Introduction to Fungi, Third Edition

512 BASIDIOMYCOTA
Fig18.21 Phylogenetic tree of the
Basidiomycota based on small
nuclear (18S) rDNA gene analyses.
The various taxonomic groups and
the chapters covering them are
indicated. Redrawn and modified
from Nishida and Sugiyama (1994),
with permission from Mycoscience.
postulated that, in evolutionary terms, basidiospores
are the equivalent of ascospores whose
development has become external instead of
taking place endogenously. Clamp connections
(Fig. 18.11), characteristic of dikaryotic hyphae
of basidiomycetes, are seen as homologous to the
croziers in ascogenous hyphae (see Fig. 8.10);
both have the same function of re-distributing
nuclei. Tehler et al. (2003) have indicated the
strength of their belief that the two groups are
closely related by classifying them together in
the Dikaryomycota. They have suggested that
this group is a sister group to the Glomeromycota
(treated in this book within the Zygomycota
as the order Glomales, see Section 7.6). Ascomycetes
and basidiomycetes probably diverged from
zygomycetes some 400 600 million years ago
(Berbee & Taylor, 2001).
Basidiomycetes have a long history. Fossil
records show that the characteristic feature of
basidiomycete hyphae, the clamp connection,
already existed some 300 million years ago in
the Carboniferous period (Dennis, 1970). Clearly
recognizable mushroom basidiocarps have been
preserved in amber about 90 94 million years
old (Hibbett et al., 1997a). This also contained
well-preserved basidiospores with prominent
hilar appendices.
18.12 Classification
The classification which we have chosen follows
that proposed by McLaughlin et al. (2001). They
have divided the phylum Basidiomycota into
four classes:
1. Homobasidiomycetes. Fungi with holobasidia,
e.g. agarics and polypores.
2. Heterobasidiomycetes. Fungi with heterobasidia,
i.e. jelly fungi and their allies.
3. Urediniomycetes. Rust fungi.
4. Ustilaginomycetes. Smut fungi.