Insect wings and body wall evolved from ancient leg segments
The origin of insect wings has long been debated. Central to this debate is whether wings evolved from an epipod (outgrowth, e.g. a gill) on ancestral crustacean leg segments, or represent a novel outgrowth from the dorsal body wall that co opted some of the genes used to pattern the epipods. To determine whether wings can be traced to ancestral, pre insect structures, or arose by co option, comparisons are necessary between insects and arthropods more representative of the ancestral state, where the hypothesized proximal leg region is not fused to the body wall. To do so, we examined the function of five leg patterning genes in the crustacean Parhyale hawaiensis and compared this to previous functional data from insects. By comparing gene knockout phenotypes of leg patterning genes in a crustacean with those of insects, we show that two ancestral crustacean leg segments were incorporated into the insect body, moving the leg's epipod dorsally, up onto the back to form insect wings. Thus, our data shows that much of the body wall of insects, including the entire wing, is derived from these two ancestral proximal leg segments. This model explains all observations in favor of either the body wall origin or proximal leg origin of insect wings. Thus, our results show that insect wings are not novel structures, but instead evolved from existing, ancestral structures. One Sentence Summary CRISPR Cas9 knockout of leg gap genes in a crustacean reveals that insect wings are not novel structures, they evolved from crustacean leg segments