DIFFERENTIATION AND DEVELOPMENT 101
medium that contained both nitrogen and glucose. On
agar plates this whole developmental sequence occurs
in a zone of about 1–2 mm diameter, located a few
millimeters behind the colony margin. Presumably, there
is a succession of physiological changes in the hyphae
that co-ordinates the developmental sequence, but only
by growing the fungus in chemostat culture was it pos-
sible to show that each stage is differentially regulated.
An even more comprehensive study of sporulation
has been made with Emericella nidulans, where the
sequential activation of many sporulation-related
genes has been demonstrated (Adams 1995). The
details are complex, but essentially the genes are sug-
gested to fall into three categories – those involved in
the switch from somatic growth to sporulation, those
that regulate the developmental stages of sporulation,
and those that govern secondary aspects such as spore
color.
Although many fungi such as Aspergillus and
Penicilliumcan sporulate in darkness, some require a
light trigger (Schwerdtfeger & Linden 2003). The most
common response is to near-ultraviolet irradiation
(NUV; 330–380 nm wavelength) which can induce
sporulation after a short (1 hour) exposure if the
colony is then kept in darkness. However, a subsequent
exposure to blue light can reverse the process because
the photoreceptor exists in alternating forms, one
responsive to NUV and one responsive to blue light.
Botrytis cinerea(Fig. 5.20), which causes gray mould
disease of strawberries and other soft fruits, behaves
in this way. It never becomes wholly committed to
sporulation, as Suzuki et al. (1977) demonstrated by
subjecting the colonies to 1-hour exposures of NUV
and blue light in different sequences. As shown in
Fig. 5.19, at almost any stage during sporulation an
exposure to blue light caused the fungus to form
hyphal outgrowths instead of continuing the develop-
mental pathway.the spore often undergoes repeated divisions, while the
spore elongates into a banana shape; the resulting
macroconidia of Fusariumhave several septa and one
nucleus in each spore compartment. These different
sporulation strategies have important consequences in
fungal genetics, which are discussed in Chapter 9.
Dipodascus geotrichum (Fig. 2.33) is a good example
of thallic conidial development. In this case a hyphal
branch grows to some length, then stops and develops
multiple septa which separate it into short compart-
ments. The septal pores are then plugged and the
middle zone of each septum is enzymatically degraded
to separate the spores.
Regulation and control of conidiationAsexual sporulation occurs during normal colony
growth, but in zones behind the extending colony
margin or in the aerial environment rather than on the
substrate. The factors that control conidial develop-
ment are difficult to study in these conditions because
development is not synchronized over the whole
colony. Ng et al. (1973) overcame this problem by
growing Aspergillus nigerin a chemostat so that the
hyphae grew synchronously, then the culture condi-
tions could be adjusted to trigger the developmental
stages. Conidiophores were found to be produced only
in nitrogen-limited growth conditions but when the
medium was carbon-rich. Apparently, this is the trig-
ger that switches this fungus from vegetative growth
to sporulation. However, the conidiophores did not
develop further unless the medium was changed to
contain nitrogen and a TCA cycle intermediate such
as citrate. Then the tips of the conidiophores swelled
into vesicles (large swollen heads – see Fig. 2.33)
which produced phialides. The production of conidia
from the phialides required yet another change – to a
Fig. 5.19Diagrammatic representation of stages in the
development of conidia of Botrytis cinerea. (a) After
exposure to near-ultraviolet (NUV) irradiation the
aerial hyphae are transformed into branched conidio-
phores which swell at the tips to form small globose
vesicles. Conidia are formed on minute projections
from these vesicles, so the mature clusters resemble
bunches of grapes – see Fig. 5.20. (b) A short expo-
sure to blue light at different times after triggering
of development by NUV causes development to be
arrested. The fungus then switches back to a hyphal
growth form – for example, the swelling of the imma-
ture conidia stops and the fungus produces narrow
hyphal outgrowths from the immature conidia.
(Based on Suzuki et al. 1977.)