Genome Evolution In Poplar Pathogens From Wild and Recently Domesticated Pathosystems

Mycospharella populorum and M. populicola are closely related fungal pathogens of poplars that have co-evolved with two different hosts. M. populorum causes two distinct diseases on poplars: leaf-spots and cankers. It infects poplars with Aigeiros parentage and is responsible for severe damage in hybrid poplar plantations. M. populicola, on the other hand, only causes leaf spots on poplars in the Tacamahaca section. To identify and understand the evolution of pathogenicity and host-specificity factors in these pathogens, their genomes were sequenced to a coverage of 18-38X. We estimate the divergence time between M. populorum and M. populicola to be ~6.4 Mya, which parallels the divergence time estimates between the Aigeiros and Tacamahaca sections (6.8 - 7.8 Mya). In accordance with the relatively recent divergence time, the whole genome comparisons show a remarkably high macro-synteny. A consistent expansion of several pathogenicity-related gene families was observed in M. populorum, suggesting a role for gene-dosage in determining its ability to cause cankers. The CAZy and peptidase profile of these two poplar pathogens was reduced compared to other Mycosphaerella genomes reinforcing the stealth pathogen hypothesis. Genes encoding hemicellulose-degrading enzymes were more numerous in M. populorum than in M. populicola. Accordingly, M. populorum grew better than M. populicola on media amended with poplar woodchips. This suggests that M. populorum maintained (or conserved) hydrolytic enzymes resulting in adaptation to infect and grow in woody stems of poplar and cause tree mortality.