Dictyostelium discoideum

Slime Molds:The Social Amoebae

10 um2 mm

105 cells

Life Cycle

Diploid amoebas

Haploid amoebas

Fusion Meiosis

starvation

Dispersal

The Dictyostelium

Genome

• 6 chromosomes• 33.6 Mb of DNA (77%[A+T])• 12,500 ORFs

Heterotrophic Feeding by

Phagocytosis

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Actin Microfilament-Based MotilityDrives Cell Movements

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Myosin II::GFP

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Actin::GFP

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The Microtubule Cytoskeleton Is Not Involved in Motility

Tubulin::GFP Bacteria in phagosomes

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Mitosis and Cytokinesis

Tubulin and Actin

Myosin I and Myosin II

Actin and Myosin II

Chemotaxis to cAMP

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Chemotaxis Triggers a

Signal Transduction

Cascade

+

cAMP

cAR

Heterotrimeric G-protein

PLC

GTP

IP3

Actin Assembly

Ca++

ER

HighCa++

LowCa++

Starving Cells Coordinate with

cAMP

Pacemaker Centers Trigger Spiral Waves of Cellular Movements

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Responding to the Call . . .

Phosphodiesteraseoutside

cAMPoutside cAR Receptor

cAMPinside

Adenylate HeterotrimericCyclase G-protein

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Pacemaker center

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Differentiation is also Controlled by cAMP

Differential Gene Expression

Prestalk cells (cAMP lower)Prespore cells (cAMP higher)

Spore differentiation factor

Light +Chlorinated hexanone

DIFSDF-2

Slug Formation

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Prestalk cells Prespore cells

(tagB::GFP) (cotB::GFP) leaders followers

Video: 1.25 hr total time

Chlorinated hexanone

Cells Assume Different Fates Depending on

Their Location in The Slug

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Fluorescent reporters driven by differentiation-

specific promoters

PrestalkA::GFP and PrestalkO::RFP

(i.e. subpopulations of stalk cells)

CulminantStructure

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).