This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Optimality theory for defense evolution (Hamilton et al. 2001 for overview), currently the best framework we have to understand the simultaneous evolution of defense and competitive ability in plants, predicts a tradeoff between defense and competitiveness because defense production is thought to divert limited resources away from competitive growth (Herms and Mattson 1992). We have tested for this evolutionary tradeoff by (1) studying the effects of genetic or environmental variation in defenses on competitive growth, and by (2) studying the effects of genetic variation in competitiveness on defense expression. Although we have detected costs of defenses manifest as reduced growth in the absence of herbivores, these costs were not detected in competitive environments (Siemens et al. 2002;Siemens et al. 2003). That is, defense production affected growth, but not competitive ability. We have also found evidence that measures of defense, such as glucosinolate concentration, were sometimes higher or competitively induced in better competitors. A competitive function for defensive genes would explain many of these results. The genomic tools of Arabidopsis allow us to determine whether competitors induce defensive genes, as one would expect for defenses that also function in competition. A genomic approach also allows a test for the predicted tradeoff between defense and competitive ability, which may further our understanding of gene regulation (Stearns and Magwene 2003).