Introduction to Fungi, Third Edition

LECANORALES
455
listed in Table 16.1, summarizing data from
Tehler (1996), Kirk et al. (2001), Ott and Lumbsch
(2001) and Grube and Winka (2002). The orders
Dothideales (Section 17.3), Hypocreales (Section
12.4) and Helotiales (Chapter 15) are not considered
here because they contain only a small
proportion of lichenized species. Some small
families of lichenized fungi (e.g. Baeomycetaceae,
Icmadophilaceae, Umbilicariaceae) are incertae
sedes at the moment, and their accurate placement
will be determined in further studies. Such
studies will have to be based on the combined
analysis of several different genes in order to
obtain a greater degree of confidence in the
resulting phylogenetic trees (Myllys et al., 2002).
Most results so far have been obtained with the
small subunit (18S) ribosomal RNA sequence.
The data summarized in Table 16.1 are too
diffuse to be fully understood at present, but we
note in passing that the occurrence of fissitunicate
asci is not always correlated with ascolocular
development (see p. 459 for an explanation),
i.e. that fissitunicate asci can be found in
perithecia and apothecia, not just pseudothecia
(see Chapter 17). Further, many orders, and
especially the Lecanorales, produce bitunicate
but non-fissitunicate asci, as do some other
Ascomycota, e.g. Helotiales and Pezizales (see
pp. 414. and 429).
16.3 Lecanorales
Members of the Lecanorales produce inoperculate
asci in apothecia. The asci are bitunicate but
non-fissitunicate. The ascus apex is thickened,
and ascospores are discharged when the outermost
wall layer breaks and the innermost layer
protrudes through the pore thus generated, to
produce an apical beak called rostrum (Figs.
8.12e,f). Rostrate ascus dehiscence is typical of
the Lecanorales.
Over 75% of all lichenized fungi belong to
this order, making it one of the largest in
the Ascomycota. Most of the best-known and
most readily collected lichens belong to the
Lecanorales. Only a few non-lichenized members
of the Lecanorales are known; these are usually
lichenicolous. This order has been divided into
42 families (Kirk et al., 2001), of which many still
have an uncertain phylogenetic position and are
currently being circumscribed by DNA sequence
comparisons. Stenroos and DePriest (1998) have
identified five suborders which together make
up a monophyletic order. We shall consider just
a few representatives to indicate the astonishing
morphological and ecological variability of lecanoralean
lichens.
16.3.1 Lecanora
About 300 species of Lecanora have been described,
mainly from temperate climates. Thalli
are usually crustose and are very common on
rock surfaces, including ancient monuments,
dry stone walls and roof tiles, as well as on the
bark of trees. The photobiont usually belongs to
the genus Trentepohlia. We have already come
across L. conizaeoides as a particularly SO 2 -tolerant
species (p. 454). Because of their exposed habitats,
Lecanora spp. often deposit light-screen
pigments in their upper cortex which give
them a bright yellow coloration. An example is
the xanthone lichexanthone; usnic acid and
pulvinic acid derivatives have also been detected.
The quantity and diversity of pigments may be
greater in thalli exposed to higher levels of
irradiation (Obermayer & Poelt, 1992). A common
example of the genus is L. muralis, showing a
typical crustose thallus with apothecia (Plate 8a).
The thalli of Lecanora (Sphaerothallia) esculenta,
a species found from northern Africa to western
Central Asia, may roll up and become detached
upon maturity, being blown about by the wind.
They are said to be edible. On occasions, windborne
lichen thalli have been so abundant that
the common name ‘manna lichen’ has been
coined for L. esculenta (Richardson, 1988).
16.3.2 Xanthoria
The most abundant species is X. parietina, which
forms bright yellow foliose thalli (Plate 8c) on the
surface of rocks, roofs, trees and farm buildings,
especially near the sea. It is particularly common
in places enriched by manure, e.g. dust from
cattle yards, or from birds. The thallus is lobed
and is attached to the substratum by short
rhizinae. The photobiont is the green alga