Abstract
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 doublesex (dsx) 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 transformer (tra), which controls sex-specific splicing of dsx. In Lepidoptera, studies of sex determination have focused on the lepidopteran model species Bombyx mori (the silkworm). In B. mori, the primary signal of sex determination cascade starts from Fem, a female-specific PIWI-interacting RNA, and its targeting gene Masc, which is apparently specific to and conserved among Lepidoptera. Tra has not been found in Lepidoptera. Instead, the B. mori PSI protein binds directly to dsx 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.
Highlights
Sexual reproduction is an evolutionarily ancient feature
The novel sex determination factors that function as primary signals have been identified in Diptera in recent years. They include Nix, which is a distant homolog of tra2 and which was identified in Aedes aegypti; YoB, which is a maleness gene in Anopheles gambiae; Mdmd, a splicing factor that was identified in Musca domestica; and MoY, the Y-linked, male-determining factor in the Ceratitis capitata, which is functionally conserved in Bactrocera dorsalis, Bactrocera oleae and many other Teprhtidae species [14,15,16,17,18]
We have discovered that SIWI is crucial for feminizing the silkworm, whereas Argonaute RISC Catalytic Component 3 (Ago3) mutants display no phenotype involved in sex determination [25]
Summary
Sexual reproduction is an evolutionarily ancient feature. Recombination of chromosomes from two parents can result in the emergence or the loss of genes during meiosis. They include Nix, which is a distant homolog of tra and which was identified in Aedes aegypti; YoB, which is a maleness gene in Anopheles gambiae; Mdmd, a splicing factor that was identified in Musca domestica; and MoY, the Y-linked, male-determining factor in the Ceratitis capitata, which is functionally conserved in Bactrocera dorsalis, Bactrocera oleae and many other Teprhtidae species [14,15,16,17,18] These factors are diverse, the primary signal mediated by these genes is transduced by a pathway that leads to alternative splicing of dsx, which controls sex differentiation eventually [19]. We suggest further directions in the Lepidoptera model B. mori that will provide insights into sex determination in other Lepidoptera species
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