Differential requirement of nanos homologs in the germline suggests the evolutionary path toward an inheritance mechanism of primordial germ cell formation in the silkmoth Bombyx

The lepidopteran insect Bombyx mori possesses unique embryogenesis characteristics among insects. nanos (nos) has conserved functions in metazoan primordial germ cell formation. Bombyx possesses four nos genes (M, N, O, P), a unique feature found in lepidopterans examined so far. Of these, maternal nosO mRNA exhibits a localization pattern: it may act as a primordial germ cell (PGC) determinant. A previous knock-out experiment of nosO showed that this localized mRNA is dispensable for PGC formation in laboratory environment and has limited involvement in PGC specification. This study examined whether other nos genes act redundantly with nosO in germline using RNAi and gene editing. Although individual embryonic RNAi exhibited no detectable phenotypic alterations, simultaneous RNAi of nosO/ nosP markedly reduced oocyte number and male fecundity. Additionally, nosP KO almost completely sterilized both sexes. Because nosP is broadly expressed in the posterior of embryos in non-germline specific manner, these results could reflect an evolutionary step taken by Bombyx toward its unique inheritance mechanisms. This study also suggests that nos genes in Bombyx do not affect anterior-posterior axis specification. This could reflect its characteristic embryogenesis.