Introduction to Fungi, Third Edition

17
Loculoascomycetes
17.1 Introduction
The characteristic feature of this group is that
the ascus is bitunicate and fissitunicate; it has
two separable walls (see p. 240). The outer wall
(ectotunica or ectoascus) does not stretch readily,
but ruptures laterally or at its apex to allow
the stretching of the thinner inner layer, the
endotunica or endoascus (Figs. 17.1a c). Asci
are generally non-amyloid. The fruit body with
asci is regarded as an ascostroma, and each
cavity in which asci develop is termed a locule.
In contrast to the Hymenoascomycetes, in which
ascocarps develop following plasmogamy and
the pairing up of two genetically dissimilar
nuclei (ascohymenial development), in the
Loculoascomycetes the ascoma is already present
before the compatible nuclei are brought together
(Barr & Huhndorf, 2001). The development of
asci in pre-formed locules is called ascolocular.
The ascostroma has therefore been defined as
an aggregation of vegetative hyphae not resulting
from a sexual stimulus (Wehmeyer, 1926).
However, Holm (1959) has questioned the accuracy
of this definition, since examples are known
where the ascocarps do develop following the
pairing of nuclei (Shoemaker, 1955). Within the
developing ascocarp, one or more locules are
formed by the downgrowth of pseudoparaphyses
(see below) and the development of asci. One or
more ostioles then develop by the breakdown
(lysis) of a pre-formed mass of tissue (lysigenous
development). Where a single locule develops, a
structure resembling a perithecium results and,
although this term is commonly used for such
loculoascomycete fruit bodies, they should
strictly be called pseudothecia (see p. 245).
Although a mature ascostroma with several
locules can be superficially similar to the perithecial
stroma of Pyrenomycetes such as
Hypoxylon (Fig. 12.11) or Claviceps (Fig. 12.27), the
difference is that in the pyrenomycete stroma
each fertile region (perithecium) is surrounded
by a wall whereas the locules of the ascostroma
are not (Alexopoulos et al., 1996). However, the
ascocarp of Loculoascomycetes does not always
take the form of a pseudothecium. In some groups
it may be an apothecium, a hysterothecium (an
elongate ascoma with a slit-like opening) or a
cleistothecium (Barr & Huhndorf, 2001). These
deviations from the classical pseudothecium are
found for example in lichenized Loculoascomycetes
(see Table 16.1).
The name Loculoascomycetes was coined
by Luttrell (1955) and corresponds to the
Ascoloculares of Nannfeldt (1932). The group
has also been named the Dothideomycetidae (see
Kirk et al., 2001). It is very large, with about 900
genera and over 7000 species. Most members are
terrestrial, growing as saprotrophs, endophytes
or parasites on the shoots and leaves of herbaceous
or woody plants and may cause diseases of
economic significance, but some grow in freshwater
or the sea and others in dung or soil.
There is a very wide range of anamorphs, some
hyphomycetous, others pycnidial (Sivanesan,
1984; Seifert & Gams, 2001).
It is most unlikely that the Loculoascomycetes,
as currently classified, are monophyletic