American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Resistance-genes affect how pathogens maintain plant abundance and diversity”

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Simon Maccracken Stump, James H. Marden, Noelle G. Beckman, Scott A. Mangan, and Liza S. Comita (Oct 2020)

Here we synthesize the Janzen-Connell hypothesis of plant diversity with the gene-for-gene model of plant defense

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Abstract

Specialized pathogens are thought to maintain plant community diversity; however, most ecological studies treat pathogens as a black-box. Here we develop a theoretical model to test how the impact of specialized pathogens changes when plant resistance (R) genes mediate susceptibility. This work synthesizes two major hypotheses: the gene-for-gene model of pathogen resistance and the Janzen-Connell hypothesis of pathogen-mediated coexistence. We examine three scenarios. First, R-genes do not affect seedling survival; in this case, pathogens promote diversity. Second, seedlings are protected from pathogens when their R-gene alleles and susceptibility differ from those of nearby conspecific adults, thereby reducing transmission. If resistance is not costly, pathogens are less able to promote diversity because populations with low R-gene diversity suffer higher mortality, putting those populations at a disadvantage, and potentially causing their exclusion. R-gene diversity may also be reduced during population bottlenecks, creating a priority effect. Third, when R-genes affect survival, but resistance is costly, populations can avoid extinction by losing resistance alleles, as they cease paying a cost that is unneeded. Thus, the impact pathogens can have on tree diversity depends on the mechanism of plant-pathogen interactions. Future empirical studies should examine which of these scenarios most closely reflects the real world.