Introduction to Fungi, Third Edition

GENERAL ASPECTS OF LICHEN BIOLOGY
451
A pre-thallus can be formed by fusion of
several genetically distinct isidia, soredia or
photo- and mycobionts, and likewise several
pre-thalli can fuse in the process of thallus
maturation. Thus, a mature thallus may contain
a jigsaw of genetically heterogeneous myco- and
photobionts (Fahselt, 1996). In other cases, e.g.
the map lichen Rhizocarpon geographicum, the
borders between incompatible thalli are demarcated
by black barrage lines. In some lichens with
a green alga as photobiont, the situation is
further complicated by the inclusion of a second
(cyanobacterial) photobiont. This is then usually
confined to discrete regions termed cephalodia
which often differ in their morphology from the
thallus containing the green photobiont (Fig.
16.4).
16.2.4 Lichenicolous fungi
The capture of a compatible photobiont partner
by a germinating fungus spore can be problematic,
especially if the photobiont belongs to the
genus Trebouxia which does not seem to be
widespread as a free-living organism. One solution
to the problem is the recovery of photobionts
from the propagules of other lichen
species. Certain photobiont strains are favoured
by many taxonomically unrelated mycobionts
(Rikkinen et al., 2002). If these lichens grow
in similar ecological situations, communities are
formed which have a high mycobiont diversity
but share the same or closely related photobiont
strains. Thus, the chances for a germinating
spore to salvage compatible photobionts from
soredia or isidia of other lichen species may be
quite high (Rikkinen, 2003).
Several mycobionts have taken the ultimate
step of poaching their photobiont from an
existing lichen thallus in order to establish
their own independent thallus (Ott et al., 1995).
Such organisms are called lichenicolous lichens,
and the phenomenon has been aptly named
‘cleptobiosis’ (Honegger, 1993). Numerous other
fungi feed on the photosynthetic products of a
lichenized photobiont without ever establishing
an independent thallus, while yet others destructively
parasitize the host lichen (Rambold
& Triebel, 1992; Richardson, 1999; Lawrey &
Diederich, 2003). Such fungi are collectively
called lichenicolous fungi, and they are often
taxonomically related to lichenized fungi.
16.2.5 The nutritional basis of the
lichen symbiosis
Hill and Smith (1972) devised a simple and
elegant method termed the ‘inhibition technique’
which has permitted the identification of
carbohydrates secreted by the photobiont. Lichen
thalli are exposed to radiolabelled CO 2 , and
after a while an excess of a single unlabelled
Fig16.4 Placopsisgelida, a foliose lichen.The
brightly coloured main thallus containing the
primary (algal) photobiont has produced a
dark central gall-like cephalodium in which
the secondary (cyanobacterial) photobiont is
localized. From Bu«del and Scheidegger (1996),
by permission of Cambridge University Press.
Original image kindly provided by
C. Scheidegger.