Chapter 5
Differentiation and development
This chapter is divided into the following major
sections:- mould-yeast dimorphism
- infection structures of plant pathogens
- sclerotia
- nutrient-translocating organs: mycelial cords and
rhizomorphs - asexual reproduction
- sexual development
Differentiation can be defined as the regulated change
of an organism from one state to another. These states
can be physiological, morphological, or both. So the
germination of fungal spores (Chapter 10) and the
switch from primary to secondary metabolism (Chap-
ter 7) are examples of differentiation. But here we focus
on the developmental changes that lead to the pro-
duction of a wide range of differentiated structures, such
as the infection structures of fungal pathogens, the reg-
ulation of sexual and asexual development, the switch
between hyphal and yeast forms of some human-
pathogenic fungi, and other developmental processes.
We consider both the underlying control mechanisms
and the functions of the differentiated structures.Mould-yeast dimorphism
Most fungi grow either as hyphae (the mycelial,
mould, or M-phase) or as single-celled yeasts (the
Y-phase). In general, yeasts and yeast phases are found
in environments with high levels of soluble sugars
that can diffuse towards the cells, or where the cells
can be dispersed in liquid films or circulating fluids
to obtain nutrients. Yeasts have little or no ability to
degrade polymers such as cellulose or proteins, etc.,
and they also have no penetrating power, unlike the
mycelial fungi which commonly have these abilities.
So, the yeast form and the mycelial form represent two
different growth strategies, suited to particular environ-
ments and conditions.
However, some fungi can alternate between a
mycelial form and a yeast-like form, in response to
environmental factors. These dimorphic fungi(with
two forms) include several pathogens of humans. For
example, Candida albicanscommonly grows as a yeast
on the mucosal membranes of humans, but converts
to hyphae for invasion of host tissues (see Fig. 1.4).
This dimorphic switch can be induced experimentally
by growing C. albicansin horse serum of low nutrient
content (Chapter 16). Similarly, the fungi such as
Metarhiziumand Beauveriaspp., which commonly par-
asitize insects, penetrate the insect cuticle by hyphae
but then proliferate in a single-celled form in the cir-
culating fluids of the host (Chapter 15). As a further
example, the vascular wilt pathogens of plants (e.g.
Fusarium oxysporum, Ophiostoma novo-ulmi) penetrate
initially by hyphae but then spread as yeast-like forms
in the xylem vessels (Chapter 14).
The switch between mycelial (M) and yeast-like
(Y) growth of dimorphic fungi occurs in response to
environmental factors, and can be reproduced experi-
mentally as shown in Table 5.1. Several opportunistic
pathogens of humans grow in the M phase as sapro-
trophs in plant and animal remains (their normal
habitat, Chapter 16) and also grow as mycelia in labor-
atory culture at 20 –25°C. But they convert to budding
yeasts or swollen cells in the body fluids or when
grown at 37°C in laboratory culture. This thermally