Constraints on the evolution of thermal sensitivity of foraging in Trichogramma: genetic trade-offs and plasticity in maternal selection.

Negative genetic correlation between performance at different temperatures or temperature-dependent mutations may promote evolution of thermal specialization in ectotherms. The first hypothesis implies that a selective change in performance at one temperature simultaneously results in change in performance at others, while the second implies a delay before observing such indirect responses. Comparison of the direction of evolution among Trichogramma lines selected for improvement of parasitization capacity at low, medium, or high temperatures indicated that a change in performance at one temperature concurrently resulted in opposite changes at distant temperatures. Unexpectedly, selection at high temperatures resulted in a decrease in adult fitness components, while adult performance expressed at cold temperatures simultaneously increased. The relationship between maternal fecundity and offspring fitness components varied across the thermal range. No correlation between these traits was present at cold or medium temperatures, but negative relationships appeared at high temperatures. We show that maternal selection resulting from a conflict between adult and offspring fitness components may have resulted in reversed evolution of the adult traits at the high end of the thermal range. Thus, genetic trade-offs in performance at different temperatures and phenotypic plasticity in maternal selection may constrain evolution of the thermal niche in Trichogramma.