Comparative genomics in the boxwood blight system: Insights into the global diversity of the mating-type locus

The ascomycete Calonectria pseudonaviculata, a destructive fungal pathogen
responsible for boxwood blight disease, was first reported in the U.S. in 2011.
In a matter of months, North American boxwood blight outbreaks occurred
across the eastern seaboard and Pacific Northwest. In order to understand the
sexual reproductive potential of C. pseudonaviculata, we have applied next
generation sequencing to assess the primary determinant of mating (MAT1)
locus. The genome of fifteen C. pseudonaviculata isolates were sequenced,
used to create a local BLAST databases in CLC Genomics, and then searched
for homologues of the idiomorphic genes MAT1-1 and MAT1-2. Only a single
MAT1 idiomorph was identified in each of the fifteen genome assemblies,
indicating that C. pseudonaviculata is a heterothallic fungus. Subsequently,
we designed specific primers to amplify differently sized regions for each
MAT1 locus. Over 200 isolates originating from four continents were analyzed
by a duplexed PCR reaction. PCR analyses revealed that the mating types
correspond with the genetic lineages G1 and G2. MAT1-2 genotype is globally
distributed, but exclusively in the G1 lineage. The MAT1-1 genotype is solely
present in five European countries from isolates of the G2 lineage. These
results indicate that the fungus is currently unable to sexually reproduce in
North America, since all the U.S. isolates belonged to the MAT1-2 genotype.