Introduction to Fungi, Third Edition

460 LOCULOASCOMYCETES
Fig17.1 Sexualreproductionin Pleosporascirpicola. (a) Young ascus withundifferentiated cytoplasm.Note the thick ascus wall.
(b)Maturingasciwithcytoplasmcleavedintoascospores.(c)Matureascuswithmuriformascospores.Theascusis shownafewseconds
before discharge; the ectotunica hasruptured (arrow), and themore flexible endotunica hasbeenforced to expandby theincreasing
turgor pressure. (d) Discharged ascospore mountedin Indian ink to show the mucilaginous extensions. Allimages to same scale.
(Berbee, 1996; Dong et al., 1998; Silva-Hanlin &
Hanlin, 1999). Molecular analyses based on a relatively
small number of representatives of ascomycete
groups suggest that the most closely related
groups are Pyrenomycetes (perithecioid fungi)
belonging to Diaporthales, Hypocreales, Microascales
and Sordariales (see Chapter 12; Spatafora,
1995). Several orders are currently included in
the Loculoascomycetes (Barr & Huhndorf, 2001;
Kirk et al., 2001) but we shall consider representatives
only of the two most important, the
Pleosporales and Dothideales. Because the
arrangement of orders and especially families
is still in a state of flux, we shall not discuss it.
17.2 Pleosporales
This is a large group of ascomycetes believed to
be monophyletic on the basis of molecular
evidence (Berbee, 1996). Several economically
important genera of plant pathogens are
included, such as Cochliobolus, Phaeosphaeria and
Pyrenophora, parasitic on grasses and cereals.
Pleospora, Lewia and Leptosphaeria are common
endophytes, saprotrophs or parasites of other
herbaceous plants. They and their anamorphs
may also have significance as human allergens,
as human pathogens and in the production
of mycotoxins. Sporormiella fruits on herbivore
dung.
The development of pseudothecia in these
forms conforms to the Pleospora type of Luttrell
(1951). Ascogonia arise within a stroma and,
in the region of the ascogonia, a group of
vertically arranged septate hyphae appears,
each hypha arising as an outgrowth from a
stromatal cell. These hyphae are capable of
elongating by intercalary growth and are
termed pseudoparaphyses (Luttrell, 1965).