Wednesday, August 5, 2009

PS 55-145: Genetic variation across a geographic cline and induced responses shape resistance to herbivores on Solanum americanum

Nash E. Turley, Daniel M. Evans, and Joshua J. Tewksbury. University of Washington

Herbivores impose selection pressure on plants, driving the evolution of constitutive and induced plant defenses. Spatial variation in the biotic and abiotic environment can alter the strength and direction of this selection, causing plants to adapt to their local environment.  We tested whether genotypes of the herbaceous plant Solanum americanum collected from a latitudinal gradient exhibited genetic variation in constitutive and induced resistance consistent with local adaptation to clinal gradients in selection.  Specifically, genotypes were collected from South Carolina to Florida and individuals of each genotype were transplanted into a common garden in South Carolina.  Half the plants were mechanically damaged to induce a response to herbivory. 
We found significant variation in herbivory among genotypes (F3,240 = 7.75, P < 0.001)  consistent with local adaptation in resistance.  The relationship was linear with levels of herbivory increasing with geographic distance between a genotype’s site of origin and the common garden.  At the extremes of the latitudinal gradient the south FL genotype received 65% more herbivory relative to the local SC genotype.  The induced response to herbivory treatment decreased levels of herbivory by 15%. All genotypes appear to respond to mechanical herbivory in a similar fashion, as the interaction between genotype and herbivory induction was not significant. Our results suggest that S. americanum genotypes are adapted to their local herbivore community and are less resistant to herbivores in environments away from their native range. This pattern appears to be driven by changes in constitutive resistance rather than induced resistance.