Genetic patterns reveal historical and contemporary dispersal of a tree pathogen

Abstract

Sphaerulina musiva (Peck) Verkley, Quaedvlieg and Crous (syn = Septoria musiva Peck, Mycosphaerella populorum Thompson) is a pathogen of poplar that causes two distinct diseases, leaf spots and cankers. This pathogen co-evolved with Populus deltoides but recent reports have linked it to infections in planted stands of P. trichocarpa, P. balsamifera and their hybrids. Reports of S. musiva have mainly come from central and eastern US and eastern Canada, the assumed endemic range of the pathogen. S. musiva was detected for the first time in the Canadian provinces of British Columbia in 2006 and in Alberta in 2009. Our objectives were to determine the source of S. musiva in British Columbia and Alberta and examine the dispersal pathways of this pathogen across North America. For this task we sequenced eight genes and extracted single nucleotide polymorphisms on a geographically diverse set of 73 strains of S. musiva. Population structure and Approximate Bayesian Computation (ABC) analyses eliminated eastern Canada as a source for these introductions. Genetic diversity estimates and ABC analyses support an eastern US centre of origin for S. musiva and two waves of dispersal into Canada. The recently detected west Canadian populations appear to have received contributions from Saskatchewan (a western Canadian population) and also, in the case of British Columbia from the mid-west US populations. These results also reveal distinct eastern and western Canadian populations. Our analyses suggest that dissemination of the pathogen appears to be associated with the natural distribution of wild P. deltoides and more recently linked to anthropogenic activities. The most parsimonious explanation for the contemporary spread of S. musiva across the landscape is via infected plant material. Our analysis of the tree disease caused by S. musiva demonstrates that a population genetics approach is essential to reveal potential sources and patterns of spread of a pathogen.

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Acknowledgments

We would like to acknowledge the reviewers whose valuable comments helped improve this manuscript. We would like to thank Harry Kope, Stephan Zeglan and Cees van Oosten for their advice on the history of poplar material imports into British Columbia. Also Dr. Cyril Dutech (INRA de Bordeaux, France) for his excellent advice regarding the findings of this manuscript. We appreciate the assistance Kevin Keenan (Queen’s University, Belfast, Northern Ireland, United Kingdom) provided in modifying his R package diveRsity for haploid organism analysis. Thank you to Dr. Glen Stanoz (University of Wisconsin-Madison, WI, USA), Dr. Jared Leboldus (North Dakota State University, ND, USA) and Dr. Louis Bernier (Laval University, QC, Canada) for providing us with some cultures. This work would not have been possible without members of the TAIGA team who assisted with the lab work. This work was supported by grants from Genome Canada and Genome British Columbia (Large Scale Applied Research Program, Grant #164) to the TAIGA project (www.taigaforesthealth.com) of the University of British Columbia.

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Correspondence to Monique L. Sakalidis.

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Sakalidis, M.L., Feau, N., Dhillon, B. et al. Genetic patterns reveal historical and contemporary dispersal of a tree pathogen. Biol Invasions 18, 1781–1799 (2016). https://doi.org/10.1007/s10530-016-1120-7

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Keywords

  • Approximate Bayesian Computation
  • Eastern cottonwood
  • Hybrid poplar
  • Mycosphaerella populorum
  • Population genetics
  • Septoria canker
  • Sphaerulina musiva