Photo credit: Ester Gaya

Photo credit: Ester Gaya

Viridothelium virens is a lichen-forming fungus belonging to the family Trypetheliaceae within the class Dothideomycetes. Lichens are obligate mutualistic ectosymbioses between filamentous fungi (the mycobiont – most commonly an ascomycete), and one or more photosynthetic partners (either or both green algae and cyanobacteria). Recent estimates of the number of lichen-forming fungal species are between 17,500 and 20,000 (i.e., about 20% of all known fungal species).

The Trypetheliaceae (ca. 200 species) represents the largest family of lichen-forming fungi within the Dothideomycetes, and have a mainly tropical and subtropical distribution. The species are almost exclusively corticolous (growing on bark), forming a crustose, endo- or epiperidermal thallus, with algae belonging to the Trentepohliaceae. Their ecology varies between species from lowland or submontane tropical rainforests and mangroves, to dry forests and savannas that have a distinct dry season. Their secondary chemistry includes lichexanthone and pigments as most common substances. In fact, the number of species with secondary substances (ca. 70) is much higher in the family Trypetheliaceae than any other group within the Dothideomycetes.

The species selected here, Viridothelium virens, is a common corticolous, pantropical species, characterized by a brown-green to yellowish smooth endoperidermal thallus. The ascomata (perithecia, as in all Trypetheliaceae) are completely immersed in a structure called pseudostromata, which is irregularly raised and usually cream-coloured, with a yellow to orange pigment. As in the rest of the family, asci are bitunicate, the hamathecium is composed of thin, anastomosing pseudoparaphyses embedded in a stiff gelatinous matrix, and the hyaline ascospores are multiseptate, with diamond-shaped septa. Its chemistry is defined by pseudostromata containing yellow to orange, K+ purple, UV+ red anthraquinone.

Viridothelium virens was chosen as a representative of symbiotic systems in the class Dothideomycetes, the largest and most diverse class (with ca. 19,000 species) within the largest fungal phylum, Ascomycota. This class has a complex evolutionary history encompassing an extraordinarily heterogeneous group of fungi with different life styles and nutritional modes. Members of this group can be plant pathogens, saprobes or lichen-forming fungi such as T. eluteriae. In this context, the sequencing of a lichenised fungus was extremely needed. Genome studies are expected to strongly contribute to a broader understanding of the evolution of the Dothideomycetes.

This is the first lichen included in JGI that has perithecia as sexually reproducing structures, instead of the more common apothecia in the Lecanoromycetes. Viridothelium virens is readily amenable to axenic cultivation. This culture came from ascospore progeny from an individual thallus (Florida, USA), and exhibited high rates of germination and rapidly propagated asexually through fragmentation.

For more information on Dothideomycetes and Trypetheliales

Del Prado et al. 2006. Molecular data place Trypetheliaceae in Dothideomycetes. Mycological research 110: 511-520
Kirk, M.P. et al. 2008. Ainsworth & Bisby's Dictionary of the Fungi. CABI, Wallingford, Oxon, UK

Nelsen et al. 2009. Unravelling the phylogenetic relationships of lichenised fungi in Dothideomyceta. Studies in Mycology 64: 135-144

Nelsen et al. 2011. New insights into relationships of lichen-forming Dothideomycetes. Fungal Diversity 51: 155-162

Schoch et al. 2009. A class-wide phylogenetic assessment ofDothideomycetes. Studies in Mycology 64: 1-15