FREQUENCY-DEPENDENT SELECTION IN A WILD PLANT-PATHOGEN SYSTEM

This study investigated the occurrence of the rust fungus Puccinia cho ndrillina in clonal populations of its host plant, Chondrilla juncea. In 16 populations spread across eastern Turkey, 48 different multilocu s isozyme phenotypes were identified in the host. Of these clones, 88% were restricted to single localities, while the remaining 12% were fo und in 2-11 populations. For 13 of the 16 plant populations the common est host clone was always infected. Indeed, at ten sites this clone wa s the only one found to carry disease. In the remaining three populati ons the rusted plants were all of the second commonest isozyme type. T he possibility of such a tight association of rust incidence with host clone frequency simultaneously across a wide geographic area is very low (P<=0.023), supporting the contention that the pathogen P. chondri llina may be imposing negative frequency-dependent selection on these C. juncea populations.