Introduction to Fungi, Third Edition

CONIDIUM PRODUCTION IN ASCOMYCETES
235
Secession of conidia, irrespective of their
mode of development, is in most cases by
dissolution of the septum or septa which
separate them from the conidiogenous cell or
from adjacent spores. This process is termed
schizolytic secession (Gr. schizo ¼ to split, divide;
lyticos ¼ able to loosen). In some other cases
secession is brought about by the collapse of a
special separating cell beneath the terminal
conidium. This is termed rhexolytic secession
(Gr. rhexis ¼ a rupture, breaking).
8.5.2 Thallic conidiogenesis
Thallic conidiogenesis (Gr. thallos ¼ a branch)
occurs by conversion of a pre-existing hyphal
element in which terminal or intercalary cells of
a hypha become cut off by septa (see Fig. 8.9). Two
kinds of thallic development have been distinguished:
holothallic (Gr. holos ¼ whole, entire)
and thallic-arthric (Gr. arthron ¼ a joint).
In holothallic development a hyphal element,
e.g. a terminal segment of a hypha, is converted
as a whole into a single conidium (see Fig. 8.9).
Secession of such conidia may be schizolytic
or rhexolytic. Microsporum spp. (anamorphic
Arthroderma), which are skin pathogens (dermatophytes)
of mammals, provide examples of this
holothallic development (see Fig. 11.6). During
thallic-arthric conidiogenesis, septa develop in
a hypha and divide it up into segments which
separate into individual cells by dissolution of
the septa (see Fig. 8.9). Geotrichum candidum
(anamorphic Galactomyces), a common soil
fungus and frequent contaminant of milk and
milk products, develops conidia in this way
(see Fig. 10.10; Cole, 1975).
The proliferation of the conidiogenous cell
or the conidiophore may occur in various ways,
for example by the formation of a new growing
point in the region of the conidiophore beneath
the point at which the first conidium was
formed. The new apex extends beyond the
point of origin of the first conidium and develops
a new conidiogenous cell. These methods of
conidiophore regeneration are discussed more
fully in relation to some of the different genera.
Fig 8.9 Holothallic and thallic arthric conidiogenesis.From de Hoog et al. (2000a), with kind permission of Centraalbureau voor
Schimmelcultures.
Holothallic conidiogenesis with rhexolytic secession Thallic arthric conidiogenesis with schizolytic secession
a. The terminal portion of a hypha is cut off by a septum. a. A terminal segment of a hypha.
b. A second septum laid down near the first cuts off a b. Septa develop, dividing the segment into several cells.
subterminal segment, the separating cell.
c. The terminal cell enlarges to form the conidium. c. The septa divide, each separating into two layers.
d. Collapse of the separating cell causes conidium secession. d. The daughter cells separate.