Local genetic differentiation for disease resistance in a selfing annual

Electrophoretic analysis of a population of the annual legume Amphicarpaea bracteata revealed that plants in two areas 50 m apart were fixed or nearly fixed for different alleles at eight polymorphic allozyme loci. This multilocus differentiation was highly correlated with variation in disease resistance to the specialist pathogen Synchytrium decipiens : in both laboratory and field experiments, all plants from one area were susceptible, and all plants from the other area were resistant to local pathogen isolates. Analysis of F2 plants from a cross between the two plant biotypes suggested polygenic inheritance of disease resistance, with a minimum of 7–8 loci contributing to the difference among parental lines. The high degree of self-fertilization in A. bracteata , together with spatially restricted seed dispersal, may account for perpetuation of the two divergent plant lineages at this site with little introgression or mixing. Although pathogens could influence the relative frequency of the two host lineages in this habitat, the restricted diversity of local pathogens implies that there may be little selection on plants for outcrossing as a mechanism to reduce pathogen impact.