
Actin
Assembly
IP3
ER
Low
Ca++
Starving Cells
Coordinate with
cAMP
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Pacemaker
Centers Trigger
Spiral Waves of
Cellular
Movements
Pacemaker
center
Responding
to the Call . . .
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Phosphodiesteraseoutside
cAMPoutside
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cAR Receptor
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cAMPinside
Adenylate
Cyclase
Heterotrimeric
G-protein
Differentiation is also
Controlled by cAMP
Differential Gene Expression
Chlorinated
hexanone
Light +
Spore differentiation factor
DIF
Prestalk cells (cAMP lower)
Prespore cells (cAMP higher)
SDF-2
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time
Slug
Formation
Chlorinated hexanone
Prestalk cells
Prespore cells
(tagB::GFP)
(cotB::GFP)
leaders
followers
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Cells Assume
Different Fates
Depending on
Their Location in
The Slug
Fluorescent reporters
driven by differentiationspecific promoters
PrestalkA::GFP and
PrestalkO::RFP
(i.e. subpopulations
of stalk cells)
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Culminant
Structure
Morphogenesis!
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The Morphogenetic
Progression (24hr)
A Few Questions for Thought
• Describe, in your own words, the Dictyostelium life cycle.
• Review your notes from the first semester on
microfilament (actin)/myosin-based cell motilities and
integrate that with the Dictyostelium model.
• Review your notes from previous quarters on cell
signaling - the roles of cAMP, G-proteins,
phosphoinositides (IP3), Calcium ions and kinases in how
cells transduce, amplify and respond to changing
environmental conditions.
• Describe the cooperative nature of culminant formation,
considering how and why certain cells contribute to the
structure by forming a stalk or spores.
•Consider how “cheaters” emerge in this process and how
that might bring an immediate reward but ultimately prove
a poor strategy for life (a very Haverfordian lesson).