Introduction to Fungi, Third Edition

42 PROTOZOA: MYXOMYCOTA (SLIME MOULDS)
46 species in four genera (Kirk et al., 2001).
The best-known example is Dictyostelium which
has been so named because the stalk of its multicellular
sorocarp appears as a network, made up
from cellulose walls secreted by the amoebae
from which it is formed. Dictyostelium spp. are
common in soil, on decaying plant material
and on dung, and can be demonstrated by smearing
non-nutrient agar with cells of a suitable
bacterial food such as Escherichia coli or Klebsiella
aerogenes, and adding a small crumb of moistened
soil to the centre of the bacterial smear.
Amoebae will creep out of the soil and consume
the bacteria. At the end of the feeding
phase, sorocarps develop and isolations can be
made (Cavender, 1990). An axenic defined
medium has been developed for D. discoideum
and has greatly facilitated experimentation
with this organism (Franke & Kessin, 1977).
Good general accounts of the dictyostelids are
those by K. B. Raper (1984), Cavender (1990) and
Alexopoulos et al. (1996). The history of research
on Dictyostelium has been recounted by Bonner
(1999). Work on D. discoideum has contributed
significantly to our understanding of the key
features of eukaryotic cell biology, especially
signalling events, phagocytosis, and the evolution
of multicellularity in animals. Consequently,
there is a vast literature on this organism.
An excellent introduction to the impact of
research on D. discoideum on general eukaryotic
biology is the book by Kessin (2001), and challenging
questions have been summarized by Ratner
and Kessin (2000). Bonner (2001) has also
provided a stimulating read.
The life cycle of D. discoideum is shown in
Fig. 2.2. Amoebae of dictyostelids are morphologically
different from those of acrasids in that
they have filose (acutely pointed) rather than
lobose pseudopodia (see Fig. 2.1b). Each spore
from a sorocarp germinates to give rise to one
uninucleate haploid amoeba which feeds by
phagocytosis of bacteria. Amoebae reproduce
asexually by division to form two haploid daughter
amoebae. As with acrasid slime moulds,
the amoebae of dictyostelids can form microcysts
under unfavourable environmental conditions.
Encystment may be triggered by the production
of ammonia, which thus functions as a
signal molecule (Cotter et al., 1992). Sexual reproduction
occurs by means of macrocysts and is
initiated when two compatible amoebae meet
and fuse. Both homothallic and heterothallic
species and strains of Dicytostelium are known.
In D. discoideum, fusion is inhibited by light and
by the presence of cAMP, but is stimulated by
ethylene (Amagai, 1992). The fusion cell is greatly
enlarged relative to the two progenitor amoebae.
This giant cell attracts unfused amoebae which
aggregate and secrete a sheath (primary wall)
around themselves and the zygote. Inside the
primary wall, the giant cell undergoes karyogamy,
and the resulting zygote feeds cannibalistically
on the other amoebae by phagocytosis
and eventually produces a secondary wall.
Cellulose seems to be the main structural wall
polymer. Meiosis is followed by mitotic divisions
and cytoplasmic cleavage, and the macrocyst
germinates to release numerous haploid uninucleate
amoebae (Nickerson & Raper, 1973;
Szabo et al., 1982).
The most striking feature of D. discoideum
is the aggregation of thousands of amoebae to
form a pseudoplasmodium with radiating arms
(Figs. 2.3a,b). This is a vegetative process not
involving meiosis or mitosis. Aggregation is
initiated when the bacterial food supply is
exhausted, and follows the gradient of a hormone
which causes directional (chemotactic)
movement of starving amoebae (Konijn et al.,
1967; Swanson & Taylor, 1982). In the case of
D. discoideum, the hormone is cAMP (Konijn
et al., 1967), but other molecules are implicated
in this role in different dictyostelids. Upon exposure
to a cAMP gradient, amoebae of D. discoideum
change their shape from isodiametric to elongated,
with the migrating tip pointing towards
the highest cAMP concentration. Migration
occurs in waves which correspond to the production
of cAMP by starving amoebae, its detection
and further synthesis by neighbouring amoebae,
and its degradation by cAMP phosphodiesterase
(Nagano, 2000; Weijer, 2004). In this way, waves
of cAMP diffuse outwards, and waves of amoebae
migrate inwards. During aggregation, amoebae
migrate to the centre or one of the arms of the
pseudoplasmodium. This is a highly co-ordinated
effort in which hundreds of thousands of