Introduction to Fungi, Third Edition

EUROTIALES
301
biseriate species Aspergillus (Emericella) nidulans
can be compared with those summarized earlier
for the two ‘model yeasts’, Schizosaccharomyces
pombe (pp. 256 259) and Saccharomyces cerevisiae
(p. 270). While the cell cycle (nuclear division)
is always followed by cell division (cytokinesis)
in these yeasts, this is not necessarily the case
in A. nidulans. For instance, when a uninucleate
conidium of A. nidulans germinates, several
mitotic divisions take place prior to and during
the emergence of the germ tube before the first
septum is laid down near the base of the germ
tube. Subsequent septa are formed at regular
intervals along the hypha, and each hyphal
segment contains about 3 4 nuclei (Fiddy &
Trinci, 1976). Only the nuclei in the apical cell
continue to divide whereas those in intercalary
compartments are arrested at the G1 stage but
become reactivated later if a lateral branch is
formed (Kaminskyj & Hamer, 1998). As the mycelium
develops and spreads horizontally on an
agar surface, conidiophores begin to grow out
vertically after about 16 h in optimal conditions,
with the first conidia being produced about 8 h
later. Thus the time span from the germination
of a conidium to the production of the new crop
of conidia is only 24 h. It is not surprising that
Aspergillus and Penicillium-type moulds are omnipresent
in our environment!
The development of the conidiophore tip has
been described in detail by Mims et al. (1988) and is
shown in Fig. 11.13. After vertical growth for
a limited distance (about 100 mm inA. nidulans),
the tip of the conidiophore swells to form the
vesicle (Fig. 11.13a). In the conidiophore, cytokinesis
is suppressed whereas nuclear division
continues, resulting in a multinucleate cell. In
contrast, nuclear division and cytokinesis are
tightly controlled beyond the vesicle stage
because a single nucleus migrates into each of
the 60 or so metulae which are formed by the
vesicle of A. nidulans (Fig. 11.13b). Each metula
then produces about two uninucleate phialides
(Fig. 11.13c). At the phialide tip, conidia are
formed (Fig. 11.13d), and here the transition
from polarized to yeast-like growth takes place.
The nucleus divides as the conidium enlarges, and
one daughter nucleus migrates along microtubules
into the spore whereas the other remains
in the phialide, ready to divide again. The signalling
events which co-ordinate the interactions
between the nuclear division cycle and cytokinesis
are beginning to be unravelled (Ye et al., 1999).
As in the case of S. cerevisiae and S. pombe,
septin-type proteins play a crucial role in organizing
morphogenesis in A. nidulans and most probably
other members of the Eurotiales, although
detailed studies are lacking. In A. nidulans, septin
rings with superimposed constricting actin rings
are found at the sites of septum formation, the
points of origin of metulae at the vesicle surface,
the point of origin of phialides, and at
the phialide tip where conidia are budded off
(Momany & Hamer, 1997; Westfall & Momany,
2002). All rings except for the last-mentioned are
transient, disappearing as soon as the septum is
complete.
11.4.2 Morphogenesis in Penicillium
Little work has been carried out on conidiogenesis
in Penicillium, although it is reasonable to assume
that the general principles will be similar to those
described above for Aspergillus (Borneman et al.,
2000). One interesting case is P. marneffei, which is
the only species displaying a switch from hyphae
to yeast cells, growing as a mycelium at 25°C and
as yeast cells at 37°C. Upon transfer of a mycelial
colony to 37°C, nuclear division becomes tightly
coupled with cytokinesis so that uninucleate
hyphal segments are formed which fragment
into arthrospores (Garrison & Boyd, 1973;
Andrianopoulos, 2002). These form yeast cells
which reproduce by fission at 37°C. Interestingly,
the same regulatory mechanism is responsible
for phialidic conidiogenesis at 25°C and for the
switch from multinucleate hyphae to uninucleate
yeast cells (Borneman et al., 2000), supporting the
notion that conidiogenesis by phialides should be
regarded as yeast-like growth.
In Penicillium cyclopium, conidiophore formation
is thought to be induced by a hormone called
conidiogenone, which is permanently produced
by growing colonies. When the concentration
of conidiogenone exceeds a particular threshold
(10 7 10 8 M), conidiogenesis, i.e. growth and
differentiation of the conidiophore, is triggered
(Roncal et al., 2002). For conidiogenesis to occur,
hyphae must usually be exposed to air. This may