Introduction to Fungi, Third Edition

290 PLECTOMYCETES
Fig11.4 Onygena. (a) Stalked gymnothecial stromata of
O. equina on a cast sheep’s horn. (b) Ascogenous hyphae and
asci of O. corvina.
gymnothecium and distribute them. Thus, the
gymnothecium may be an adaptation to dispersal
by arthropods.
The asci are formed loosely throughout the
gymnothecium. Asci are eight-spored and evanescent,
releasing their ascospores passively. Ascus
development is similar to that in other higher
ascomycetes, with the cytoplasm being delimited
by two membranes between which the ascospore
wall is laid down. The inner membrane
eventually becomes the ascospore plasma membrane
(Ito et al., 1998). The anamorphic states are
usually more readily seen than the teleomorph
and typically consist of rhexolytic arthrospores,
although thick-walled chlamydospores are also
sometimes present.
Members of the Onygenales are cosmopolitan,
although many individual species have a very
limited distribution. Thankfully, this is true especially
of many of the human pathogens. Most
species, including the pathogenic ones, are soilborne
and associated with keratin-containing
substrates such as hair, hooves, feathers and
the dung of carnivores (Hubalek, 2000). An
excellent review of the order has been compiled
by Currah (1985); Geiser and LoBuglio (2001) and
Sugiyama et al. (2002) have discussed phylogenetic
aspects.
11.3.1 Onygenaceae
This family contains 22 genera and 57 species
and includes the most important human pathogens.
The anamorphic states are arthrosporic
with rhexolytic secession (e.g. Malbranchea;
Fig. 11.3f), or solitary terminal spores are produced
which may be unicellular (Chrysosporiumlike)
or multicellular. The ascospores carry
ornamentations (spines, pits or reticulations).
The gymnothecia often have a few conspicuously
large coiled hyphae (see Fig. 11.3a). Kwon-Chung
and Bennett (1992), de Hoog et al. (2000a) and
Sigler (2003) have given accounts of the most
important pathogens; these are associated
with the teleomorph genus Ajellomyces (Guého
et al., 1997), although gymnothecia are seldom
formed and the species are better known by
their anamorphic names. Histoplasma capsulatum,
Blastomyces dermatitidis and Paracoccidioides
brasiliensis are particularly closely related to
each other, and this grouping has been given
family status by Untereiner et al. (2004), with
Coccidioides immitis being less closely related and
retained in the Onygenaceae. We shall consider
these four pathogenic species together because
of their medical importance. It is not permitted
to work with them in standard laboratories
because they are among the handful of fungi
currently listed in hazard category 3 (Kirk et al.,
2001). Teleomorph genera other than Ajellomyces
are Auxarthron, Amauroascus and Onygena; the
latter, being the type of the family, is also briefly
considered (p. 293).