SYMBIOSIS - Cyanobacteria - page 364

and desiccation, the walls of the air cavities are lined by the fungal hyphae that are covered by the
rodlet layer indicating the presence of hydrophobins. Three genes DGH1 , DGH2 and DGH3 governing
the production of hydrophobins DGH1, DGH2, and DGH3, respectively have been identifi ed in D .
glabratum . Of these three hydrophobins, DGH1 is of 14-kDa and most abundant protein. The N-terminal
sequence of this protein has been used to carry out cDNA cloning by RT-PCR. The co-amplifi cation
of cDNA fragments that encode hydrophobins DGH2 and DGH3 was also possible when the cDNA
encoding the signal peptide was cloned by RACE-PCR. These three hydrophobins also share 54-66%
amino acid identity (Trembley et al ., 2002a). The differential expression of these three genes depended
on age and location of the particular hydrophobin in the thallus.
xi) Co-speciation versus algal switching : Ahmadjian (1987) advocated the hypothesis of co-evolution
for lichens especially when one or both symbionts appear obligate and specialized. However, this
hypothesis has not been put to test. Thompson (1994) suggested that co-evolution directly requires
an assessment of increased fi tness resulting from genetic change. Another alternative suggested for
co-evolution is to indirectly demonstrate parallel cladogenesis or co-speciation of symbiont lineages
(Page and Hafner, 1996). This can be an acceptable proposition if there are highly specifi c associations
between the algal and fungal partners to the extent the algal partner is vertically transmitted
throughout the same fungal lineage. If horizantal transfer of algal partners through a fungal lineage
takes place then the concept of cladogenesis or co-speciation could be rejected. Evidences in support of
horizantal transfer of algal partners through fungal lineages was also designated as “algal switching”
by Piercey-Normore and DePriest (2001) who tested this by using nuclear ITS phylogenies of algal
and fungal partners from 33 natural lichen associations of Cladoniaceae predominantly harbouring
the green alga Asterochloris . Random amplifi ed polymorphic DNA analysis with 23 primers showed
little polymorphism among Asterochloris assemblages suggesting that a low level of variation exists
across the entire algal genome. This clade, designated as clade I comprising Asterochloris assemblages
also overlaps with the sequences of Trebouxia glomerata , T . irregularis and T . pyriformis along with
33 natural lichen-forming algae. It was also shown that clade I genotypes associate with lichen-
forming fungi of different orders such as Stereocaulon (Stereocaulaceae, Lecanorales), Pycnothelia
papillaria (Cladoniaceae, Lecanorales) and Anzia carnionivea (Trapeliaceae, Agyriales). On this basis
they rejected parallel cladogenesis and co-speciation and proposed that switching of highly selected
algal genotypes occurs repeatedly among lichen symbioses.
xii) Diversity of cyanolichens : McCune (1993) has developed a conceptual framework for
understanding the distribution of epiphytic lichens along with three gradients such as time, height
and moisture. This is known as 'similar gradient hypothesis' that predicts a special sequence of
successional events in lichen fl ora as the particular stand matures. Chlorolichens and electorioid
lichens are present in young regenerating stands and as the stand matures these are replaced by
cyanolichens followed by bryophytes. Cyanolichens constitute 42% of the lichen community of
Northwest old-growth Pseudotsuga - Tsuga forests. Cyanolichens include all macrolichens, mainly
populations of Lobaria oregana that are concentrated in the “light transition zone”. This extends
from about 13 to 37 m height in an overall canopy height of 50-60 m, as revealed by the vertical
stratifi cation studies of these forests (McCune et al ., 1997). Other cyanolichen representatives that
inhabit these areas are other species of Lobaria , Nephroma , Peltigera and Pseudocyphellaria . Some of
these are indicators of acidic deposition (Denison et al ., 1977; James et al ., 1977; Gauslaa, 1995) and
ecological continuity (Rose, 1976, 1988; Goward, 1994). Cyanolichens are considered to be important
source of nitrogen for forest ecosystems because of their ability to fi x atmospheric nitrogen through
their cyanobionts (Pike, 1978; Antoine, 2004) and more particularly in old-growth temperate forests