Insect egg‐killing: a new front on the evolutionary arms‐race between brassicaceous plants and pierid butterflies

Evolutionary arms‐races between plants and insect herbivores have long been proposed to generate key innovations such as plant toxins and detoxification mechanisms that can drive diversification of the interacting species. A novel front‐line of plant defence is the killing of herbivorous insect eggs. We test whether an egg‐killing plant trait has an evolutionary basis in such a plant–insect arms‐race. Within the crucifer family (Brassicaceae), some species express a hypersensitive response (HR)‐like necrosis underneath butterfly eggs (Pieridae) that leads to eggs desiccating or falling off the plant. We studied the phylogenetic distribution of this trait, its egg‐killing effect on and elicitation by butterflies, by screening 31 Brassicales species, and nine Pieridae species. We show a clade‐specific induction of strong, egg‐killing HR‐like necrosis mainly in species of the Brassiceae tribe including Brassica crops and close relatives. The necrosis is strongly elicited by pierid butterflies that are specialists of crucifers. Furthermore, HR‐like necrosis is linked to PR1 defence gene expression, accumulation of reactive oxygen species and cell death, eventually leading to egg‐killing. Our findings suggest that the plants’ egg‐killing trait is a new front on the evolutionary arms‐race between Brassicaceae and pierid butterflies beyond the well‐studied plant toxins that have evolved against their caterpillars.