Introduction to Fungi, Third Edition

21
Heterobasidiomycetes
21.1 Introduction
The class Heterobasidiomycetes is approximately
synonymous with the terms ‘Phragmobasidiomycetes’
or ‘jelly fungi’ and contains fungi with
the following characteristics.
1. The dolipore septum is complex, i.e. it is
surrounded by a parenthesome. Parenthesomes
are also found in the Homobasidiomycetes (Chapters
19 and 20), but not in the Urediniomycetes
(Chapter 22) and Ustilaginomycetes (Chapter 23).
2. The basidia of Heterobasidiomycetes may
be strongly lobed and often divided by transverse,
oblique or longitudinal septa. Such basidia are
loosely termed heterobasidia, especially if they
arise directly from hyphae instead of teliospores
as in most Urediniomycetes and Ustilaginomycetes.
If the basidia are septate, they are also
called phragmobasidia. The sterigma of the
heterobasidium is unusually prominent and is
often termed epibasidium (Martin, 1945). In
contrast, the basidia of Homobasidiomycetes
are club-shaped and always single-celled.
3. The fruit bodies of most Heterobasidiomycetes
are simpler in architecture than those of
Homobasidiomycetes, and the hymenium is not
normally protected by a roof- or shelf-like architecture.
In compensation, these simple fruit
bodies are generally able to survive drying and
rehydration, with fresh crops of basidiospores
produced after each rehydration event. Fully
hydrated basidiocarps are typically greatly
swollen and gelatinous, hence the term ‘jelly
fungi’ for the Heterobasidiomycetes.
4. The basidiospores of most species are
capable of producing secondary spores which
may be ballistoconidia, passively released conidia
or yeast cells.
Species included in this class show considerable
morphological diversity, and taxonomic
concepts have been in a state of flux. The first
workers to emphasize the importance of basidial
morphology were Patouillard (1887) and
Brefeld (1888). Most orders currently included
were placed here by Martin (1945) and Bandoni
(1984), and these are listed in Table 21.1.
The inclusion of these groups is supported
by DNA-based phylogenetic studies (Weiss &
Oberwinkler, 2001). The life cycles of Heterobasidiomycetes,
as far as they are known, show
an alternation of monokaryotic and dikaryotic
stages. The two broad subclasses, Heterobasidiomycetidae
and Tremellomycetidae, can be distinguished
by their monokaryotic phase being
mycelial or yeast-like, respectively. All heterobasidiomycete
yeasts discussed in Chapter 24
seem to belong to the Tremellomycetidae
(Wells & Bandoni, 2001). Thorough and authoritative
circumscriptions of the Heterobasidiomycetes
have been written by Wells (1994) and Wells
and Bandoni (2001).
Ecologically, Heterobasidiomycetes are associated
with wood and other decaying plant
matter, either as saprotrophs or as mycoparasites
of saprotrophic fungi. Some species, especially
in the Ceratobasidiales (Section. 21.2), play dual
roles as necrotrophic pathogens of various plants
and mycorrhizal associates of orchids. Heterobasidiomycetes
are mainly terrestrial.