Aggregating Dictyostelium use chemotaxis to find each other.
Each amoeba begins to secrete pulses of the chemical cyclic AMP
(that human cells use as a second-messenger inside-the-cell hormone)
And each amoeba re-orients its locomotion toward whatever direction the concentration of cyclic AMP is highest.
So they attract each other.
The first evidence for this chemotaxis was in the 1940s;
Dictyostelium were cultured on sheets of cellophane, through which only small molecules can diffuse
Scientists noticed that when amoebae aggregated on one side of the cellophane, other amoebae aggregated exactly on the opposite side!
(because the "attractant" molecule was diffusing through)
This experiment is easy to duplicate in the lab.
Another scientist tried making water flow slowly sideways across glass surfaces past aggregating Dictyostelium,
and found they could only detect aggregation centers from the downstream side.
This and related observations proved beyond doubt that chemotaxis was being used,
but another twenty years of research were needed to find out what chemical they use as the attractant. (1947 to 1967).
(Don't think you have to know what chemical is used to prove
HOW it is used.
Discoveries usually go in the opposite direction.
There are many other species of slime mold amoebae, that use different chemicals to attract each other, & nobody knows what.
Slugs" crawl toward light.
Eventually the amoebae at the front differentiate into stalk cells
The rest of the amoebae differentiate into spores with hard shells.
As this happens, the cells rearrange, like an inside-out fountain.
The stalk cells form a tall stiff rod,
Spore cells crawl up the outside of this rod, and form a mass
Functionally, this has the same purpose as a mushroom, holding the spores up in the air, so they are more likely to blow away.
Spores later hatch out,
if they are lucky somewhere there are bacteria to eat.
The stalk structure is very similar to a notochord:
swollen vacuolated cells, wrapped tightly in fibers
(cellulose instead of collagen, however)
Mutant amoebae without myosin can still crawl, but can't cleave in the normal way.
Much research has been done on this.
Of the many kinds of amoebae, their locomotion is the closest to the mechanism of locomotion used by animal body cells.
An interesting feature of differentiation of stalks and spores is that the proportions
(length to width ratio, and also % of cells that become stalk and % that become spores) "regulates"
A slug with 5,000 cells forms a tiny miniature stalk, etc.
an exact scale model of what is formed by a slug with 500,000 cells.
Similarly, if you separate the first 4 cells of a sea urchin embryo,
each one will form a one-fourth sized "scale model" pluteus.
Two one-cell stages pushed together form a double-size pluteus
Embryologists call such phenomena "regulation".
Urchins can regulate over an 8-fold size range,
But Dictyostelium regulates over 100-fold or more.
Later parts of this course will cover different theories of how this works.
The mechanism might be different or the same in animals.
Mutations in Dictyostelium can change the proportions of spores versus stalk.
Nobody has yet found the mechanisms.
Another good question is: do the slugs use the same mechanism of locomotion as the individual amoebae?
And when the future spores crawl up the outside of the stalk:
is that also the same propulsion mechanism?
And what force wraps the cellulose fibers around the stalk cells?
Someone will discover these things. Who will do it? How?
1) the CD that came with your textbook has some good video clips
2) the web site for the Dictyostelium genome project is at
and includes some good introductory material
3) The following are some books, in order of publication date.
Bonner, John Tyler. The Cellular Slime Molds. Princeton University Press, 1959 (also a second edition in 1967)
This is the classic descriptive book about Dictyostelium and related organisms. It's still worth reading.
Raper, Kenneth B. The Dictyostelids. Princeton University Press, 1984
Raper was a UNC graduate (class of 1929 undergraduate, and he later got an honorary degree as well).
In 1933 he collected a sample of Dictyostelium from some decaying leaves in a forest near Asheville NC.
This isolate was named Dictyostelium discoideum. Raper worked out conditions for maintaining in laboratory culture,
and it became the strain that has been most thoroughly studied (and is the one whose DNA has been sequenced.)
Maeda, Y., K. Inouye, and I. Takeuchi, editors. Dictyostelium. A Model System for Cell and Developmental Biology. Universal Academy Press, Tokyo, 1996.
This is a "symposium volume" containing chapters by many different authors on various topics in Dictyostelium research.
Kessin, Richard H. Dictyostelium. Evolution, Cell Biology and the Development of Multicellularity. Cambridge University Press, 2000.
This is a recent book by a single author that covers the topics currently being studied in Dictyostelium.
Bonner, John Tyler. Lives of a Biologist. Adventures in a Century of Extraordinary Science. Harvard University Press, 2002.
This is Bonner's autobiography, and is very interesting, enjoyable reading.