May 30, 2014

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A proposed model for the sex determination pathway in the silkworm Bombyx mori. The 29-base piRNA acts as a switch setting the insect’s sex to female, switching off the masculinizing gene Masc. © Munetaka Kawamoto

The silkworm Bombyx mori, a lepidopteran model insect that was the backbone of the Japanese sericulture industry, has two sexes as do humans. However, while human sex is determined by the presence or absence of a Y sex chromosome, silkworm sex is determined by the presence or absence of a W sex chromosome. The silkworm W chromosome plays a dominant role in female determination, suggesting the existence of a dominant feminizing gene in this chromosome. Despite many years of research, however, no gene has so far been identified.

Associate Professor Susumu Katsuma of the University of Tokyo Graduate School of Agricultural and Life Sciences and colleagues report that a single W chromosome-derived female-specific PIWI-interacting RNA (piRNA) of just 29 bases is the feminizing factor of B. mori, acting as a switch, and answering a question that has puzzled researchers for eighty years. This piRNA is of a type of short RNA that is never transcribed into a protein and is generally only found in germ cells (a type of cell that gives rise to gametes). Further, the research group demonstrated that the female-determining piRNA is produced by the cyclical process termed the “ping-pong cycle” that also produces the piRNA from messenger RNA (mRNA) of the masculinizing gene, Masc.

This study describes a novel sex determination system in which a single small RNA is the primary determiner of sex in an organism. This finding will enable us to manipulate the sex of lepidopteran insects and to develop novel strategies for insect pest control.

Paper Information

Takashi Kiuchi, Hikaru Koga, Munetaka Kawamoto, Keisuke Shoji, Hiroki Sakai, Yuji Arai, Genki Ishihara, Shinpei Kawaoka, Sumio Sugano, Toru Shimada, Yutaka Suzuki, Masataka G. Suzuki, Susumu Katsuma, “A single female-specific piRNA is the primary determiner of sex in the silkworm,” Nature, doi:10.1038/nature13315.