Introduction to Fungi, Third Edition

44 PROTOZOA: MYXOMYCOTA (SLIME MOULDS)
Fig 2.3 Dictyostelium discoideum development. (a) Aggregation of amoebae. (b) Aggregation, enlarged. (c) Amoebae feeding
on bacteria; note their isodiametric shape. (d) Aggregating amoebae; note their elongated shape. (e) Late aggregation stage.
(f,g) Migration stage. (h) Culmination; the spore mass is rising around the stalk. (i) Spore mass almost at the apex of the stalk.
(j) Mature sorocarps.
The ability of free-living individual amoebae
of Dictyostelium to aggregate into the multicellular
slug has led to dictyostelid slime moulds
being called social amoebae (Kessin, 2001). This
phenomenon gives rise to interesting and fundamental
questions. To give an example, since
amoebae in the anterior end of the slug become
stalk cells and are thus excluded from perpetuation
as spores, cells skiving off to the rear of the
slug and thereby avoiding self-sacrifice would
have a selective advantage. ‘Cheater strains’ are
indeed known from nature and the laboratory;