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86 CHAPTER 5

regulated dimorphismis a significant factor in human

pathogenesis. By contrast, the dimorphic saprotrophic

Mucorspecies (e.g. M. rouxii, M. racemosus) do not

respond to temperature changes but respond to oxy-

gen levels. They grow as budding yeasts in anaerobic

conditions but as mycelia in the presence of even

low concentrations of oxygen. Ustilago maydis and

other plant-pathogenic smut fungi (Basidiomycota)

are yeast-like in their monokaryotic phase but hyphal

in the dikaryotic form (Chapter 2) so their transition

is governed genetically.

This range of responses shows that there is no com-

mon environmental cue that governs the M–Y trans-

ition, and instead we need to consider the underlying

control mechanisms (Gow 1994; Orlowski 1995). In this

regard, the studies on cell polarity of budding yeasts

and fission yeasts (see Fig. 4.12) are particularly import-

ant because several of the genes that regulate polarity

have been identified.

Control of the dimorphic switch

The usual approach to identifying the underlying

basis of dimorphism is to grow a fungus in conditions

that are, as nearly as possible, identical except for one

factor that changes the growth form. Then populations

of M and Y forms can be compared for differences

in biochemistry, physiology, or gene expression. Even

so, it is difficult to establish an obligatory, causal rela-

tionship between these differences and a change of cell

shape, because the altered environmental factor might

cause coincidental changes in biochemistry and gene

expression. In fact, most of the differences that have

been found to date are quantitative rather than qualit-

ative. Some examples are given below.

Differences in wall composition

The wall components of M and Y forms sometimes

differ:

• in Mucor rouxiithe Y form has more mannose than
  the M form;


• in Paracoccidioides brasiliensisthe Y form has α-1,3
  glucan, whereas the M form has β-1,3 glucan;


• in Candida albicansthe M form has more chitin
  than the Y form;


• in Histoplasma capsulatumand Blastomyces dermatitidis
  the M form has less chitin than the Y form.

Table 5.1Some environmental or genetic factors that cause transitions between mycelial growth or swollen, yeast-like
growth.

Conditions for swollen or

Fungus Conditions for mycelial growth yeast-like growth

Human pathogens

Histoplasma capsulatum 20–25°C 37°C

Blastomyces dermatitidis 20–25°C 37°C

Paracoccidioides brasiliensis 20–25°C 37°C

Sporothrix schenckii 20–25°C 37°C

Coccidioides immitis 20–25°C 37°C

Candida albicans Low nutrient levels High nutrient levels

Saprotrophs
Mucor rouxii and some Aeration Anaerobiosis
other Zygomycota

Plant pathogens
Ophiostoma ulmi Calcium Low calcium
Some nitrogen sources
Phialophora asteris Flooding with water
Ustilago maydis Dikaryon Monokaryon

Insect pathogens
Metarhizium anisopliae Solid media Submerged liquid culture
Beauveria bassiana Solid media Submerged liquid culture