Characterization of mating type genes in heterothallic Neonectria species with emphasis on N. coccinea, N. ditissima, and N. faginata

Neonectria ditissimaandN. faginataare canker pathogens involved in an insect-fungus disease complex of American beech (Fagus grandifolia) in North America commonly known as beech bark disease (BBD). In Europe, bothN. ditissimaandN. coccineaare involved in BBD on European beech (Fagus sylvatica). Field observations across the range of BBD indicate ascospores to be the dominant spore type in the environment. Several studies report a heterothallic (self-sterile) mating strategy forNeonectriafungi, but one study reported homothallism (self-fertility) forN. ditissima. As such, investigations into mating strategy are important for understanding both the disease cycle and population genetics ofNeonectria. This is particularly important in the United States given that over timeN. faginatadominates the BBD pathosystem despite high densities of nonbeech hosts forN. ditissima. This study utilized whole-genome sequences of BBD-associatedNeonectriaspp. along with other publicly availableNeonectriaandCorinectriagenomes and in vitro mating assays to characterize mating type (MAT) locus and confirm thallism for select members ofNeonectriaandCorinectria. MAT gene-specific primer pairs were developed to efficiently characterize the mating types of additional single-ascospore strains ofN. ditissima, N. faginata, andN. coccineaand several other related species lacking genomic data. These assays also confirmed the sexual compatibility amongN. ditissimastrains from different plant hosts. Maximum likelihood phylogenetic analyses of bothMAT1-1-1andMAT1-2-1sequences recovered trees with similar topology to previously published phylogenies ofNeonectriaandCorinectria. The results of this study indicate that allNeonectriaandCorinectriatested are heterothallic based on our limited sampling and, as such, thallism cannot help explain the inevitable dominance ofN. faginatain the BBD pathosystem.