Threshold-dependent gene regulation and partial assortative mating determine wing dimorphism of an Insect

Wing dimorphism is a fantastic life-historical trait of insects which indicates the developmental trend of populations, going as migrants or staying as residents. The alternative wing morph enhances adaptability of insects to changing environments. However, the underlying mechanism for maintaining wing dimorphism remains vague. The brown planthoppers Nilaparvata lugens, a serious pest of rice, are either short-winged or long-winged. The shift between two wing morphs is determined by two insulin receptors. So, this pest is a better model organism to explore the mechanism of wing dimorphism. Here, the short- and long-winged strains of N. lugens were selected. Rates of a selected wing morph were linearly increased in populations, and the selection response of the short-winged morph was stronger than that of the long-winged. Selection enhanced the migratory or resident propensity. Directional selection for the long-winged morph resulted in longer and lighter forewings, while selection for the short-winged morph generated shorter and thicker wings and higher fecundity. Relative expression levels of wing development genes InR1, InR2, and Foxo were positively correlated with the short-winged rate in the short-winged strain, while they were negatively correlated with the long-winged rate in the long-winged strain at specific stages. The change rate of expression levels was approximately two times as high as that of the wing morph rates in populations, indicating a high threshold-dependent gene expressed regulation of wing morph. A partial assortative mating behavior occurred between the nearly pure-bred lineages of short- and long-winged strains. The short-winged males preferred to mate with short-winged females while the long-winged males mated with the short- and long-winged females equally. The threshold-dependent mode may be more flexible to adjust wing morphs according to environmental conditions and the partial assortative mating promotes the genetic exchange between two wing morphs which drive the maintenance of wing dimorphism.