In a study of African butterflies, researchers have discovered a new molecular mechanism of how the sex of an embryo is determined. This mechanism differs from the traditional sex determination pathway found in most insects. Typically, the sex of offspring is determined by a gene on the female-specific W chromosome, which inhibits a gene on the male Z chromosome. However, in the Squinting Bush Brown butterfly, researchers found that the sex determination switch does not rely on sex-limited genes or chromosomes. Instead, it recognizes differences in the sequence of the Masculinizer gene in males to produce healthy males.
When an embryo has one Z chromosome, it develops into a female, even without the presence of a W chromosome. However, if both copies of the Masculinizer gene are identical in a genetically male embryo, it is sent down the female developmental pathway and dies as an embryo due to gene expression imbalances. This results in genetically male butterflies producing 50% fewer sons when mating with individuals that share an identical Masculinizer gene copy, making it advantageous to be a rare variant of the gene.
The diversity of sex-determining mechanisms in nature is highlighted by the discovery of this alternative switch in the Squinting Bush Brown butterfly. The researchers suspect that this mechanism evolved as a response to male-killing bacteria that manipulate the host’s sex-determination machinery to promote their own transmission through females. This unique sex determination mechanism raises questions about its evolution and whether similar mechanisms have emerged independently in other butterfly and moth species.
The study’s corresponding author, Professor Ilik Saccheri, emphasizes the importance of understanding the evolutionary drivers of sex determination mechanisms. By reconstructing the evolutionary history of this alternative switch, exploring similar mechanisms in other species, and investigating the functioning of the Masculinizer same/different mechanism in depth, researchers hope to gain insight into the diverse ways in which nature produces males and females. This discovery underscores the complexity of sex determination in animals and the intriguing adaptations that have evolved over time.
