Symbiosis and DNA methylation in the Cladonia lichen fungus
In eukaryotes, the modification of DNA by addition or removal of specific methyl groups is thought to affect gene activity and differentiation. We began to investigate the relationship between DNA methylation and differentiation in lichens, organisms in which the symbiosis between a fungus and a unicellular alga or cyanobacterium generates remarkable morphological and biochemical complexity. Restriction analysis of DNA from the lichen Cladonia grayi indicated that overall DNA methylation is low in the fungus cultured in absence of the alga and high in the natural lichen. Within the lichen, however, fungal DNA methylation is not uniform: it is high in the body of the goblet-shaped thalli (podetia) and in the vegetative propagules (soredia), somatic tissues in which the fungus is associated with the alga; it is low in the fungal ascocarps (apothecia) which develop without algae on the upper rim of the podetia and produce meiotic spores. Methylation remains low in the mycelia derived from spores in axenic culture. These results suggest a correlation between symbiosis and methylation of fungal DNA in Cladonia. DNA methylation was also observed in two other lichens tested, a Parmelia and an Usnea. To relate the overall genomic changes found in Cladonia to the behavior of individual genes, we evaluated through Southern blotting the methylation of four fungal genes presumably involved in the production of lichen secondary compounds: three appear more methylated in the lichen than in the isolated fungus, while one shows the opposite behavior.
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