Population Genomics Reveals Population Structure and Mating-Type Loci in Marssonina brunnea

Marssonina brunnea is an important fungal pathogen of poplar trees. We collected 32 M. brunnea f.sp. multigermtubi (MbMu) and three M. brunnea f.sp. monogermtubi (MbMo) isolates from four poplar species in three Chinese regions and performed genome resequencing. An annotation of SNPs of MbMu indicated that the SNPs potentially have a functional effect on 69.2% of the predicted genes. Using the SNP dataset of nonredundant isolates, a structure and principal component analysis revealed that MbMu and MbMo belong to two genetically distinct populations. By contrast, subpopulation structures could not be found among MbMu isolates. A neighbor-net analysis and a homoplasy index test provided evidence of recombination among MbMu isolates. The short distance (109-174 bp) of linkage disequilibrium half-decay supported the presence of a high level of recombination in the MbMu population. The genetic architectures of the MAT loci of MbMu and MbMo were revealed by searching genome assemblies or by homology-based cloning, and a BLAST search verified each isolate carrying one of the two opposite MAT loci. This study revealed that the MbMu population contains a wide range of functional variants, shows high-frequency recombination, and exhibits heterothallic mating systems, indicating high evolutionary potential and a resultant threat to poplar plantations.