Introduction to Fungi, Third Edition

ONYGENALES
293
disturb the soil. One hot spot of infection is the
San Joaquín Valley in Southern California, where
coccidioidomycosis is known as ‘valley fever’. The
Californian population of C. immitis is reproductively
isolated from populations elsewhere and
has recently been given the status of a separate
species, C. posadasii (Fisher et al., 2002). A readable
account of the history of C. immitis has been given
by Odds (2003), and Cox and Magee (2004) have
covered general aspects of its interactions with
the mammalian host.
Onygena
This is perhaps the most unusual yet least
researched member of the Onygenaceae. Its
fructification (Fig. 11.4a) is interpreted as a
stalked ascostroma, i.e. an aggregate of several
gymnothecia at the tip of a sterile stalk which
may be 1 cm or more in length. The stalks are
phototropic during growth. At maturity, the
peridium of the stroma ruptures, thereby exposing
the ascospores. There are two species,
O. corvina which is associated with animal hair
and bird feathers, and O. equina growing on the
hooves and horns of herbivorous mammals
(Currah, 1985). Both species are strongly keratinolytic,
and although little work has been
published on their biological features, we can
assume that keratinolysis proceeds as in other
Onygenales. According to Kunert (2000), the key
feature is the ability to use keratin as the sole
source of both carbon and nitrogen. The vast
surplus of nitrogen is released into the environment
as ammonia, thereby generating an alkaline
pH of 9.0 or higher. The cystein-rich keratin also
contains sulphur in excess of the growth requirements
of keratinolytic fungi. This is often released
as sulphate, thereby buffering the pH increase
caused by the release of ammonia.
Like many protein-degrading fungi, Onygena
produces a cadaverous smell in culture, and
Currah (1985) has suggested that this might
attract carrion flies if produced in nature. This
would make Onygena an insect-dispersed fungus.
11.3.2 Ar throdermataceae
There are only two genera in this small family.
Ctenomyces (Fig. 11.5), with only one species
(C. serratus), has a Chrysosporium anamorph
in which one-celled microconidia are formed as
terminal or intercalary cells of hyphae. Species of
Arthroderma also sometimes produce Chrysosporium-like
microconidia (Fig. 11.6b) but are better
known by their macroconidial synanamorphs
Epidermophyton, Microsporum and Trichophyton.
These are multicellular with transverse septa,
and are spindle-shaped or cylindrical. They are
typical and readily recognized (Fig. 11.6a). The
perfect state has not been found for many
of these anamorphs, but they are suspected to
be phylogenetically close to Arthroderma (Gräser
et al., 1999; Hirai et al., 2003). Howard et al. (2003)
have given a useful summary of Arthroderma and
its associated anamorphs, listing 47 species. The
teleomorph of the Arthrodermataceae is a typical
gymnothecium with a basket of branching and
anastomosing hyphae enclosing spherical asci
which release their spores passively. Characteristic
appendages are often present, e.g. in
Ctenomyces (Fig. 11.5), a keratinolytic species associated
with feathers. The combed appendages
may serve to attach the gymnothecium to bird
feathers for dispersal (Currah, 1985).
Members of the Arthrodermataceae are generally
keratinolytic, i.e. they degrade skin and hair.
Howard et al. (2003) distinguished between species
primarily associated with man (anthropophilic),
animals (zoophilic) or the soil (geophilic). Because
of their ability to grow on the skin, hair and nails
of animals, the Arthrodermataceae are collectively
called dermatophytes. Diseases caused by
dermatophytes are colloquially known as ‘ringworm’,
whereas they are called tinea within the
medical profession, with descriptive terms such
as capitis, barbae, corporis and pedis added to
describe mycoses of the scalp, beard, general
body, or feet, respectively (Howard et al., 2003).
These infections are usually confined to the outer
(dead) skin regions and are relatively easily
controlled either by the superficial (topical)
application of creams containing a wide variety
of drugs, or by oral treatment especially with
triazoles or terbinafine (see p. 278; Weitzman &
Summerbell, 1995; Gupta et al., 1998). Griseofulvin,
produced by Penicillium griseofulvum (see
p. 302), was one of the first oral and topical
drugs and is still in use today, especially in