Growth, Differentiation and Sexuality

206 U. Ugalde
Fig. 11.2.A–C Drawing showing the typical pattern of a fungal
colony resulting from a single germinated spore (A).
The peripheral zone contains no anastomosis of hyphae,
whereas the central zone shows anastomosis and the formation
of a matrix. Image adapted from Buller (1933). Confocal
images showing hyphal organisation at the periphery (B)
and at distal regions (C) inacolonyofNeurospora crassa,
with distinctly different patterns of growth, branching and
anastomosis. Images kindly provided by Hickey et al. (2002)
grow away from their origin, as well as from each
other, has been the subject of much study (Glass
et al. 2004). A clear depiction of this pattern is
provided by Hickey et al. (2002) in Neurospora
crassa (Fig. 11.2B) but the chemical signals
driving this guiding system remain unknown.
Chemotropism along increasing concentration
gradients of nutrients (and therefore away from
lower nutrient-containing zones) has found little
supporting evidence (Gooday 1975). However,
positive aerotropism (tropism towards higher
oxygen concentrations) has been shown to occur
in early studies (Robinson 1973a,b,c). This guiding
cuewouldnotonlydirecthyphaeawayfromthe
centre of the colony but also away from each other.
No evidence has yet emerged on the existence
of autoregulatory signals guiding this growth
pattern, but they cannot be ruled out at this time.
Fresh biophysical, biochemical and cytochemical
determinations will be required to confirm the
identity of this important guiding system.
In contrast to the outward tropism shown
by leading hyphae at the colony edge, subapical
regions generate lateral branches which fuse with
each other (Gooday 1999, and Glass and Fleißner,
Chap. 7, this volume). Detailed studies with
fluorophore-labelled Neurospora crassa hyphae
(Hickey et al. 2002) show that the formation of
lateral branches is induced by the remote presence
of other hyphae and moreover, once formed,
they attract each other (Fig. 11.2C). The chemical
signals which operate in these two processes
(formation of lateral branches, and their homing
and fusion) could be different, and still remain
unidentified. Early projections on autoregulatormediated
positive autotropism by Müller and
Jaffe (1965) in Botrytis cinerea postulated that
a diffusible growth stimulator, with a half life
of 10 s and acting at a radius of 10 μm around
each hypha, would fit in with their observations.
This projection resembles similar ones presented
later by other authors (Gooday 1975). Flow cell
studies using Aspergillus oryzae have shown that
substitution of fresh medium with medium which
was re-circulated resulted in increased apical
extension growth and increased branching (Spohr
et al. 1998). This has supported the suggestion
that a relatively stable, self-produced growth and
branching inducer or inducers were present in the
medium. Further studies along these lines should
clarify the nature of these endogenous signals, and
whether they influence colony morphogenesis.
In addition to positive autotropism (perpendicular
to the growth axis) at subapical zones, a negative
tropism (along the growth axis) inhibiting
backgrowth towards older parts of the colony has
also been reported in mycelial colonies. Studies in
static agar cultures by Bottone et al. (1998) showed
that cultures of Mucor spp. and Aspergillus fumigatus
on membrane filters over agar plates, later
removed along with the membranes, left a clear
circular patch of agar, into which backgrowth was
precluded. The effect of nutrient depletion on this
phenomenon was ruled out by several controls,
and treatment of the cleared region with chloroform
removed the inhibition, leading the authors
to propose that an endogenous lipidic factor
was involved. In addition, backgrowth took
place only when the central patch was replaced by
fresh medium and separated from the surrounding
colony by a “moat”. This measure avoided back
diffusion of the inhibitor from the surrounding
biomass. In addition to this experimental evidence,
theoretical models could only emulate colony morphogenesis
by incorporating the action of a selfproduced
inhibitor of backgrowth in the kinetic
algorithms (Indermitte et al. 1994). The chemical
identity of this signal remains to be determined.
Previous reports by Park and Robinson (1964)
had described activities of self-produced staling
compounds causing vacuolation in cultures of A.
niger. The same investigators (Robinson and Park
1966) later described that citrinin acts as a selfproduced
agent causing hyphal narrowing (a feature
attributed to hyphae from older regions) and