Introduction to Fungi, Third Edition

448 LICHENIZED FUNGI (CHIEFLY HYMENOASCOMYCETES: LECANORALES)
Fig16.1 SEM view of a section of
the stratified fruticose thallus of
Anaptychiaciliaris.The globose cells
of the photobiont (Trebouxia)are
located in a loose layer underneath
the thick mucilaginous cortex
produced exclusively by the
mycobiont hyphae. From Bu«del &
Scheidegger (1996), by permission
of Cambridge University Press.
Original image kindly provided by
C. Scheidegger.
and a medulla, but lacking an inner cortex
(Fig. 16.1).
16.2.2 Reproduction of lichens
Sexual reproductive features of lichenized ascomycetes
are largely equivalent to those found
in non-lichenized groups. Both homothallic
and heterothallic mating systems are known.
Apothecia with inoperculate asci are especially
common, but perithecia and pseudothecia also
occur. In contrast, no cleistothecia or apothecia
with operculate asci are known among lichenized
fungi. The taxonomic affinities of the
various orders of lichenized ascomycetes are
summarized in Table 16.1. Conidial states, if produced,
are often pycnidial. In most cases, the
photobiont is excluded from the fertile regions
of the mycobiont so that ascospores or conidia
are released without photobiont cells. A new
lichen thallus can be established from fungal
spores only by re-lichenization with suitable
photobiont cells. It is unclear how frequent this
is in nature.
Most lichen thalli therefore produce vegetative
propagules containing both symbionts.
These can be very variable, and an extensive
terminology has evolved to describe them (see
Büdel & Scheidegger, 1996). The most common
vegetative propagules are soredia, i.e. small
clumps of hyphae enclosing a few algal cells
(Fig. 16.2). They are produced over the entire
surface of the thallus (Plate 8e; Fig. 16.8) or in
differentiated structures called soralia. Soredia
are usually hydrophobic and are dispersed by
wind, perhaps following their initial detachment
by the impact of a rain drop. Isidia are larger,
upright cylindrical structures which contain
both symbionts. They serve to increase the
surface area of the lichen thallus but can also
become detached and then function as vegetative
propagules. In some lichens such as Cladonia,
squamules broken off a vegetative thallus are
capable of establishing a new thallus. Animals
can also play a role in dispersing lichens. Meier
et al. (2002) have shown for Xanthoria parietina
(Plate 8c) that mites feeding on lichen thalli can
spread both the mycobiont and photobiont via
their faecal pellets. Since this lichen does not
produce soredia or isidia, dispersal by invertebrates
could be significant.
16.2.3 Establishment of a lichen thallus
A germinated mycobiont spore in nature may
be able to survive for a while as a mycelium
of limited spread, or it can undergo a loose
association with free-living algae not suitable for
an intimate and permanent lichen symbiosis
(Ahmadjian & Jacobs, 1981; Ott, 1987). The initial
stage of the lichen symbiosis is therefore nonspecific.
It results in mycobiont hyphae making
contact with and growing around individual
photobiont cells (Figs. 16.3a,b).