Genome assembly and methylome analysis of the white wax scale insect provides insight into sexual differentiation of metamorphosis in hexapod

Scale insects are hemimetabolous, showing “incomplete” metamorphosis and no true pupal stage. Ericerus pela, commonly known as the white wax scale insect (hereafter, WWS), is a wax-producing insect found in Asia and Europe. WWS displays dramatic sexual dimorphism, with notably different metamorphic fates in males and females. Males develop into winged adults, while females are neotenic and maintain a nymph-like appearance, which are flightless and remain stationary. Here we report the de novo assembly and analysis of the WWS genome. From these data, we constructed a robust phylogenetic analysis of 24,923 gene families from 16 representative insect genomes, which indicates that holometabola evolved from hemimetabolous insects in the Late Carboniferous, about 50 million years earlier than previously thought. To study the distinct development of males and females, we analyzed the methylome landscape in either sex. Surprisingly, WWS displayed high levels of methylation (4.42%) when compared to other insects. We observed differential methylation patterns for genes involved in steroid and sesquiterpenoids production as well as related fatty acid metabolism pathways. We show here that both males and females produce distinct profiles of ecdysone (the principal insect steroid hormone) and juvenile hormone (a sesquiterpenoid), consistent with their different development fates. Our results provide a comprehensive genomic and epigenomic resource of scale insects that provide new insights into the evolution of metamorphosis and sexual dimorphism in insects.