Sperm Production Induced in Female Silkworms

Scientists reveal key gene that determines maleness

Sperm production in female silkworms A. Testis-like gonads are found in female silkworms carrying the Masc-R gene (left photo). An enlarged image (middle photo) reveals the testis-like tissues are filled with small follicles similar to those found in testes. The tissues contain considerable numbers of sperm bundles (right photo, black arrows). B. The testis of a normal male silkworm is composed of four chambers (left photo), and each chamber is filled with a large number of follicles containing sperm (middle photo). The testis contains a large number of long, narrow sperm bundles (right photo). Each bundle holds several hundred sperm. C. The ovary of a normal female silkworm is composed of four tubular ovarioles. © 2016 Masataka Suzuki.

A group of University of Tokyo researchers and their collaborators embedded a male sex gene into the genome of female silkworms and found that they developed male reproductive glands and started producing sperm, as well as displaying other male characteristics. This finding may eventually lead to the development of pesticides targeting the Mascgene, considered the determinant of maleness in silkworms, and help pave the way for controlling agricultural pests through the sterilization of moths.

Sex of the silkworm is determined by small RNA molecules called piRNAs—a mechanism found only in silkworms. The piRNAs are produced by the Femgene located on the female sex chromosome, or W chromosome, which represses the Masc gene believed to determine maleness. Thus, silkworms carrying Fem are thought to repress the male gene, thereby developing into females, but it was unclear whether Masc induced maleness.

The research group led by Associate Professor Masataka Suzuki at the University of Tokyo Graduate School of Frontier Sciences and their collaborators designed a gene called Masc-R, which is resistant to piRNAs, and observed what would happen if they introduced the Masc-R gene into the female silkworm genome. They found that females embedded withMasc-R developed abnormal ovaries and the number of eggs they produced fell drastically, close to zero. The abnormal ovaries contained male sex gland-like tissues and, more notably, the tissues contained a considerable number of sperm. External genital organs and external features of abdominal segments also showed signs of partial masculinization.

The researchers concluded that the results strongly suggest the ability of Masc to induce maleness in the silkworm. The number of genes identified thus far as determining maleness in silkworms is still limited. Therefore, this study is valuable in showing the role of a gene determining maleness in butterflies and moths.

“The biological mechanism for sex determination in silkworms is very unique. Indeed, Masc is a novel male-determining gene unlike any other,” says Suzuki. He continues, “I was unsure whether Masc in fact determined maleness in the silkworm, but was relieved to find out that it does through this study.Masc is truly an amazing gene.”

The current outcome is the result of joint research with a team led by Researcher Megumi Kasashima at the Transgenic Silkworm Research Unit, Institute of Agrobiological Sciences, National Agriculture and Food Research Organization (NARO), and Associate Professor Susumu Katsuma and others at the University of Tokyo Graduate School of Agricultural and Life Sciences.