Bicyclus anynana embryonic dosage compensation. Zygosity-based sex determination in a butterfly drives hypervariability of Masculinizer

The central paradox of sex determination is why nature has devised many different ways of producing alternative sexes. We describe a novel mechanism for Lepidoptera that functions without a female-specific gene. Zygosity of a Z-linked gene (Masc) is the primary switch in the Afrotropical butterfly Bicyclus anynana, heterozygotes developing into ZZ males and hemizygotes into WZ (or Z0) females. However, Masc homozygosity in genetic ZZ males initiates female development, mismatched dosage compensation, and embryonic death. Consequently, selection against homozygotes has favoured rarity, driving an explosion in the number of Masc alleles, with 205 different coding sequence haplotypes detected in a sample of 243 females. Our discovery of a structurally distinct primary switch highlights the fascinating diversity of sex-determining mechanisms and underlying evolutionary drivers.