Introduction to Fungi, Third Edition

TREMELLALES
605
conjugation of compatible yeast cells. The mating
system has been described as ‘modified tetrapolar’
(Bandoni, 1963) because there are only two
alleles at the A locus but multiple alleles at B.
This system is uncommon in the Basidiomycota,
which usually have multiple alleles at both loci,
aiding outbreeding (see p. 507). The modified
tetrapolar system of Tremella is, however, also
found in Ustilago maydis, and the designation
of loci is equivalent (p. 643). Thus, the A locus
controls conjugation and the B locus growth of
the resulting dikaryon. Both A alleles of Tremella
encode peptide-type hormones (Sakagami et al.,
1981; Ishibashi et al., 1984) and receptors for
the hormone of the opposite mating type. In
T. mesenterica, these hormones are linear peptides
called tremerogen A-10 (12 amino acids) and
tremerogen a-13 with 13 amino acids. Both
peptides are derivatized with a farnesyl unit.
Mating occurs by formation of conjugation tubes
which requires the presence of the hormone
of the opposite mating type; each hormone
is produced constitutively, irrespective of the
presence or absence of a compatible mating
partner (Bandoni, 1965). Yeast cells with opposite
alleles at A but like B alleles will conjugate but
fail to initiate dikaryotic hyphal growth.
The fruit bodies of Tremellales are usually
formed on wood, often in association with those
of other fungi (Asco- and Basidiomycota) or
with lichen thalli, which may be parasitized
(Diederich, 1996; Chen, 1998). Parasitism is by
intimate hyphal contact via haustorial branches
(Figs. 21.10b,c; see below). The fruit bodies of
Tremellales are highly variable in size, ranging
from a limited hymenium on the mycelium of
putative hosts to large structures (several centimetres)
surrounding host basidiocarps or growing
near them. Although often considered as
saprotrophs, no special capacity to degrade wood
seems to have been recorded.
Accounts of the Tremellales have been written
by Bandoni (1987), Bandoni and Boekhout
(1998) and Chen (1998).
21.5.1 Tremella
This is a large genus of some 80 species. Detailed
descriptions of representatives of all species
groups have been given by Chen (1998). One of
the commonest and most thoroughly examined
species is T. mesenterica, whose yellow or orange
gelatinous fruit bodies are readily seen on
various woody hosts such as oak, willow, gorse
and beech (Plate 11i). Variations in the intensity
of fruit body coloration could be due to a
stimulation of carotenoid synthesis by high
light intensity because fruit bodies exposed to
sunlight are often more deeply coloured than
those in the shade (Wong et al., 1985). The fruit
bodies of T. mesenterica are usually associated
with those of Peniophora in the field, and
Zugmaier et al. (1994) have shown that hyphae
of Peniophora spp. are parasitized in vivo and in
vitro. Several Tremella spp. are more obviously
mycoparasitic than T. mesenterica. For instance,
T. globospora produces its fruit bodies within the
perithecia of Diaporthe, and T. encephala overgrows
the fructifications of its host, Stereum, which
remain as a firm core in the Tremella basidiocarp.
There are also several other genera within the
Tremellales which parasitize other fungi (Bauer,
2004; Figs. 21.10b,c).
Parasitism is mediated by modified hyphae
which Olive (1947) called ‘haustorial branches’.
They consist of a swollen binucleate hyphal
segment, delimited at its base by a clamp
connection which puts forward one or several
long thin filaments (Fig. 21.11a). The association
of the swollen segment with the term ‘haustorium’
is unfortunate because the haustorial
branch is formed outside the host cell. Where
‘haustorial filaments’ of Tremella or related
fungi contact the hypha of a suitable host, the
host wall is dissolved and a micropore is formed
which establishes direct cytoplasmic contact
between the filament and the host, apparently
by fusion of the two plasma membranes
(Figs. 21.10b,c).
The life cycle of T. mesenterica is complex and
not yet fully understood (Fig. 21.12). Haploid
basidiospores are discharged from long epibasidia
by the surface tension catapult mechanism.
Basidiospores failing to escape from the hymenium
can germinate by repetition to form
ballistoconidia (Ingold, 1982b). When landing on
a suitable substrate, the basidiospore germinates
by forming several buds which remain attached