Tradeoffs between alternate forms of defense (e.g. constitutive and induced defenses, trichomes and secondary chemicals), growth and defense, and reproduction and defense have all been documented in a variety of plant systems. Thee tradeoffs influence natural selection on defense traits by determining the net benefit of a particular strategy in light of its costs, and the environmental context. If tradeoffs result in balancing selection, multiple defensive strategies may be maintained over time within or across populations.
To understand how alternative chemical defense strategies in the wild mustard Boechera stricta (Brassicaceae) may be maintained over space and time, we conducted an experiment testing for tradeoffs in the efficacy of defenses in different ecological contexts. Using near-isogenic lines varying only in a small portion of the genome (<<1 centiMorgan) that influences defensive chemistry, we constructed artificial arrays of two contrasting defense genotypes and allowed them to experience six weeks of natural field conditions, including ambient herbivory and abiotic stresses. We then assessed all plants for survival, growth, and herbivore defense to test for variable performance of these contrasting genotypes in response to genotype frequency, biotic environment (insect abundance and community composition), and abiotic environment (natural drought stress).
Results/Conclusions
Our study showed that the plant defense genotype that confers greater herbivore resistance in most environments (regardless of genotype frequency or insect community context) also has decreased survival following natural drought stress. This suggests that tradeoffs between defense and survival may contribute to phenotypic diversity of this trait across populations of Boechera stricta. Furthermore, by using known genotypes with predictable phenotypes, we demonstrate that measuring natural selection only on surviving plants can bias outcomes toward demonstrating directional selection on ecologically important traits, even if balancing selection may actually be driving patterns of diversity observed in nature. This echoes previous work demonstrating the importance of considering “the invisible fraction” in studies of evolutionary ecology, in which some individuals die before the trait(s) of interest are measured in the population.