International Association for Lichenology

Scott Kroken

Scott Kroken

The Mason Hale Award was presented to Scott Kroken in the country of his ancestors. Initially, Scott was not educated as a lichenologist. During his masters thesis at the University of Wisconsin at Madison he studied resistant cell wall materials of bryophytes and charophytes. After moving to Berkeley he then became interested in the population genetics of lichens at the DNA-level, which was a poorly explored field at that time, except for the work in Cladonia by Paula DePriest. With this background, Scott started a doctoral thesis with John Taylor as supervisor, and he selected an interesting case in lichenology. The title of his thesis was “Cryptic speciation and the role of sex in the lichenized fungus Letharia”.

The genus Letharia includes a classic species pair with the fertile L. columbiana and the sterile L. vulpina. Most lichenologists distinguished these two species on the basis of their dispersal strategies. Scott was interested whether this concept would also be corroborated by molecular studies and started to investigate the evolutionary history and population biology of both the mycobionts and the photobiont.

Scott’s approach was different from other infraspecific approaches at that time. He did not rely on a single genetic locus, but tried to find as many gene loci as possible to look for congruence of the gene genealogies. To find appropriate gene loci was a pioneering work which involved primer design in a heterologous approach and the development of homologous loci using arbitrary primers (anonymous loci). Absence of congruence between the genealogies of these multiple loci signified genetic exchange, whereas congruent parts of the tree topologies indicated clonality or indirectly, the lack of genetic exchange. Using this approach, Scott found evidence for genetic isolation and he suggested six phylogenetic species in the Letharia columbiana/vulpicida complex.

The thesis is divided into three major chapters: A gene genealogical approach to determine species boundaries and reproductive mode, a demonstration of obligately outbreeding in a frequently sexual species and a rarely sexual species, and the symbiosis with the green alga Trebouxia. Scott also detected outcrossing in two of the phylogenetic species by comparing the genotypes in maternal thalli and apothecia. Several phylogenetic species were also suggested for the photobionts of Letharia, which all belong to the Trebouxia jamesii complex. Scotts thesis paved the way for the following generation to further explore lichen population genetics and population history of both partners in the symbiotic system.

Martin Grube