Introduction to Fungi, Third Edition

MYXOMYCETES: TRUE (PLASMODIAL) SLIME MOULDS
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have been revived after more than 50 years’
storage in a herbarium (Elliott, 1949).
2.5.2 Orders of myxomycetes
Myxomycetes are currently grouped into five
orders, all of which are frequently found either
in nature or upon incubating suitable plant
material on moist filter paper.
The Echinosteliales (e.g. Echinostelium,
Clastoderma) contain the smallest known
myxomycetes. They form protoplasmodia, with
each protoplasmodium giving rise to only one
sporangium. The Echinosteliales resemble the
protostelids from which they are probably
derived (Frederick, 1990; Spiegel, 1991; see
Fig. 2.5).
The Liceales (e.g. Lycogala, Dictydium, Cribraria,
Reticularia) are common on the bark of dead
trees. Some of the smaller species produce protoplasmodia,
but most have phaneroplasmodia.
Various types of sporophores are formed; the
aethalia of Lycogala epidendron (Plate 1b) and
Reticularia (= Enteridum) lycoperdon (Plates 1c,d)
are particularly common.
The Trichiales (e.g. Arcyria, Trichia, Hemitrichia)
are ubiquitous on fallen logs. The plasmodia are
intermediate between aphanoplasmodia and
phaneroplasmodia. Fructifications in Trichia floriforme
are well-defined sporangia which contain
an internal meshwork of threads, collectively
called the capillitium. The peridium breaks
open at maturity, and the spores are released
over time by the twisting of the capillitial
threads which thus act as elaters (Fig. 2.11).
Arcyria denudata produces reddish sporangia
on rotting wood (Plate 1e). Another member,
Hemitrichia serpula, produces plasmodiocarps.
The Physarales (e.g. Physarum, Fuligo) produce
the largest plasmodia. Physarum polycephalum has
been used extensively in fundamental research
on cell biology, for example on the nature
of protoplasmic streaming, or the synchrony of
nuclear division in a large plasmodium comprising
thousands of nuclei (see below). The plasmodia
are typical phaneroplasmodia, each of
which produces numerous sporangia at maturity
(Plate 1f). Fuligo septica forms particularly large
sporophores (aethalia) which are bright yellow
and are frequently seen on decaying wood
(Plate 1g).
The Stemonitales include such genera as
Comatricha and Stemonitis. Stemonitis spp. produce
clusters of stalked sporangia from aphanoplasmodia
which are visible on rotting wood
(Plate 1h).
2.5.3 Physarum polycephalum as an
experimental tool
This species has been used to investigate several
aspects of cell biology. The conspicuous cytoplasmic
shuttle streaming in the veins of its large
phaneroplasmodia is a fascinating phenomenon
and has been examined extensively. The pulse is
caused by actin myosin interactions controlled
by Ca 2þ (Smith, 1994). It is brought about not
by the direct binding of organelles to actin
cables, but by the constriction and relaxation
of an actin myosin skeleton lining the veins.
Several proteins interacting with actin and
myosin are directly or indirectly regulated by
Ca 2þ , but the most important effect of Ca 2þ
is on one of the myosin light chains. This is a
regulatory subunit which directly binds Ca 2þ .
In contrast to most animal actin myosin
systems which are stimulated by Ca 2þ , that of
Physarum is inhibited, i.e. contraction occurs
at low Ca 2þ concentrations, and relaxation at
higher concentrations. Ca 2þ -inhibited actin
myosin interaction also occur in plant cells
where they are visible as cytoplasmic streaming.
Nakamura and Kohama (1999) have written a
thorough review of the actin myosin system
in Physarum.
Mitotic division of all nuclei throughout
the plasmodium of P. polycaphalum occurs in a
synchronized manner, and Physarum was one
of the pioneer organisms in which the existence
of the cell cycle was demonstrated. Synchrony of
mitosis is regulated by a protein kinase which
catalyses the phosphorylation of H1 histones,
leading to the condensation of chromosomes at
the onset of mitosis (Bradbury et al., 1974; Inglis
et al., 1976). This protein kinase is now known to
be homologous to the cdc2 product in the fission
yeast Schizosaccharomyces pombe (see Fig. 9.5;
Langan et al., 1989).