Sex determination in moths and butterflies: Masculinizer as key player

Knowledge about sex determination in Lepidoptera is starting to unfold just over a decade after the discovery of the primary sex determination trigger in the silkworm Bombyx mori. The silkworm has a W-dominant sex determination mechanism with a PIWI-interacting RNA (piRNA) precursor gene called Feminizer (Fem) as the primary trigger. The emerging view is that the silkworm is unsuitable to predict primary triggers in other Lepidoptera species, despite its role as model organism. However, the Z-linked gene named Masculinizer (Masc), which is targeted by Fem piRNA in the silkworm, plays a key role in sex determination in all species studied so far. This conserved role of Masc at the beginning of the sex determination cascade differs from what is known in other insects, where the cascade is initially diverse and becomes increasingly conserved towards the end, where doublesex (dsx) is alternatively spliced into a female or male variant. Sex-specific dsx splicing is also conserved in Lepidoptera, while the other genes which make up the sex-determining cascade are yet to be revealed in full detail. The sex determination mechanisms in two species are highlighted because, unlike the silkworm, they do not rely on a primary trigger from the W chromosome. The moth Samia cynthia ricini uses the ratio of Z chromosomes to autosome sets to determine sex. The butterfly Bicyclus anynana has a sex determination more similar to the honey bee than to the silkworm, with the zygosity of a hypervariable region of Masc determining whether individuals become female or male.