Introduction to Fungi, Third Edition

46 PROTOZOA: MYXOMYCOTA (SLIME MOULDS)
Useful treatments of the group have been
written by Olive (1967, 1975) and Spiegel (1990).
Protostelids are ubiquitous on decaying plant
parts in soil and humus, as well as on dung
or in freshwater. They occur in all climatic
zones from the tundra to tropical rainforests.
Protostelids produce amoebae with filose pseudopodia
(Fig. 2.1b), feeding phagocytotically on
bacteria, yeast cells or spores of fungi. Some
species also produce small plasmodia, thereby
providing structural affinities to both the cellular
and plasmodial slime moulds. Sporulation
occurs by the conversion of a feeding amoeba
or plasmodium into a round prespore cell
which then rises at the tip of a delicate acellular
stalk, ultimately forming one or several spores
in a single sporangium. It is possible to isolate
protostelids by transferring a spore from its
stalk onto a weak nutrient agar plate with
appropriate food organisms.
Protostelium is a typical member of the group
(Fig. 2.5). The sporocarp consists of a long, slender
stalk about 75 mm long, bearing a single spherical
spore about 4 10 mm in diameter. The spore
is deciduous and readily detached. Upon germination,
a single uninucleate amoeba with thin
pseudopodia emerges. The amoeboid stage
feeds voraciously on yeast cells and may also
feed cannibalistically on amoebae of the same
species. Development of the sporocarp probably
follows the generalized pattern described by
Olive (1967) and summarized in Fig. 2.6. When
feeding stops, the amoeba rounds off and
heaps its protoplasm in the centre to form the
‘hat-shaped’ stage (Fig. 2.6b). A membranous,
pliable, impermeable sheath develops over the
surface of the cell. When the protoplast contracts
into the central hump, the sheath collapses
at the margins, forming the disc-like base to
the stalk of the sporocarp. This may be the
structural equivalent of the hypothallus of the
Myxomycetes (see p. 48). Within the protoplast,
a granular basal core, the steliogen, differentiates
and begins to mould a hollow tube
(Figs. 2.6d,e). As the tube extends at its tip,
the protoplast migrates upwards, always seated
on top of the growing tip. The entire structure
remains covered by the sheath. Tube extension
is an actin myosin-driven process (Spiegel
et al., 1979). Ultimately, the steliogen is left
behind at the tip of the stalk to form an apophysis
(Fig. 2.5a), and the protoplast secretes a cell
wall and becomes the spore.
Variations of this pattern occur within
the protostelids. For instance, some species
produce spores which are discharged forcibly
(e.g. Spiegel, 1984). In Ceratiomyxa fruticulosa,
a species which may or may not belong to
the Protosteliomycetes (Spiegel, 1990; Kirk et al.,
2001; Clark et al., 2004), numerous spores are
formed externally on a sporocarp (Figs. 2.7a,b)
and are the product of meiosis. They germinate
to release a single quadrinucleate protoplast
(Figs. 2.7c e) which divides repeatedly to
produce a clump of four and later eight haploid
cells, the octette stage (Figs. 2.7f,g). Each of these
cells releases a motile cell (a swarmer) which
has one or two whiplash-type flagella (Fig. 2.7h).
Fig 2.5 Protostelium sp. (a) Two sporocarps, one
immature, the other with a detached spore. Note the
apophysis beneath the spore. (b) Empty spore case after
germination. (c) Amoeboid phase.