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Periodicity in Host Availability Does Not Account for Evolutionary Branching as Observed in Many Plant Pathogens: An Application to Gaeumannomyces graminis var. tritici

First and fourth authors: Department of Biomathematics and Bioinformatics, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, UK; second author: Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EA, UK; and third author: INRA–Agrocampus Rennes, UMR BiO3P, BP 35527, F-35653 Le Rheu Cedex, France.

Periodicity in host availability is common in agricultural systems. Although it is known to have profound effects on plant pathogen abundance, the evolutionary consequences of periodicity for the pathogen population have not previously been analyzed. An epidemiological model incorporating periodic absence of the host crop is combined with the theory of adaptive dynamics to determine whether or not seasonality in host presence plays a role in the occurrence of evolutionary branching, leading to coexisting yet genetically distinct pathogen phenotypes. The study is motivated and illustrated by the specific example of take-all disease of wheat, caused by the pathogen Gaeumannomyces graminis var. tritici, for which two coexisting but genetically distinct types and a trade-off related to seasonality in host presence have been identified. Numerical simulations are used to show that a trade-off between the pathogen transmission rate and the survival of the pathogen between cropping seasons cannot account for the evolutionary branching observed in many pathogens. Model elaborations show that this conclusion holds for a broad range of putative mechanisms. Although the analysis is motivated and illustrated by the specific example of take-all of wheat, the results apply to a broad range of pathogens.

Additional keywords: coexistence, seasonal trade-off.