Sizing the fungal and algal genomes of the lichen Cladonia grayi through quantitative PCR

Using a method based on quantitative PCR, we determined that the nuclear genome sizes for the mycobiont and photobiont of the lichen Cladonia grayi are 28.6 Mb and 106.7 Mb, respectively. This is the first genome size determination for lichens, and suggests that between 20,000 and 25,000 genes function in C. grayi. The mycobiont genome size is near the middle of the range observed within the Pezizomycota, the subphylum containing all known ascomycete lichen fungi. The genome size of the photobiont (the green alga Asterochloris sp.) is near the lower end of its class, the Trebouxiophyceae. Genomes in this size range can be sequenced at relatively low cost with current pyrosequencing-based methods. The genome sizing method requires very small amounts of precisely quantified DNA and should be applicable to any lichen whose symbionts can be reliably isolated from one another. Since the symbionts used in this project were isolated from soredia, the lichen's vegetative propagules, we also describe a method for the establishment of axenic symbiont cultures from large numbers of soredia.