The Making of Tree Pathogens: A Genomic Dissection

Tree pathogens cause some of the most devastating outbreaks in forests. In spite of their importance, we know relatively little about their origin and sources and the basic common characteristics of tree pathogens. We applied a genomics approach to understand how the poplar canker pathogen, caused by

Mycosphaerella populorum (anamorph: Septoria musiva syn. Sphaerulina musiva), evolved to cause one of the most severe diseases of poplars across various hosts and landscapes. This pathogen has recently expanded geographically and has been increased its host range to additional species such as the black cottonwood, Populus trichocarpa and the balsam poplar, P. balsamifera. Genome comparison between M. populorum, which causes both leaf spots and cankers, and M. populicola, a close relative that also attacks poplar but does not cause cankers, indicates that M. populorum has acquired a unique set of genes via horizontal gene transfer (HGT) that is important to its ability to attack woody hosts. Population genomic analysis of M. populorum samples from across its distribution range highlights an unusually high and uneven level of diversity even in recently introduced populations. The patterns suggest both extensive migration and possibly additional unsampled ghost populations that could have contributed to the recent expansions. We hypothesize that a jump to P. balsamifera could be driving speciation by imposing selection at the mating type locus, causing a shift from sexual to asexual reproduction, followed by isolation and drift. These results highlight very dynamic mechanisms leading to host colonization and incipient speciation and specialization in this tree pathogen.