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vi) Fossil records : Hallbauer et al . (1977) described Thuchomyces lichenoides consisting of only
mycobiont that has been assigned to Precambrian period. Pelicothallos reported from Tertiary period
was in fact only a photobiont and the mycobiont was lacking in this 'lichen' (Sherwood-Pike, 1985).
Ziegler (1992) described apothecia-like structures on a lichen thallus from Triassic sediments.
Spongiophyton is another lichen thallus described from Lower Devonian period of North America
(Stein et al., 1993), that did not reveal the presence of either mycobiont or photobiont. All these
fossil records fall short of the description of true lichen symbiosis due to the absence of one or the
other partners in the thallus (Taylor and Taylor, 1993). The fi rst convincing unequivocal evidence
for the presence of a lichen symbiosis (mycobiont and photobiont together) in a new fossil genus
Winfrenatia , a cyanolichen from Rhynie Chert of the Lower Devonian, was described by Taylor et
al . (1997). The mycobiont was composed of superimposed layers of coenocytic hyphae present as a
network. The cells of a coccoid cyanobacterium with rings of mucilage are entangled in the spaces
of the net. The division of the photobiont in three planes characteristic of coccoid cyanobacteria
resulted in cell clusters presumably up to 64 cells. The presence of endospores and soredia as the
reproductive structures was also indicated. Phylogenetically, the mycobiont is suggested to be closer
to Zygomycetes while the photobiont is almost similar to Gloeocapsa and Chroococcidiopsis .
vii) Nature of symbiosis : Lichen symbiosis has been traditionally considered to be a mutualistic
association (Nash, 1996). The mycobiont is benefi ted to a large extent as it can meet nearly 70-80%
of its carbon requirements from the photobiont (Smith, 1980; Tapper, 1981). Reports on the supply of
nutrients from the mycobiont to the photobiont do not exist in literature (Hill, 1994). The argument
that the photobiont is benefi ted by getting a shelter in an otherwise hostile outer environment seems
no longer acceptable because the photobiont's growth and multiplication are very much restricted in
the lichen symbiosis than when it exists freely in nature (Honegger, 1993; Ahmadjian, 1993). These
observations generated interest in defi ning the role of mycobiont as nothing but a parasite and the
concept that lichen symbiosis is a form of controlled parasitism gained strength.
Comparative phylogenetic analysis has been carried out by matching the sequences of 16S
rRNA and 23S rRNA genes of lichenized and non-lichenized fungi of Ascomycota (representing
~75% species). These results suggest that the evolution of free-living non-lichenized Ascomycota
had taken place due to losses of the lichen symbiosis. The genera Penicillium and Aspergillus must
have originated from such lichenized mycobionts (Lutzoni et al ., 2001). By combining Bayesian
phylogenetic tree sampling methodology and a statistical model of trait evolution, Lutzoni et al .
(2001) estimated the rates of gains or losses of lichenization on each of the 19,900 phylogenetic trees
and suggested that during evolution of Ascomycota there have been at least 1.5 times as many
losses of the lichen symbiotic state than gains. The rates of loss of lichen symbiotic habit led to the
development of non-lichenized fungi called as lichenicolous fungi (that can dwell on or in lichens
as parasites, commensals or saprophytes).
On the basis of frequency of heterocysts in cyanobionts, relative proportion of green algal cells
vs cyanobacterial cells per unit of fungus as variables, a theoretical model has been predicted to
explain the cost of lichen symbiosis. The model explains that the mycobiont derives maximum benefi t
by altering the role of the photobiont in tripartite lichens. The mycobiont meets the negligible cost
of differentiating cephalodia (to accommodate the cyanobiont), thereby allowing the increase in
frequency of heterocysts so as to restrict its growth. The reported heterocyst frequencies in bipartite
(2 to 8%) and tripartite lichens (10 to 55%) reported in literature support the above predictions. The
mycobiont thus derives maximum benefi t from the cyanobiont by meeting its nitrogen requirements
whereas the green alga meets the carbon requirements (Hyvärinen et al ., 2002). These observations
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