Introduction to Fungi, Third Edition

12
Hymenoascomycetes: Pyrenomycetes
12.1 Introduction
The Pyrenomycetes are defined here according
to Samuels and Blackwell (2001) as fungi which
produce non-fissitunicate or occasionally prototunicate
asci usually in flask-shaped ascomata
(perithecia), less frequently in cleistothecia. The
sub-class Pyrenomycetes is one of several groups
belonging to the huge and heterogeneous class
Hymenoascomycetes. The characteristic feature
of this class is that the asci develop in an
ascohymenial way, i.e. the ascoma is formed after
plasmogamy and the pairing of nuclei have
occurred, and the asci therefore arise from a
hymenium. This is in contrast to asci being
formed singly (Archiascomycetes, Hemiascomycetes),
scattered throughout the fruit body (Plectomycetes),
or formed in a locule within a preformed
fruit body (Loculoascomycetes). Although
the term ‘Pyrenomycetes’ is not generally understood
in a taxonomic sense at the present,
Samuels and Blackwell (2001) pointed out the
monophyly of a core group of orders, including all
those which we shall describe in this chapter
(summarized in Table 12.1).
The development of the perithecium follows
several different schemes defined by Luttrell
(1951), which are described in more detail for
the different orders. Following fertilization and
plasmogamy, the ascogonium gives rise to
ascogenous hyphae while the perithecial wall is
formed by hyphae arising from the ascogonial
stalk or elsewhere. Sterile hyphae growing up
from the basal fertile region (paraphyses) and
periphyses which line the inner surface of the
ostiole, may be present. The development of the
opening of the perithecium is typically schizogenous,
i.e. it is formed by the pushing apart
of tissue by the periphyses at the apex of the
perithecium. This is in contrast to lysigenous
development, e.g. in the pseudothecial neck of
Loculoascomycetes (see p. 459). Perithecia may be
formed singly or in a perithecial stroma.
Ascospore discharge is by active turgor-driven
liberation or occurs passively, with the ascospores
oozing out of the perithecial ostiole as
a tendril (cirrhus).
Although the core orders belonging to the
Pyrenomycetes appear to be monophyletic, the
fungi considered here follow numerous different
lifestyles. Many species grow saprotrophically
in terrestrial habitats, while others are associated
with plants, covering a wide range from
mutualistic or commensalistic endophytes over
biotrophic pathogens through to hemibiotrophic
and necrotrophic pathogens. Animals, especially
insects, are also parasitized. Numerous biologically
active metabolites are produced, including
alkaloids, antibiotics and phytotoxins.
12.2 Sordariales
The order Sordariales is a substantial group of
ascomycetes containing some 7 families, 115
genera and over 500 species. We shall study
representatives of only 2 families, the
Sordariaceae (6 genera, 37 spp.) and
Chaetomiaceae (15 genera, 150 spp.).