Genetic Variation in Chemical Defence Affects Protection of an Herbivorous Insect Against Predation

Genetic variation contributes to intraspecific differences in the chemical defence in many insect species, yet the underlying genetic mechanisms remain poorly understood. The horseradish flea beetle, Phyllotreta armoraciae, sequesters glucosinolates from its horseradish host plant and activates them using endogenous myrosinase enzymes. Of the three known myrosinases in P. armoraciae, PaMyr1 functions primarily in adults, whereas PaMyr2 and PaMyr3 are responsible for myrosinase activity in larvae. Here, we identify natural genetic variation at the myrosinase locus that gives rise to three distinct myrosinase haplotypes, only one of which retains a functional PaMyr3 gene. This variation affected PaMyr gene expression and myrosinase activity in larvae but not in adults. Larvae expressing both PaMyr2 and PaMyr3 showed elevated myrosinase activity toward 2-propenyl glucosinolate, the major glucosinolate in horseradish. Gene expression and biochemical analyses indicate that elevated myrosinase activity results from a subfunctionalization of PaMyr3, which confers greater catalytic efficiency rather than higher total myrosinase abundance. Importantly, PaMyr3-expressing larvae were less susceptible to a model generalist predator in laboratory assays, suggesting a selective advantage under high predation pressure. Consistent with this hypothesis, the PaMyr3-containing haplotype occurred at higher frequency in a natural population than in long-term laboratory populations lacking predators. Together, our results link structural genetic variation to intraspecific differences in insect chemical defence with potential consequences for predator-prey interactions in natural populations.