Aggregation and slug formation
At the beginning of each film, the entire visual field is covered with
bacteria except near the left end, where slime mould cells are intermingled
with the bacteria upon which they feed.
As the slime mould cells devour the bacteria and rapidly increase in number, they
gradually extend their territory towards the right.
In the regions where the bacteria have been eaten up, some of the slime mould cells
start to send out chemotactic signals periodically. The signal substance is sensed by
the surrounding cells, which in turn rapidly make and secrete the same
substance. They also start to move towards the direction the original
signal came from.
In this way, periodical chemotactic signals propagate outwards, while the cells
move inwards. The opposite directions of cell movement and wave
propagation are clearly seen in the high time-resolution movie
(see Movie 2 shown below).
The cells gradually gather and pile up to form cell aggregates, which then
elongate to form motile structures called "pseudoplasmodia" or more
commonly "slugs". The anterior end of the slug is called the "tip".
In the two species shown here, each slug leaves a thin string-like thing
trailing behind. This is the "stalk" which is made up with cells each filled
with a large valuole and covered with a rigid cell wall, almost like plant
cells. The stalk goes all the way up to the tip, where the cells in
contact with the end of the stalk rapidly develop large vacuoles and
cell walls to become mature stalk cells.
In D. discoideum, the stalk is not formed
until the slug starts to transform into a fruiting body. So, slugs of that species
move in a manner much more like real slugs.
Why are the waves of chemotactic signals visible?
The dark and light
stripes seen to be moving in the films reflect the shape changes of the
cells. When a cell receives a signal, it sort of shrinks first, then
extends protrusions ("pseudopods") towards the signal source and becomes
elongated in that direction. It continues to proceed for a short period
(a few minutes) towards the signal source. It then stops and resumes the
non-elongated shape until the next wave comes. Because all the cells along
the wavefront do this synchronously, even the slight changes in the
scattering of light caused by minute cells result in visible,
The chemotactic substance is known to be cyclic AMP for D. rosarium
shown here and some other species of the genus Dictyostelium such
as D. discoideum.
Species of the genus Polysphondylium uses a modified dipeptide called glorin
as its chemoattractant during the aggregation process.