Introduction to Fungi, Third Edition

594 HETEROBASIDIOMYCETES
Table 21.1. Orders and selected genera currently
included in the Heterobasidiomycetes. After Wells
and Bandoni (2001).
Subclass Heterobasidiomycetidae
1. Ceratobasidiales (see below)
Ceratobasidium (anam. Rhizoctonia ¼ Ceratorhiza).
Thanatephorus (anam. Rhizoctonia ¼ Moniliopsis).
2. Tulasnellales (see Fig. 21.1b)
Tulasnella (anam. Rhizoctonia)
3. Dacrymycetales (p. 598)
Calocera
Dacrymyces
Ditiola
4. Auriculariales (p. 601)
Auricularia
Exidia
Pseudohydnum
Sebacina
SubclassTremellomycetidae
1. Tremellales (p. 60 4)
Cryptococcus and Bullera (yeast forms;Table 24.1)
Filobasidiella neoformans (see p.660)
Tremella
2. Christianseniales
3. Filobasidiales (yeast forms; seeTable 24.1)
4. Cystofilobasidiales (seeTable 24.1)
Itersonilia (see p.493)
Phaffia, Xanthophyllomyces (see p.665)
5. Trichosporonales (yeastforms; seeTable 24.1)
21.2 Ceratobasidiales
The most important members of this family
belong to the anamorph genus Rhizoctonia which
we shall consider in detail. Sneh et al. (1996)
and Roberts (1999) have compiled important
reference works on this group. The secondary
hyphae of Rhizoctonia have conspicuous dolipore
septa which are visible even with the light
microscope (Tu et al., 1977). Electron microscopy
studies have revealed the parenthesomes to be
perforated by several large pores (Müller et al.,
1998b). Clamp connections are not found in
Rhizoctonia but may be present in other members
of the order. The hyphae of Rhizoctonia are highly
characteristic. Branches typically arise at a right
angle to the leading hypha, with the branch
point slightly constricted and a septum located a
little way into the branch (Fig. 21.2a). Depending
on the species, the compartments of vegetative
hyphae are binucleate or multinucleate; uninucleate
hyphae are uncommon. Teleomorphic
states are rare and conidia are not produced,
but sclerotia of the loose type (see Fig. 1.16a) are
frequently seen. The form-genus Rhizoctonia has
now been broken up into several taxa which
correlate with different teleomorphs (Moore,
1987; Andersen & Stalpers, 1994). For instance,
Moniliopsis has multinucleate hyphae and is
referred to the teleomorph genus Thanatephorus
whereas the hyphae of Ceratorhiza (teleomorph
Ceratobasidium) are binucleate (Tu et al., 1977;
Vilgalys & Cubeta, 1994). Fungi resembling
Rhizoctonia transgress the boundaries of
orders, with some teleomorphs referable to the
Tulasnellales. Indeed, a few Rhizoctonia-like fungi
have even been assigned to the Ascomycota.
Whereas hyphae of Rhizoctonia are readily
recognized as such, they offer few microscopic
features for species identification, and a system
based on anastomosis groups, i.e. the ability of
a given isolate to undergo plasmogamy with
hyphae of defined tester strains, has been
developed (Sneh et al., 1991). Such pairings may
yield three different responses, with intermediate
reactions also possible. (1) In genetically
identical or closely related strains, anastomosis
leads to perfect fusion. (2) In less closely related
members of the same anastomosis group, anastomosis
is followed by death of the fusion cell due
to vegetative incompatibility. (3) No anastomosis
occurs between members of different anastomosis
groups. The best-studied taxon, R. solani,
contains about a dozen anastomosis groups,
some of which have been further divided into
subgroups according to biochemical or other
criteria. Although the individual anastomosis
groups within R. solani and other taxa (e.g.
Ceratorhiza) correlate with phylogenetic clusters
obtained by DNA-based phylogeny (Kuninaga
et al., 1997; Gonzalez et al., 2001) and also to
a certain extent with the range of plant hosts