Background Symbiosis often gives organisms the ability to expand ecological niches which are inaccessible as individuals. In lichen symbiosis, mutualistic relationships between lichen-forming fungi and algae and/or cyanobacteria produce unique features that make lichens adaptive to wide range of environments. This study revealed the fungal-algal interaction specific to the symbiosis in lichen using Usnea hakonensis as a model system.
Results The whole genome of U. hakonensis, the fungal partner, was sequenced by using the culture isolated from a natural lichen thallus. Isolated cultures of the fungal and the algal partners were co-cultured in vitro for three months, and the thalli were successfully resynthesized into visible protrusions. Transcriptomes of resynthesized and natural thalli (symbiotic states) were compared to that of isolated cultures (non-symbiotic state). Sets of fungal and algal genes up-regulated in both symbiotic states were identified as symbiosis-related genes.
Conclusion From the predicted functions of these genes, we identified the genetic background of two main features fundamental to the symbiotic lifestyle in lichen. First is an establishment of fungal symbiotic interface: (a) production of a hydrophobic layer that ensheaths fungal and algal cells; and (b) remodeling of cell walls at fungal-algal contact sites. Second is a symbiosis-specific nutrient flow: (a) the algal supply of photosynthetic product to the fungus; and (b) the fungal supply of phosphorous and nitrogen compounds to the alga. Since both features are widespread among lichens, our result may indicate the genetic basis of lichen symbiosis.