Lecture 19: Slime molds

Plasmodial slime molds

Myxomycetes (Amoebozoa)

Myxomycetes

• ~500 described species
• phagotrophic nutrition
• four life-cycle stages:

four life-cycle stages of myxomycetes

1. three types of uninucleate single cells:
spore, myxamoeba, swarm cell
2. multinucleate somatic phase - plasmodium
--moves and exhibits a reversible cytoplasmic streaming
3. resistant stage consisting of a sclerotium
4. reproductive phase: stationary sporophores containing
walled spores

Spores

• haploid, resistant to unfavorable conditions
• germinate in rain water
• germination results in one or more myxamoebae or
swarm cells

Myxamoebae

are the basic unit of structure of slime
molds and are uninucleate, naked, haploid amoeba

Swarm cells

are unicellular motile cells of the
Myxomycetes that possess from 1 to 4 anteriorly
attached whiplash flagella

Swarm cells

• Mxamoebae may convert into swarm cells - reversible
• Absorb dissolved food material or ingest fungi &
bacteria
• Swarm cell can withdraw flagella and transform into a
myxamoeba

Formation of Zygotes and Plasmodia

• Swarm cells and myxamoeba can both function as
gametes - fuse to form a zygote
• Zygote is initially flagellated, swims, retracts flagella
and transforms into an amoeba-like cell
• Zygote grows; its nucleus undergoes successive
mitotic divisions without cytokinesis -- becomes
transformed into a multinucleate plasmodium

Plasmodium

• No cell wall
• Enveloped by a gelatinous slime sheath
• Inside the slime sheath is the plasma membrane
• Feed on bacteria, spores of fungi and plants, protozoa
and decaying organic matter

Sclerotium

a hardened structure composed of tissue
that resists unfavorable conditions.
• capable of long periods of dormancy

Sclerotia

• Under unfavorable environmental conditions a
plasmodium can become converted into a sclerotium
• When conditions are favorable, a plasmodium emerges

Cellular slime molds

Dictyostelia (Amoebozoa)

Dictyostelia

• ~350 spp. described
• Grow as unicellular amoebae and aggregates
• Myxamoebae are the basic unit of structure of slime
molds and are uninucleate, naked, haploid amoeba
• Fructifications are inconspicuous, delicate and
ephemeral
• Terrestrial – soil, leaf litter
• Nutrition: consume bacteria

Amoeba growth

• Typically uninucleate
• Unicellular (exc. aggregations)
• Lacking a cell wall
• Pseudopods
• Lacking a definite form
• mobile

pseudoplasmodium

is a structure formed by the
massing of separate myxamoebae.

Pseudoplasmodial growth

A pseudoplasmodium is a structure formed by the
massing of separate myxamoebae.
Separate myxamoebae do not fuse.

“Slimy” or aggregate parts of the life cycle

• Component amoebae do not fuse
• Retain their individuality while cooperating as members
of the pseudoplasmodium
• Acrasin (e.g. cyclic AMP): pheromone responsible for
aggregation of myxamoebae to form a mound stage

Acrasins

• Chemical messengers in slime
molds
• Edmund Spenser’s epic poem
“The Faerie Queen” 1590
• Acrasia seduced men against their
will and then transformed them
into beasts

“Slimy” or aggregate parts of the life cycle

• The mound stage of the pseudoplasmodium
transforms into the mobile slug, or grex stage
• 10 to 50,000 individual amoebae make up a single slug
• Grex results in the differentiation of myxamoebae –
stalk and spore mass
• Amoebae in the stalk will die and their genetic
information won’t be passed on

Farming

Bacterial prey dispersed with spores of Dictyostelium

Model Organism
Dictyostelium discoideum

A model for studying:
-cytokinesis
-signaling (e.g. cAMP)
-chemotaxis
-phagocytosis
-motility
-cell sorting
-cell differentiation