The Sex Determination Cascade in the Silkworm.

In insects, sex determination pathways involve three levels of master regulators: primary signals, which determine the sex; executors, which control sex-specific differentiation of tissues and organs; and transducers, which link the primary signals to the executors. The primary signals differ widely among insect species. In Diptera alone, several unrelated primary sex determiners have been identified. However, the () gene is highly conserved as the executor component across multiple insect orders. The transducer level shows an intermediate level of conservation. In many, but not all examined insects, a key transducer role is performed by (), which controls sex-specific splicing of . In Lepidoptera, studies of sex determination have focused on the lepidopteran model species (the silkworm). In the primary signal of sex determination cascade starts from , a female-specific PIWI-interacting RNA, and its targeting gene , which is apparently specific to and conserved among Lepidoptera. has not been found in Lepidoptera. Instead, the PSI protein binds directly to pre-mRNA and regulates its alternative splicing to produce male- and female-specific transcripts. Despite this basic understanding of the molecular mechanisms underlying sex determination, the links among the primary signals, transducers and executors remain largely unknown in Lepidoptera. In this review, we focus on the latest findings regarding the functions and working mechanisms of genes involved in feminization and masculinization in Lepidoptera and discuss directions for future research of sex determination in the silkworm.