Introduction to Fungi, Third Edition

232 ASCOMYCOTA (ASCOMYCETES)
Fig 8.5 Two basic modes of development during
conidiogenesis of a hyphal apex (a): blastic (b) and thallic (c).
From de Hoog et al. (2000a), with kind permission of
Centraalbureau voor Schimmelcultures.
state of Discosphaerina; see Fig. 17.25). In certain
yeasts including Saccharomyces, the blastic development
of new daughter cells is known as
budding (see Fig. 10.3). Two kinds of blastic
development have been distinguished:
1. Holoblastic. All the wall layers of the
conidiogenous cell contribute to the wall of
the newly formed conidium (see Fig. 8.6b).
Aureobasidium pullulans (Fig. 17.25) and Tricladium
splendens (conidial Hymenoscyphus; see Fig. 25.12)
are examples. In some genera with dark (i.e.
melanized), relatively thick-walled conidiophores
such as Stemphylium and Alternaria (anamorphs
of Pleospora), the conidia develop holoblastically,
but a narrow channel persists in the wall of
the conidiogenous cell through which cytoplasm
had passed as the spore expanded. This type of
development has been described as porogenous
(Luttrell, 1963) or tretic (Ellis, 1971a) and the
conidia are sometimes termed porospores or
poroconidia (see Figs. 17.10 17.13; Carroll &
Carroll, 1971; Ellis, 1971b).
2. Enteroblastic. The wall of the conidiogenous
cell is rigid and breaks open. The initial of
the conidium is pushed through the opening
and is surrounded by a newly formed wall
(Fig. 8.6c). Two types of enteroblastic development
have been distinguished, phialidic and
annellidic (see Fig. 8.7). In phialidic development
a basipetal succession of conidia (phialospores,
phialoconidia) develops from a specialized conidiogenous
cell, the phialide (Gr. diminutive of
phialis ¼ flask), usually shaped like a bottle
with a narrow neck. Phialides are formed singly
Fig 8.6 Two alternative types of conidiogenesis starting from
an undifferentiated hyphal apex (a). In holoblastic
conidiogenesis (b), the entire wallbecomes inflated to form the
conidium initial. In enteroblastic conidiogenesis (c), the
conidium initial develops through a hole in the rigid outer wall.
From de Hoog et al. (2000a), with kind permission of
Centraalbureau voor Schimmelcultures.
or in clusters at the tip of a conidiophore or,
more rarely, laterally. There may be one or
several nuclei in a phialide. As shown in Fig. 8.8
for Thielaviopsis basicola, the initial of the firstformed
phialoconidium is surrounded by the
apical wall of the phialide and is, in reality,
holoblastic. The phialide wall breaks transversely
near its tip and the first conidium, surrounded
by a newly formed wall and capped by the wall
from the broken tip of the phialide, is pushed
out (Hawes & Beckett, 1977; Ingold, 1981). The
new wall material which encases the phialoconidium
is secreted in the form of a cylinder from
the surface of the cytoplasm deep within the
phialide, a process known as ring wall building
(Minter et al., 1983a). Before the conidium is
extruded, a septum develops within the phialide
below its neck, at the base of the conidium.
The upper part of the wall of the now open-ended
phialide persists as a small collar, the collarette.
The nucleus or nuclei within the phialide
continue to divide mitotically. A second conidium
develops below the first, and is surrounded
by newly secreted wall material. This conidium is
also cut off by a septum and pushed out. Part of
the newly secreted wall material may persist
around the inside of the neck of the phialide as
periclinal thickening. The process is repeated
so that many phialoconidia may develop from
a single phialide. In phialides which have
developed several conidia, the periclinal