Abstract:  Sex determination is an outstanding question in developmental and evolutionary biology. The sex determination cascade in most of known insects is: primary signaling element→key gene of sex determination→double switch gene→sex differentiation gene. In spite of this pattern, different insects have different sex determination genes and regulatory mechanisms, especially the primary signaling element of sex determination. Since the primary signal of Drosophila melanogaster was discovered, the primary signal of mosquito, bee, wasp (Nasonia vitripennis), silkworm (Bombyx mori) and other model insects has been determined successively. There are many kinds of primary signals, including the dose of sex chromosomes, male-determining factors (M factors), heterozygosity of alleles and maternal imprinting, which make it more difficult to study non-model insects to some extent. Even so, the downstream regulatory mechanism is relatively conserved. In particular, the transformer (tra)+transformer2 (tra2)→doublesex (dsx)/fruitless (fru) pathway is fairly common in most insects. Tra produces alternative splicing by sensing primary signals, and with the help of tra2, tra regulates the splicing of itself and the downstream dsx and fru, and maintains gender development. Acting as the terminal ‘double-switch’, dsx is the most conserved gene in the insect sex determination cascade. dsx is highly conserved in regulating bisexual development, courtship behavior, genitalia and formation of sexual dimorphism. As a switch gene of Drosophila sexual behavior, fru is involved in regulating almost all male sexual behaviors of Drosophila. Its function has been verified in a variety of insects such as Bactrocera dorsalis, Aedes aegypti and B. mori, and it has become a typical gene for understanding the complex sexual behavior of insects. Understanding the sex determination cascade in insects, and clarifying the function and interaction of each sex determination gene are essential to elucidate the mechanism of sex determination. It provides a theoretical basis for revealing the general law of insect sex determination, and promoting the basic research on the upstream regulatory mechanism of insect sex determination, and realizing the genetic manipulation of insect sex determination.

Key words: Insect, sex determination cascade, primary signal, key gene; alternative splicing