Canadian Forest Service Publications

Horizontal gene transfer and gene dosage drives adaptation to wood colonization in a tree pathogen. 2015. Dhillon, B.; Feau, N.; Aerts, A.L.; Beauseigle, S.; Bernier, L.; Copeland, A.; Foster, A.; Navdeep G.; Henrissat, B.; Herath P.; LaButti, K.M.; Levasseur, A.; Lindquist, E.A.; Majoor, E.; Ohm, R.A.; Pangilinan, J.S.; Pribowo, A.; Saddler, J.N.; Sakalidis, M.L.; de Vries, R.P.; Grigoriev, I.V.; Goodwin, S.B.; Tanguay, P.; Hamelin, R.C. Proc. Natl. Acad. Sci. USA 112:3451-3456.

Year: 2015

Issued by: Laurentian Forestry Centre

Catalog ID: 35938

Language: English

Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1073/pnas.1424293112

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Some of the most damaging tree pathogens can attack woody stems, causing lesions (cankers) that may be lethal. To identify the genomic determinants of wood colonization leading to canker formation, we sequenced the genomes of the poplar canker pathogen, Mycosphaerella populorum, and the closely related poplar leaf pathogen, M. populicola. A secondary metabolite cluster unique to M. populorum is fully activated following induction by poplar wood and leaves. In addition, genes encoding hemicellulose-degrading enzymes, peptidases, and metabolite transporters were more abundant and were up-regulated in M. populorum growing on poplar wood-chip medium compared with M. populicola. The secondary gene cluster and several of the carbohydrate degradation genes have the signature of horizontal transfer from ascomycete fungi associated with wood decay and from prokaryotes. Acquisition and maintenance of the gene battery necessary for growth in woody tissues and gene dosage resulting in gene expression reconfiguration appear to be responsible for the adaptation of M. populorum to infect, colonize, and cause mortality on poplar woody stems.

Plain Language Summary

This study demonstrated that small genetic differences between two close species of fungi may explain why one species, Mycosphaerella populicola, causes simple leaf spots, whereas the other, Mycosphaerella populorum, can cause deadly cankers to form on tree trunks.

The Mycosphaerella populorum fungus and its natural host, the eastern cottonwood, have evolved together and adapted to each other. In natural forests, this fungus does not cause significant damage to its indigenous host. However, in plantations of hybrid or exotic poplar clones, this pathogen is aggressive, even going so far as to cause cankers to form on trunks and shape defects that can be lethal and may destroy plantations.

By comparing all of the Mycosphaerella populorum genes and those of its close parent, Mycosphaerella populicola, the researchers were able to identify genetic factors that may explain the growth of M. populorum in wood and the resulting formation of cankers.

The findings show that small genetic variations may explain how a benign pathogen, which lives in harmony with its indigenous host, can become a major cause of economic damage in plantations of hybrid or exotic poplars.