Introduction to Fungi, Third Edition

ONYGENALES
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infected larvae rather than hygienic behaviour
per se which is instinctive (Masterman
et al., 2001). Larvae become infected by A. apis
by ingestion. Many types of commercially available
honey contain viable spores of A. apis
(Anderson et al., 1997).
11.3 Onygenales
This order of the Plectomycetes is of utmost
significance to medical mycologists because it
contains most of the true human pathogens, i.e.
fungi able to cause disease in otherwise healthy
and immunocompetent individuals. Some taxonomic
confusion has arisen because many of the
serious pathogens have been known for a long
time only in their anamorphic form and continue
to be called by their anamorphic names. The
current Dictionary of the Fungi (Kirk et al., 2001)
recognizes three families Arthrodermataceae,
Gymnoascaceae and Onygenaceae but it
excludes the family Myxotrichaceae which is of
uncertain placement (incertae sedis), possibly
belonging to the Helotiales (Tsuneda & Currah,
2004). Since members of this last family have
many features in common with the other three,
we will consider them briefly here. Including the
Myxotrichaceae, there are some 120 species in the
Onygenales.
Defining features of the Onygenales are that
their ascoma consists of loosely interwoven and
often thick-walled hyphae which sometimes bear
complex and species-characteristic appendages
(Figs. 11.3a, 11.5, 11.8, 11.9). Such a cage-like
ascoma is termed gymnothecium, and the
meshwork of hyphae making up the basket
(peridium) is called reticuloperidium. Greif
and Currah (2003) have shown that the reticuloperidium
can be pierced by the stiff hairs of
arthropods such as flies, and gymnothecial
appendages may also be caught by the limbs
of flies during grooming. Movements by the
animals shake the ascospores out of the
Fig11.3 Onygenaceae. (a) Quarter-segment of a gymnothecium of Ajellomyces capsulatus showing coiled appendages. (b) Ascospore
of A. capsulatus. (c) Tuberculate macroconidia of Histoplasma capsulatum. (d) Microconidia of H. capsulatum. (e) The‘pilot wheel’ stage
of Paracoccidioides brasiliensis. One giant yeast cell is producing several buds. (f) Malbranchea-type arthroconidia.The conidia are
released by rhexolytic secession, i.e. conidia are spaced apart by sterile cells which eventually disintegrate. (a,c e) to same scale.
Redrawn and modified from de Hoog et al. (2000a), with kind permission of Centraalbureau voor Schimmelcultures.