Silkworms' Sex-Determing Factor is an RNA Molecule Instead of a Protein

Research is being conducted by the multibillion dollar silk industry on the genetic determination of sex in silkworms to maximize efficiency and profit. The industry aims to breed only male silkworms because they weave higher-quality cocoons. New research has determined that this controlled all-male population may be possible due to the unique process silk moths Bombyx mori use for sex determination. This process involves a W chromosome in females that produces short RNA molecules to determine sex instead of proteins: the first sex chromosome seen in nature to do this.

Instead of the XY sex chromosome system in mammals, these insects have the WZ system, in which the female has one of each and the male has a pair of Z chromosomes. Scientists at the University of Tokyo have determined that the W chromosome in Bombyx mori codes for a specific RNA molecule-which they named "Fem". This short RNA molecule breaks down a corresponding molecule made by the "Masculinizer" gene on the Z chromosome. When the researchers silenced the "Masculinizer" gene in silkworm embryos, female tissue was formed. The results also showed that the contents of the W chromosome were deadly to embryonic males, suggesting that the "Fem" RNA coded by the W chromosome determines the sex of female silkworms. 

The implications of this process may however be problematic for creating an all-male population. Blocking the "Fem" molecule did not turn female embryos into males, and inhibition of "Masculinizer" killed male embryos.

Article:  http://www.nature.com/news/silkworm-sex-factor-is-no-ordinary-gene-1.15221

Silkworm Genome: http://silkworm.genomics.org.cn