Introduction to Fungi, Third Edition

14
Hymenoascomycetes: Pezizales (operculate
discomycetes)
14.1 Introduction
The order Pezizales contains the operculate
discomycetes which are the most readily recognized
cup fungi. The order is large, containing
some 15 families, about 160 genera and over
1100 species (Kirk et al., 2001). Most are terrestrial
and saprotrophic on soil, burnt ground, decaying
wood, compost or dung, but some form sheathing
mycorrhiza (ectomycorrhiza) with trees
(Maia et al., 1996). A somewhat exceptional
case is Rhizina undulata, which causes root rot
of conifers in plantation situations, usually starting
from areas affected by recent fires (Callan,
1993). Whilst most species of Pezizales produce
epigeous fruit bodies above ground level and
have active ascus discharge mechanisms with
wind-dispersed ascospores, the truffles (e.g. Tuber
and Terfezia) form subterranean (hypogeous)
ascomata. The dispersal of truffles relies on
the ripe ascomata being eaten by rodents and
other mammals attracted by their strong odour.
The ascospores survive digestion and defaecation.
There are also aquatic Pezizales, growing on
wood in streams or other wet places. An overview
of the Pezizales may be found in Pfister and
Kimbrough (2001). Keys to genera are given
by Korf (1972) and Dissing et al. (2000).
The ascocarp is generally an apothecium
(p. 245) which can range in diameter from less
than one millimetre to several centimetres. It is
often cup-shaped or disc-like, fleshy, sometimes
stalked, and frequently brightly coloured.
The asci are, in most cases, cylindrical with
a well-defined lid called operculum (see
pp. 239 241) which is the characteristic feature
of the Pezizales. Members of this order are
therefore often referred to as ‘operculate discomycetes’.
The asci are interspersed by filamentous
paraphyses, the tips of which often contain
carotenoids giving the apothecia their striking
yellow, orange or red colours. Several unusual
carotenoids are known in nature only from
apothecia of Pezizales (Gill & Steglich, 1987).
The ascus wall appears distinctly two-layered
under the light microscope, but the two layers do
not separate during ascospore discharge as they
do in functionally bitunicate (i.e. fissitunicate)
ascomycetes (see p. 239). The ascus of the
Pezizales is thus bitunicate but non-fissitunicate.
In many Pezizales the ascus wall is amyloid, i.e. it
stains blue or purple with Melzer’s iodine
(an aqueous solution of iodine and KI). The
blue-staining properties are associated with an
outer mucilaginous layer which may extend for
the whole length of the ascus or may be confined
to an apical region (Samuelson, 1978a,b). The
presence or absence of the amyloid staining
property may help in distinguishing certain
genera (see Hansen et al., 2001). The septal pore
formed at the base of the ascus has characteristic
features which may be useful in classification
(Kimbrough, 1994). The ascospores are colourless
to reddish-brown, globose to ellipsoidal, unicellular
(i.e. non-septate) and may be uninucleate,