Growth, Differentiation and Sexuality

394 H.A.B. Wösten and J.G.H. Wessels
II. Development of Emergent Structures
A. Formation of a Feeding Submerged
Mycelium
Fruiting bodies develop from a vegetative submerged
mycelium. Formation of this submerged
mycelium starts with the germination of an
asexual or sexual spore. Hyphae growing out of
these spores grow at their tips, while branching
subapically (Wessels 1986, 1990). Since hyphae
can also fuse by a process called anastomosis,
a mycelium is formed which represents a network
ofhyphae,allowingfortranslocationofmolecules
in different directions (see The Mycota, Vol. VIII,
Chap. 6). The vegetative mycelium colonizes and
assimilates the substrate, often by degrading
polymeric components by extracellular enzymes
which are secreted at tips of growing hyphae
(Wösten et al. 1991; Moukha et al. 1993; Wessels
1993b). The degradation products can serve as
nutrients not only for those hyphae which have
taken up these molecules but also for the mycelium
as a whole. This explains why some fungi can grow
for considerable distances over non-nutritive surfaces,
their growth being supported by transport
of water and nutrients from a food base (Jennings
1984; see The Mycota, Vol. I, 1st edn., Chap. 9).
Translocationofwaterandnutrients,togetherwith
tip growth, are also instrumental for development
of aerial structures, the most conspicuous being
the fruiting bodies. It is clear that growth of such
emergent structures requires massive transport
of materials from the substrate mycelium. Mathematical
models have been constructed which take
most of these activities of the whole mycelium
into account (Edelstein and Segel 1983). In such
models, initiation of fruiting bodies is viewed as
the development of focal points of high-density
growth of branching hyphae with their growth
axis away from the substrate (Edelstein 1982).
Recently, a mathematical model was described
which visualises the formation of the fruiting
body. Remarkably, formation of this complex
fungal structure can be simulated by successive
waves of hyphal attraction and repulsion. As long
as all filaments behave in the same way at the same
time, there is no need for a global or organ level
control in formation of the mushroom. The shape
of the mushroom would be determined by genes
activating the repulsion and attraction, and which
would operate in a clockwork fashion (Meskauskas
et al. 2004; Money 2004).
Redistribution of active cytoplasm has been
considered as most characteristic for fungal
mycelia (Gregory 1984). In Schizophyllum commune
(Wessels 1965) and Coprinus cinereus
(Coprinopsis cinerea) (Moore 1998), fruiting-body
primordia can arise at the expense of polymeric
constituents of the supporting mycelium, whereas
expanding fruiting bodies grow at the expense
of polymeric constituents of both supporting
mycelium and abortive fruiting-body primordia.
This is accompanied by redistribution of active
cytoplasm (Ruiters and Wessels 1989a). At the
moment, it is not clear how much of this apparent
translocation of cytoplasm is due to movement of
cytoplasm as such or to degradation and resynthesis
of cellular components. Cell turnover, involving
translocation of breakdown products, becomes
very evident in the later stages of fruiting-body
development, which can occur in the absence of
external nutrients (see Sect. IV.F).
Jennings (1984) has suggested that mass flow
occurs through hyphae because of the existence
of a gradient in hydrostatic potential created by
sourcesandsinksofassimilates.Itisclearthat
rapidly growing structures emerging into the air
represent powerful sinks for assimilates and water.
In fact, such structures are generally quite isolated
from the environment by the presence of hydrophobic
coatings, a point emphasized by Rayner
(1991). Hydrophobic coatings are the result of selfassembly
of hydrophobins (see Sect. IV.A) but also
may be due to other proteins or phenolic substances
polymerised by the action of phenoloxidases or
peroxidases (see Sect. IV.E).
B. Formation of Fruiting Bodies
from the Submerged Mycelium
Formation of fruiting bodies is a highly complex
developmental process. A generalized scheme for
formation of agaric fruiting bodies such as those of
C. cinereus (Moore 1998; Kües 2000) is as follows:
after a “critical mass” of submerged mycelium has
been formed, hyphae escape the substrate to grow
into the air. These hyphae form aggregates, which
are called hyphal knots or nodules. These knots
may result from a single hypha which branches intensely
or they arise from branches of neighbouring
aerial hyphae which grow towards and alongside
each other. Within the knots hyphae aggregate,
forming a fruiting-body initial. These initials (also
referred to as secondary hyphal knots; Walser et al.