Evolution of fungal genomes
At least 6,000 species of fungi form ectomycorrhiza (EM). Analyses of DNA-based phylogenies have shown that the ancestors of the EM species were free-living and that the symbiotic ability has evolved repeatedly from saprophytic (i.e. free-living) precursors. On the genome level, there are three, compatible mechanisms: changes in gene content, structural differences in gene products, and quantitative differences in gene expression. Insights into these mechanisms have been obtained by analyzing the genome of Laccaria bicolor which was the first species of an EM fungus to be fully sequenced. The project was funded by the Joint Genome Institute (JGI) at the Department of Energy (DOE) in the US.
The analysis showed that L. bicolor has a relatively large genome, and that the large genome size is partly due to an expansion of gene family sizes. Among the largest gene families are protein kinases and RAS small GTPases, which are key components of signal transduction pathways. Other parts of the proteome, most noticeable plant cell wall degrading enzymes, has been lost in the L. bicolor genome when compared to saprophytic species.
Together with DOE/JGI, we have recently sequenced the genome of the EM fungus Paxillus involutus. Within a large international project led by Francis Martin (INRA, Nancy), this genome is compared with a number of other EM and saprophytic fungi. This effort will provide important insights into the evolutionary processes that lead to the diversification of various lifestyles in fungi.