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How Silkworms Determine Sex: A Molecule Called Fem piRNA

By Matt Goren2 min read

Sex Decided by a Small RNA

In silkworms, sex is not settled by a protein-coding gene. It is settled by a tiny RNA molecule that destroys another gene's message. The system was worked out in Bombyx mori and it is unlike the one in mammals in almost every respect.

The Chromosomes Are the Other Way Round

In mammals, males carry the odd chromosome — XY, with females XX. In silkworms and other Lepidoptera it is reversed: females are ZW and males are ZZ. The W chromosome is the female-determining one, and it is the mother, not the father, whose chromosome decides.

The Mechanism

The chain is short and unusually elegant:

  1. The W chromosome carries a gene called Feminizer (Fem).
  2. Fem does not make a protein. It makes the precursor of a piRNA — a very small RNA molecule.
  3. That Fem piRNA, loaded into a PIWI protein, finds and cuts the messenger RNA of a gene on the Z chromosome called Masculinizer (Masc).
  4. Masc, when it survives, drives male-specific splicing of a gene called doublesex, and the animal develops as male.

So maleness is the default that Masc produces, and femaleness is what happens when a piRNA from the W chromosome silences it. Sex in this animal is decided by an act of destruction.

Why It Mattered Beyond Silkworms

piRNAs were known mainly as a defence system, silencing transposons — the mobile genetic elements that would otherwise disrupt a genome. Finding one acting as the master switch for sex was a genuine surprise, and it established piRNA-mediated sex determination as a real mechanism rather than a curiosity.

What CRISPR Confirmed

Gene editing has since been used to test the model directly. Disrupting genes in this pathway with CRISPR/Cas9 produces partial sex reversal — genetically female animals shifting Masc and doublesex expression toward the male pattern. Knocking out the machinery predicted to matter changes the sex of the animal, which is about as clean a confirmation as biology offers.

Related: the silkworm genome · what silk is made of