Thursday, August 5, 2010 - 2:50 PMOOS 45-5: Genetically based clines in plant defenses
Anurag A. Agrawal1, Ellen C. Woods1, Stephen B. Heard2, and Nash E. Turley3. (1) Cornell University, (2) University of New Brunswick, (3) North Carolina State University
Background/Question/Methods
While genetically-based latitudinal trends in primary plant metabolism (i.e., traits involved in resource acquisition and allocation) may be driven by climate, clinal variation in secondary metabolism or other traits that mediate interspecific interactions have been less well studied. The overlay of interspecific interactions may synergize or oppose natural selection imposed by climate. For example, in addition to phenology and life-history traits following latitudinal clines, it has long been predicted that in more tropical latitudes, plant defenses will be stronger compared to more temperate latitudes because northern climates are constrained by abiotic factors, limiting opportunities for coevolution. This study focuses on how populations of native common milkweed, Asclepias syriaca (Apocynaceae), have genetically adapted along a latitudinal gradient in response to abiotic and biotic factors. We took a common garden approach to investigate genetically-based latitudinal clines in plant allocation to primary and secondary metabolism. Specifically, we grew several full-sib families of common milkweed from each of 22 populations spanning over 10 degrees of latitude (>1,500 km) in three common gardens.
Results/Conclusions
We find heritable variation in all 10 traits examined, with latitudinal clines evident for latex exudation (a potent physical and chemical defense), plant growth, and allocation to sexual and asexual reproduction. Counter to predictions, plants from the North were more defended and grew more vigorously. Although a bioassay with specialist monarch butterfly caterpillars revealed no latitudinal trend in "resistance", latex was a significant predictor of larval growth. Thus, we conclude that the key plant trait responsible for resistance has evolved along the latitudinal gradient. We use statistical approaches to dissect the relative role of abiotic and biotic environmental components contributing to the genetically-based latitudinal trends in milkweed primary and secondary metabolism.
While genetically-based latitudinal trends in primary plant metabolism (i.e., traits involved in resource acquisition and allocation) may be driven by climate, clinal variation in secondary metabolism or other traits that mediate interspecific interactions have been less well studied. The overlay of interspecific interactions may synergize or oppose natural selection imposed by climate. For example, in addition to phenology and life-history traits following latitudinal clines, it has long been predicted that in more tropical latitudes, plant defenses will be stronger compared to more temperate latitudes because northern climates are constrained by abiotic factors, limiting opportunities for coevolution. This study focuses on how populations of native common milkweed, Asclepias syriaca (Apocynaceae), have genetically adapted along a latitudinal gradient in response to abiotic and biotic factors. We took a common garden approach to investigate genetically-based latitudinal clines in plant allocation to primary and secondary metabolism. Specifically, we grew several full-sib families of common milkweed from each of 22 populations spanning over 10 degrees of latitude (>1,500 km) in three common gardens.
Results/Conclusions
We find heritable variation in all 10 traits examined, with latitudinal clines evident for latex exudation (a potent physical and chemical defense), plant growth, and allocation to sexual and asexual reproduction. Counter to predictions, plants from the North were more defended and grew more vigorously. Although a bioassay with specialist monarch butterfly caterpillars revealed no latitudinal trend in "resistance", latex was a significant predictor of larval growth. Thus, we conclude that the key plant trait responsible for resistance has evolved along the latitudinal gradient. We use statistical approaches to dissect the relative role of abiotic and biotic environmental components contributing to the genetically-based latitudinal trends in milkweed primary and secondary metabolism.