Population studies of the wild tomato species Solanum chilense reveal geographically structured major gene-mediated pathogen resistance

Natural plant populations encounter strong pathogen pressure and
defense-associated genes are known to be under selection dependent on the
pressure by the pathogens. Here we use populations of the wild tomato
Solanum chilense to investigate natural resistance against Cladosporium
fulvum, a well-known ascomycete pathogen of domesticated tomatoes. Host
populations used are from distinct geographical origins and share a
defined evolutionary history. We show that distinct populations of S.
chilense differ in resistance against the pathogen. Screening for major
resistance gene mediated pathogen recognition throughout the whole species
showed clear geographical differences between populations and complete
loss of pathogen recognition in the south of the species range. In
addition, we observed high complexity in a homologues of Cladosporium
resistance (Hcr) locus, underlying the recognition of C. fulvum, in
central and northern populations. Our findings show that major gene
mediated recognition specificity is diverse in a natural
plant-pathosystem. We place major gene resistance in a geographical
context that also defined the evolutionary history of that species. Data
suggest that the underlying loci are more complex than previously
anticipated, with small-scale gene recombination being possibly
responsible for maintaining balanced polymorphisms in the populations that
experience pathogen pressure.