Growth, Differentiation and Sexuality

280 R. Fischer and U. Kües
tein kinase, both needed for the control of entry
into the conidiation programme. When defect,
they cause constitutive onset of conidiation (Kothe
and Free 1998). Other analysed defects leading to
inappropriate sporulation in submerged culture
are in genes for sugar transporters (Madi et al.
1997) and in heterotrimeric G-proteins (Yang et al.
2002; Kays and Borkovich 2004; Krystofova and
Borkovich 2005). A mutant in the Gα protein GNA3
resembles in phenotype mutants in the adenylate
cyclase gene cr-1. Phenotypic defects in both
genes can be corrected by addition of cAMP, and
GNA3 possibly regulates the protein levels of these
enzymes. By contrast, defects by the Gα protein
GNA1 are not overcome by addition of cAMP.
GNA1 is necessary for response to extracellular
cAMP, and the protein has been suggested to act in
sensing cell densities and/or the nutritional status
(Kays and Borkovich 2004). Other genes affecting
conidiation have been studied, some in connection
with the molecular clock. A detailed description of
the role of all developmental genes characterized
in N. crassa cannot be given in this review, and the
reader is therefore referred to earlier reviews in
this field (Perkins and Barry 1977; Borkovich et al.
2004; The Mycota, Vol. III, 2nd edn., Chap. 11).
C. Basidiomycetes
1. Cytology
Asexual sporulation in basidiomycetes is not uncommon
(Kendrick and Watling 1979) but usually
rather neglected. Only asexual sporulation of C.
cinereus has been studied in more detail. This fun-
gusproducestwotypesofasexualspores,chlamydospores
and oidia.
Large, thick-walled chlamydospores are produced
for survival in aging cultures, preferentially
in brown-coloured matting at the agar–air interface
(Fig. 14.8). The spores have variable shapes,
from oval to irregular inflated. There appear
to be two different modes for generation – the
classical endogenous chlamydospore formation
within hyphal cells (see above), and an exogenous
blastocyst mode where an inflated bud receives
the compressed cytoplasm from a hyphal cell
(in a factual sense, not chlamydospores; see
definitions by Clémençon 1997). When produced
on the dikaryon, chlamydospores are binucleate
and germinate with either one or two germ tubes
to give a new dikaryotic mycelium or two distinct
monokaryons respectively (Anderson 1971; Kües
et al. 1998a, 2002b).
Oidiation in C. cinereus in most instances starts
with lateral bulging at an aerial hypha. Following
nuclear division, a daughter nucleus migrates
into the protrusion and a septum is laid, separating
the young, elongating sporophore (oidiophore)
from the hyphal foot cell. The full-grown oidiophore
towers at a more or less perpendicular angle
(80–85 ◦ ) into the air, reaching a length of 50 μm
and more (on average, about 20–30 μm). At its base,
it is broader than is the case for vegetative hyphae
but it tapers with length. At the tip of the matured
structure, oidial hyphae bud off, one after the
other. Usually, they are about 8–12 by 2 μm in size.
Each oidial hypha is supplied with a nucleus coming
from successive divisions of the nucleus within
the oidiophore stalk. Subsequent to uptake of a nucleus,
the oidial hypha separates from the stem cell
Fig. 14.8. A–D Chlamydospores of
Coprinus cinereus. A Chlamydospores
formed in the mycelial matting of
amonokaryon.B, C Chlamydospores
formed within hyphal cells by the
classical mode of chlamodospore
production (B bright field optic, C
phase contrast). D Blastocyst mode of
production. The scale bar represents
50 μm