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Asterochloris sp.
Closeup of young Cladonia grayi growing on soil. Photo by Chris Parrish
Fungus-alga interactions
Early fungus-alga interactions in vitro,. Photo by Suzanne Joneson.

Cladonia grayi is a member of the Cladoniaceae , a well-studied world-wide family of stalked-cup lichens classified within the Lecanoromycetes, a class that includes more than 70% of the lichen-forming fungal diversity. Lichens are stable and differentiated associations of specialized fungi (mycobionts), most often ascomycetes, and unicellular or filamentous green algae or cyanobacteria (photobionts). Widespread across terrestrial ecosystems, lichens are among the most successful mutualistic symbioses and are currently estimated to comprise about 20% (20,000 species) of all known Fungi. There are far fewer photobiont than mycobiont species, as the same algal species can be in different lichens. Therefore the name of the lichen is also the name of the mycobiont. The photobiont associated with C.grayi is Asterochloris sp. a single celled member of the largest family of lichen-forming green algae, the Trebouxiaceae.

Whereas non-lichen fungi spend most of their life within the substrates from which they derive nutrients mutualistically, parasitically, or saprophytically, lichens are permanently exposed and adapted to extreme oscillations in moisture, temperature, and radiation. Contained and propagated within lichens are their primary sources of carbon and nitrogen, the photobionts. Much of what is unique to lichen morphology, physiology, cell biology and biochemistry springs from these basic nutritional and ecological adaptations. While it is clear what the mycobiont gets from the photobiont, it is not as clear what the latter gets in return. Many lichen fungi produce large quantities of unique secondary metabolites that appear to play central roles in the symbiosis but may also have potential for biotechnological use.

The morphological plasticity of the C. grayi lichen results in a variety of differentiated structures: the podetia, cup-like structures that support the spore-producing fungal apothecia; the soredia, packets of fungus and alga serving as somatic propagules; the squamules, whose cross-section displays a typical fungus-alga-fungus layered structure. The C. grayi mycobiont and photobiont can be grown separately in culture,a feature that not all lichens share. This allowed the individual sequencing of the symbionts genomes (36 Mbp for the fungus and 56 Mbp for the alga). The C. grayi fungus culture came from a single spore isolate; the Asterochloris sp. culture came from algal cells isolated from C. grayi soredia. The early stages of the symbiotic interaction can be reproduced in the laboratory by co-culturing the isolated symbionts.

In nature the C. grayi mycobiont can reproduce somatically through soredia or sexually through ascospores formed in the lichen’s apothecia. In culture, however, its slow growing and compact mycelia can be only propagated asexually through fragmentation. The alga is not known to have a sexual stage. It propagates somatically either through motile zoospores or by generating non-motile daughters within the cell which are then released through the mother’s lysis.

Genomics of symbiotic systems focuses on what could be called a "genosystem" , where one genome modifies the operation of a completely different one and is affected by it in return. The genomics of lichens will allow us to understand a central example of co-evolution, in which the stable association of two or more organisms has created new avenues for developmental and functional adaptations. Lichen genomics will also contribute an important comparison with other co-evolved fungus-plant systems, from mycorrhizal associations to plant pathogens.

For more information on lichen biology

Lutzoni, F. and Miadlikowska, J 2009. Lichens. Quick Guide. Current Biology 19;R502-R503

Nash, T.H. 2008, Lichen Biology. Cambridge University Press, 489 pages

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