Abstract:
Diplodia pinea (=Sphaeropsis sapinea) is a well-known and economically important latent pathogen of
Pinus spp. in many parts of the world. Despite intensive scrutiny, its sexual state has never been observed
and the fungus has thus been considered exclusively asexual. It was, therefore, surprising that a recent
population genetics study showed high genotypic diversity and random association of alleles in a number
of populations, suggesting that the pathogen has a cryptic sexual stage. Using the genome sequence of
two individual D. pinea isolates, we interrogated the structure of the MAT locus in this fungus. The results
suggested that D. pinea is heterothallic (self-sterile) with complete and apparently functional copies of
the MAT genes containing the a-1 and HMG domains present in different isolates. In addition to the
MAT1-2-1 and MAT1-1-1 genes, we found a MAT1-1-4 gene in the MAT1-1 idiomorph and a novel
MAT1-2-5 gene in the MAT1-2 idiomorph. Importantly, the frequencies of occurrence of both idiomorphs
in populations examined were not significantly different from a 1:1 ratio, which would be expected in
sexually reproducing populations. Although the sexual state has never been observed, the results strongly
suggest that D. pinea has a cryptic, heterothallic sexual cycle.
