Introduction to Fungi, Third Edition

AURICULARIALES
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of basidiospores in C. viscosa. Freshly discharged
basidiospores are aseptate (Fig. 21.5b) but they
soon develop one septum. Further development is
by direct germination or formation of globose
microconidia from basidiospore segments and
from haploid monokaryotic hyphae (Figs. 21.5c,d),
as described above for D. stillatus.
21.4 Auriculariales
The order Auriculariales has been subject to
numerous taxonomic rearrangements. As currently
understood, its members produce both
mono- and dikaryotic mycelia with dolipores
and parenthesomes lacking perforations (see Fig.
18.10b). Basidia are septate (Wells, 1994; Wells &
Bandoni, 2001). The Auriculariales are distinguishable
from the Tremellales (Section 21.5)
which have yeast-like monokaryotic stages, and
from the Ceratobasidiales and Dacrymycetales
which have aseptate basidia. Although taxonomic
adjustments continue to be made, there is now
little doubt that the order Auriculariales should
contain both Auricularia with its transversely
septate basidia, and Exidia and Pseudohydnum,
which have basidia with longitudinal septa,
the so-called tremelloid basidia (Weiss &
Oberwinkler, 2001).
There are great variations in fruit body size
and shape. Clamp connections may be present
or absent, depending on species. Most species
have a bifactorial mating system with multiple
alleles at both loci (Wells, 1987, 1994; Wong &
Wells, 1987; Wong, 1993). Depending on environmental
conditions, basidiospores are typically
able to germinate in several different ways,
e.g. by repetition (ballistospore formation), as
hyphae, or by forming sickle-shaped (lunate)
microconidia. Members of the Auriculariales
are saprotrophic on wood, causing intensive
white-rots (Worrall et al., 1997).
21.4.1 Auricularia
The Jew’s ear fungus A. auricula-judae forms
rubbery, ear-shaped fruit-bodies on branches of
elder (Sambucus) (Plate 11h) and is a weak
pathogen, growing on the wood and pith of
living branches and on dead wood. A wide range
of other hosts has been reported, on which
A. auricula-judae causes a rapid white-rot similar
to that produced by members of the polyporoid
clade (see Plate 10a; Worrall et al., 1997).
A section through the flesh of a fruit body
shows a hairy upper surface, a central gelatinous
layer containing narrow clamped hyphae, and
a broad hymenium on the lower side (Fig. 21.6a).
Details of basidiocarp anatomy are useful in
classification (Lowy, 1952). The fruit body can dry
to a hard brittle mass, but on wetting it quickly
absorbs moisture and discharges spores within
a few hours. The basidia are cylindrical and
become divided into four cells by three transverse
septa (Fig. 21.6b). Each cell of the basidium
develops a long cylindrical epibasidium which
extends to the surface of the hymenium and
terminates in a conical sterigma bearing a
basidiospore which is monokaryotic. At 20 mm
or more in length, the Auricularia basidiospore
is one of the largest objects propelled by the
surface tension catapult mechanism (see Pringle
et al., 2005), and ballistospore discharge is easily
observed with thin slices of basidiocarp material
placed sideways on water agar (see Webster &
Hard, 1998b). Auricularia auricula-judae is heterothallic
with a bifactorial (tetrapolar) mating
system, and there are indications of multiple
alleles (see Wong, 1993).
Germination of the basidiospore can proceed
in several different ways (Fig. 21.6c), and such a
variability is common in the Auriculariales
(Ingold, 1982a, 1984b). Washings from the hymenial
surface of fruit bodies contain basidiospores
undergoing repetitious germination by means
of a sterigma which produces another ballistospore.
Ingold (1982a) interpreted this as a second
chance for the spore to get away from the fruit
body. Basidiospores alighting on a nutrient-poor
surface such as tap water agar lay down three
transverse septa, and each of the resulting four
cells may emit one or more extensions (denticles)
which produce a cluster of lunate microconidia.
Alternatively or additionally, germination may
occur directly by means of a germ tube, and
this mode of germination is found especially
on slightly richer media such as cornmeal agar.
Septa are laid down, the first one bulging