Fluctuating selection

As contributor to the work of Sonja Lecic

Insecticide resistance is a classic example for rapid evolution. Because in the absence of insecticides resistance alleles confer a significant cost, their frequencies in natural populations are considered to reflect the intensity of insecticide treatment. The neurotransmitter Acetylcholine is metabolized by Ace, which is a common target of insecticides. Resistance alleles show seasonal and clinal variation consistent with more intense insecticide treatment in the warm when insects are more abundant. Here, we use laboratory natural selection of a natural Drosophila simulans population to study the cost of resistance in insecticide free environment. Interestingly, the cost of resistance differs dramatically in hot and cold conditions. While resistance alleles are only weakly selected in hot (s=-0.012), in the cold they bare a high cost (s=-0.058). We propose that selection for neuronal signaling homeostasis drives the environmentally modulated fitness of natural Ace alleles: lower enzymatic activity in the cold requires more and highly active ACE, while at higher temperature lower quantities and less active ACE molecules are favored. Hence, neuronal signaling in natural populations can be modulated by Ace expression levels and the frequency of resistance alleles to accommodate for environment-specific signaling requirements. Using experimental evolution with tightly controlled environments, we demonstrate a previously unappreciated role of temperature dependent selection on neuronal signaling as an evolutionary force driving seasonal allele frequency changes of Ace in natural Drosophila populations.