The Microbial Self 87“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanityinterrupted, and a strong contraction of the body ensues, in an effort
to remove the noxious stimulus while still remaining in the same place,
ready to resume its normal activity any moment. But if this is unsuc-
cessful after many tries, the last resort appears, which is to abandon the
feeding place and swim away to a new location no longer threatened by
the stimulus, a much more drastic effort than all the preceding steps.
It should be noted that the nature and intensity of the stimulus remains
unchanged throughout the experiment. The reactions are such that one
step does not automatically lead to the next. At each stage the stentor
needs to evaluate the outcome of the previous steps. Only if the pre-
ceding steps are ineffective will a new response be brought into play.
The chain of reactions terminates at any point as soon as the stimulus is
removed. These organisms exhibit a rudimentary short-term memory, as
each new response is modified by the earlier ones.11,12
4.2.3 Slime molds: A link between unicellularity
and multicellularity
Slime molds are unique among protists in that they are capable of
switching between single-cell and multi-cell existence. The species
Dictyostelium discoideum normally lives in soil and moist leaf litter,
feeding on bacteria and yeast cells. When food supply is abundant,
they live as independent amoebas and divide every few hours. When
food is in short supply, they undergo a series of changes to ensure their
survival (Fig. 4.3). First, individual cells start to synthesize and emit
a chemo-attractant, cyclic AMP, in a random manner. Next, the cells
aggregate into several local centers with synchronized cyclic AMP
pulses. Eventually one center dominates in pulsing, into which all
others coalesce to form a single aggregate. The cells are held together
by adhesion molecules, resulting in a “slug” of about 2-4 mm long.
Each slug behaves as one organism and glides on the ground, leaving
behind a track of slime (hence the name).Once arriving in a favorable
spot, the slug stops moving and differentiates into a fruiting body,