The potential for and constraints on the evolution of compensatory abilityin Asclepias syriaca

To investigate the potential for and constraints on the evolution of compen satory ability, we performed a greenhouse experiment using Asclepias syriac a in which foliar damage and soil nutrient concentration were manipulated. Under low nutrient conditions, significant genetic variation was detected f or allocation patterns and for compensatory ability. Furthermore, resource allocation to storage was positively, genetically correlated both with comp ensatory ability and biomass when damaged, the last two being positively, g enerically correlated with each other. Thus, in the low nutrient environmen t, compensatory ability via resource allocation to storage provided greater biomass when damaged. A negative genetic correlation between compensatory ability and plant biomass when undamaged suggests that this mechanism entai led an allocation cost, which would constrain the evolution of greater comp ensatory ability when nutrients are limited. Under high nutrient conditions , neither compensatory ability nor allocation patterns predicted biomass wh en damaged, even though generic variation in compensatory ability existed, instead, plant biomass when undamaged predicted biomass when damaged. The d ifferences in outcomes between the two nutrient treatments highlight the im portance of considering the possible range of environmental conditions that a genotype may experience. Furthermore, traits that conferred compensatory ability did not necessarily contribute to biomass when damaged, demonstrat ing that it is critical to examine both compensatory ability and biomass wh en damaged to determine whether selection by herbivores can favor the evolu tion of increased compensation.