A W chromosome-derived feminizing piRNA in pyralid moths demonstrates convergent evolution for primary sex determination signals in Lepidoptera

Abstract

Background The primary signals of sex determination in insects are diverse and evolve erratically. This also appears to be the case with moths and butterflies. In the silkworm Bombyx mori, female development is triggered by a W-linked Feminizer (Fem) piRNA by degrading the mRNA of the Z-linked Masculinizer (Masc) gene, which controls male development. However, Fem piRNA is absent in the related Trilocha varians. Since a mechanism similar to that in the silkworm has evolved in the phylogenetically distant Plutella xylostella, we investigated whether this mechanism is also conserved in other groups of Lepidoptera. Results We examined the W chromosome of Ephestia kuehniella for non-coding feminizing genes and identified a putative feminizing piRNA and many partial copies of the EkMasc gene. The piRNA is generated by a repetitive W-linked sequence named E. kuehniella Moth-overruler-of-masculinization (EkMom). EkMom piRNA shows high similarity to a region of the Z-linked EkMasc genes and is expressed at the onset of female development, suggesting that it is likely the primary trigger of sex determination. We then mapped small RNA-seq data from embryos of the related Plodia interpunctella to the PiMasc gene and identified a single small RNA, a putative PiMom piRNA, able to target PiMasc, and with high sequence identity to EkMom piRNA. Both the PiMom and EkMom repeats are present in high copy number and form a single cluster on the W chromosome. Conclusions Our study provides multiple lines of evidence that a small piRNA is the putative primary sex-determining signal in two pyralid moths and highlights a possible pathway for the origin of feminizing piRNAs in Lepidoptera. A similarity between the proposed mechanism for female sex determination in E. kuehniella and P. interpunctella (Pyralidae) and P. xylostella (Plutellidae) and the Fem piRNA function in B. mori (Bombycidae) suggests convergent evolution of feminizing piRNAs in Lepidoptera.