LOCAL POPULATION DIFFERENTIATION FOR COMPATIBILITY IN AN ANNUAL LEGUME AND ITS HOST-SPECIFIC FUNGAL PATHOGEN

Severe attack by the fungal pathogen Synchytrium decipiens frequently occurs in natural populations of the annual plant Amphicarpaea bracteata (Leguminosae) in eastern North America. Field transplant experiments indicate that there is significant population differentiation in the plant-fungus association over distances of 1 km or greater: plants transplanted back into their population of origin become heavily infected, while foreign plants from populations 1 or 100 km away experience little or no infection, even though these foreign plants are subject to heavy fungal attack in their native populations. To investigate the fine structure of population differentiation, progeny of A. bracteata plants collected at six sites at 30 m intervals along a transect were inoculated with a single strain of S. decipiens in a controlled environment. Fungal lesions were initiated in all 36 plant progeny groups tested, yet there was highly significant, 5-fold variation among plants from different sites in the mean number of fungal lesions developing per plant. In addition, all fungal lesions aborted without maturing spores on all plants from one site on the transect. Fungal lesion abortion rates averaged only 9% on plants from the other five sites. Such local population differentiation in plant-pathogen compatibility may be related to A. bracteata's high degree of self-pollination. Limited long-distance recombination in A. bracteata due to self-pollination and spatially restricted pollen flow may be a major factor preventing the evolution of increased plant resistance to fungal attack.