A Mucin2-Like Gene, NlMuc2, is Required for Early Embryonic Development in Nilaparvata lugens

In Nilaparvata lugens, a serious and widespread rice pest, high fecundity is one of the important reasons for large populations and outbreaks. As embryogenesis is a critical process associated with insect fecundity, this study investigated the function of the Mucin2-like gene (NlMuc2) in the embryogenesis of N. lugens. The results showed that NlMuc2 was highly expressed in female reproductive organs and in 1-day-old eggs. Knockdown of NlMuc2 via RNA interference (RNAi) in 3rd instar nymphs resulted in increased mortality. In 5th instar nymphs, NlMuc2 knockdown led to a dramatic reduction in egg hatchability from 86.0% to 24.0%, offspring numbers from 330.4 to 81.5, egg count from 382.4 to 217.0, and resulted in 86.0% of eggs exhibiting inverted embryos. In newly emerged females, NlMuc2 knockdown retarded ovarian development and decreased the number of mature eggs, with 51.0% of eggs containing inverted embryos. Transcriptome sequencing analysis of eggs revealed that numerous genes were downregulated after NlMuc2 knockdown, with 16 and 15 downregulated genes enriched in the Wnt and MAPK pathways, respectively. Temporal and spatial expression profiling of selected differentially expressed genes, including Axin, δ-Catenin, Glypican-4-like, and Ror-like in the Wnt pathway, and MKK4 in the MAPK pathway, showed expression patterns similar to NlMuc2. Knockdown of Ror-like, Glypican-4-like, or MKK4 reduced the total number of eggs. Knockdown of Axin or MKK4 reduced the egg hatchability. A similar phenotype of eggs with inverted embryos was also observed in eggs laid by dsMKK4-and dsAxin-treated females. Thus, NlMuc2 is involved in embryonic development mainly by regulating the Wnt and MAPK signaling pathways. These findings may provide new targets for pesticide design and RNAi-based control of N. lugens, and will also provide new insights into insect embryonic development and the function of insect mucins.