March 19, 2007

Embryology Biology 441 Spring 2007 Albert Harris



The "Cellular Slime Mold" Dictyostelium discoideum is a very useful "model organism" for research on

    a) Cell differentiation (only 3 or 4 cell types)

    b) "Amoeboid cell locomotion"

    c) pattern formation, specifically regulative adjustments of sizes and shapes of parts in proportion to the whole.

    d) Chemotaxis (guidance of cell locomotion by chemical gradients)

Hundreds of laboratories around the world concentrate their research on this one species, which was discovered by a UNC alumnus named Kenneth Raper.

The life cycle:

i) Live as small, single-celled amoebae, which eat bacteria.

(These are much smaller than the amoebae you have seen in teaching laboratories, and their movement is more like human white blood cells)

ii) If and when they run out of food, all these amoeboid cells become sensitive to chemotactic attraction by a certain chemical which they also secrete. (cyclic AMP) The amoeboid cells also become sticky to each other.

They aggregate as pulsing streams of cells, but do NOT fuse.

iii) Hundreds to 100,000s of amoeboid cells form a "slug"

iv) After crawling for a few cm., each slug "fruits" =rearranges, with anterior cells differentiating into stalk cells and the great majority of cells differentiating into spore cells, located at the top of a stalk (somewhat like a mushroom) where stalk cells can blow away in the wind)


Chemotaxis was proven to occur long before anyone knew what chemical is the attractant.


A) When Dictyostelium amoebae were simultaneously cultured on both sides of cellophane 'dialysis tubing', then aggregation occurred toward exactly the same centers on both sides of the sheets.

B) When slow water currents flowed past aggregating Dictyostelium amoebae, then they cannot detect the gradient from the downstream direction.

By these methods, in the mid-late 1940s, John Bonner conclusively proved that chemotaxis occurred.

But not until the 1960s was the attractant discovered to be c-AMP. This discovery depended on dialysis tubing in a clever way.

Incidentally, different chemotactic attractant substances are used by other species of Dictyostelium, and related organisms, of which thousands of species are have been studied. Some may use peptides; but in general they haven't been studied yet.

Notice that it was possible to prove chemotaxis long before knowing what chemical is the attractant!

In fact, the methods used to prove chemotaxis were later essential parts of the methods used to identify this chemical.


Regulation! If you cut a slug into halves, or tenths, or Hundredths, then each one will regulate and form a fruiting body that is a scale model of any other.
(about 15% stalk cells is normal; but there are mutant variants)

As Mrs./Dr. Slifkin discovered, abnormally higher or lower percentages of cells will differentiate as stalk cells if the front or tail ends of slugs are mechanically separated from the rest a short time before fruiting occurs. Adjustment of cell fate requires some time. She once taught in this department.


Length/Width ratios of slugs are somehow related to ratios of spore cells to stalk cells.

But the mechanism hasn't been discovered, or imagined.

(neither what mechanism controls slug shape, nor how this is related to control of differentiation into stalk cells instead of spore cells)


Prof. Lindsay Olive of this department was the world's leading expert on different kinds of slime molds.


My own first significant research discoveries, as a undergraduate marine biologist in Florida in 1964, were about another kind of "slime mold amoeba" that causes diseases in sea grasses.

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