Introduction to Fungi, Third Edition

19
Homobasidiomycetes
19.1 Introduction
Fungi included in the Homobasidiomycetes
possess holobasidia, in contrast to the heterobasidia
(phragmobasidia) of the
Heterobasidiomycetes, rusts and smuts
(see Chapters 21 23). The traditional classification
of the Homobasidiomycetes, founded by the
nineteenth century Swedish mycologist Elias
Fries, was based on a number of different
arrangements of the hymenium on the hymenophore.
The most common types, shown in
Fig. 19.1, are (A) agaricoid, i.e. gill-bearing
(lamellate); (B) poroid, i.e. bearing pores instead
of gills; (C) hydnoid, i.e. with a toothed or spiny
hymenium; (D) clavate, with a club-shaped
or coralloid fruit body, the outside of which
is covered by the hymenium; (E) resupinate,
i.e. with a flattened (corticioid) hymenium
appressed to the underside of solid surfaces;
and (F) epigeous or (G) hypogeous gasteroid or
secotioid (non-ballistosporic) hymenophores
(see Chapter 20). It has long been suspected
that the different hymenial arrangements have
evolved separately in unrelated fungal groups,
i.e. that they represent examples of convergent
evolution. They can be interpreted as different
ways of maximizing the hymenial area for a
given amount of fungal tissue (Pöder, 1983;
Pöder & Kirchmair, 1995). Examples of similar
hymenophore arrangements in unrelated fungi
are seen in the tubular hymenia characteristic
of Boletus (Fig. 19.21) and Trametes (Fig. 19.26)
or the toothed hymenia found in the
homobasidiomycete Hydnum (Fig. 19.1c) and the
heterobasidiomycete Pseudohydnum (Fig. 21.9b).
Quite possibly, only one or a few genes are
involved in the morphogenetic events resulting
in these various hymenial structures, as has been
shown for the transition of gill-bearing fungi to
gasteromycetes (see pp. 578 580). There is much
other evidence, e.g. in fruit body construction,
ultrastructure, chemical reactions of basidiocarps,
basidium cytology, spore colour and
morphology, and molecular sequence data to
support the view that gross morphological
characters of hymenophores are unsatisfactory
criteria on which to base a natural classification
system.
Hibbett et al. (1997b) and Hibbett and Thorn
(2001), in their preliminary attempts at a
more natural classification, found the
Homobasidiomycetes to be distributed amongst
eight phylogenetic clades (Fig. 19.2), and this
scheme has been confirmed and extended in
subsequent work (e.g. Binder & Hibbett, 2002;
Moncalvo et al., 2002). The eight clades may
ultimately be given the status of orders, and
although it is tempting to use some of the
existing order names in synonymy, e.g.
Agaricales for the euagarics clade, the necessary
emendations of orders have not yet been
carried out. Therefore, we shall use the clade
system for the time being. One serious limitation
of it is that all large-scale attempts at
Homobasidiomycete phylogeny undertaken to
date have been based on nuclear and mitochondrial
ribosomal DNA, and confirmation by
comparing other genes will be required before