Department of Entomology, Comstock Hall, Cornell University. Ithaca, NY. 14853, USA.
The degree of specialization determines in large part the pattern in the distribution of organisms in nature. The synergism that can occur between ecological specialization and genetic subdivision may lead to rapid evolutionary divergence of populations, and may be an important mechanism leading to rapid "ecological speciation" (Schluter, 1998). However, little is known about the targets of selection involved in the evolution of ecological specialization. Sympatric populations of the pea aphid, Acyrthosiphon pisum, are partially reproductively isolated as a direct consequence of their specialization on alfalfa or clover (Via, 1991a, b; 1999). Adaptation to either host appears to be driven by behavioral acceptance of either (Caillaud and Via, submitted). Recent work further suggests that acceptance of either host is chemically mediated and involves taste. We performed a bioassay-guided fractionation of clover and alfalfa plant extracts. In the bioassay, the aphid probing behavior and larviposition on whole plants were reproduced. We found that specialization on alfalfa and clover is correlated with the presence of host-specific allelochemicals that trigger feeding and larviposition by the aphids. No evidence of deterrency from the non-host was found. We are currently isolating and characterizing the host-plant allelochemicals involved in ecological specialization in pea aphids. Identification of these compounds will open the route to the investigation of differences in taste between alfalfa and clover specialists.