Molecular mechanisms of sex determination in Lepidoptera: current status and perspectives

Moths and butterflies (Lepidoptera) are the largest group of organisms with female heterogamety and the sex chromosome system WZ/ZZ (female/male) or exceptionally Z0/ZZ. However, the genetic basis of sex determination in Lepidoptera remained unknown for a long time until the sex-determining pathway was discovered in 2014 in the silkworm Bombyx mori. In this species, the dominant W chromosome carries a Feminizer (Fem) gene encoding a precursor of a Fem piRNA that promotes femaleness by downregulating the expression of a Z-linked gene, Masculinizer (Masc). In the W chromosome absence, Masc promotes male development and controls dosage compensation. In the 10 years since this discovery, significant progress has been made in understanding the molecular mechanisms of sex determination in Lepidoptera. Data from recent studies discussed in this review suggest a conserved role for Masc in male sex determination and dosage compensation in the clade Ditrysia, which comprises the majority of Lepidoptera. Although the primary sex-determining signals are not conserved, the presence of feminizing piRNAs of different origins in distantly related species suggests convergent evolution of a similar mechanism of female sex determination. A unique exception is zygosity-based sex determination in the butterfly Bicyclus anynana, where the primary signal is the state of the hypervariable Masc gene. In other species with a dispensable W chromosome, such as the silkmoth Samia cynthia, sex is determined by the Z:A ratio, but a molecular mechanism is not yet known. Overall, the available data suggest considerable diversity in the upstream molecular mechanisms of sex determination in Lepidoptera.