Introduction to Fungi, Third Edition

450 LICHENIZED FUNGI (CHIEFLY HYMENOASCOMYCETES: LECANORALES)
Fig16.2 Cladonia pyxidata. Soredia containing algal cells
surrounded by fungal hyphae.
Fig16.3 Interactions between photobiontcells (Trebouxia) and mycobiont hyphae. (a,b) Early stages in the establishment of a lichen
thallus in Cladonia cristatella. (a) Single algal cell at point of contact with the mycobiont. (b) Ensheathment of algal cell by the
mycobiont. Both images redrawn and modified from SEM images of Ahmadjian and Jacobs (1981). (c) Formation of tubular
intracellular haustoria by the mycobiont of a simple crustose lichen (e.g. Lecanora conizaeoides).The same photobiont cell may be
penetrated repeatedly. (d) Formation of an intraparietal haustorium by the mycobiont of a stratified lichen. Exchange of nutrients
occurs apoplastically; the walls of both mycobiont and photobiont are surrounded by a common hydrophobin sheath produced by
the former. (c,d) schematic drawings based on the results of Honegger (1986).
If partners are compatible, a pre-thallus
is formed, i.e. a crustose non-stratified cluster
of photobiont cells ensheathed by hyphae of
the mycobiont (Trembley et al., 2002). A prethallus
is also formed by germinating isidia
or soredia. Under suitable conditions, the prethallus
enlarges and stratifies into a mature
thallus; the first sign of this step is the secretion
of mucilage by hyphae at the periphery of the
pre-thallus (Honegger, 1993).
The factors determining photobiont
mycobiont specificity and thus permitting the
development of a pre-thallus and mature thallus
are not yet known. In addition to genetic determinants,
environmental factors must also play
a crucial role as indicated by the difficulties
encountered when trying to grow pre-thalli
into fully differentiated thalli in the laboratory
(Ahmadjian, 1993). Recognition is probably a
continuous and multi-step process mediated
by surface molecules such as lectins, and facilitated
by the embedding of both bionts in a
gelatinous matrix of fungal origin (Ahmadjian,
1993).