AsianScientist (Jun 5, 2014) – The 80 year-long search for the sex-determining factor in silkworms has finally ended with the discovery of the Piwi-interacting RNA (piRNA) Fem. This research has been published in Nature.
The sericulture (silk farming) industry has long sought ways to breed only males, which produce more silk of a higher quality. Unlike mammals which use the XY system, most insects and some birds and reptiles use the WZ system, where males are ZZ and females are WZ. Although it is known that the W chromosome is important for sex determination in silkworms (Bombyx mori), scientists have thus far been unable to identify the proteins that might be involved, hampered by the fact that the W chromosome contains mostly repetitive sequences and transposons rather than protein-coding sequences.
Turning their attention instead to RNA, Susumu Katsuma and colleagues at the University of Tokyo performed in-depth sequencing, comparing the RNA expression of male and female B. mori embryos. They found one transcript that was consistently expressed in females but absent in males, a piRNA molecule they named Feminizer (Fem).
piRNAs, working together with a class of regulatory proteins called piwi, are non-coding RNA molecules involved in gene regulation. Blocking the expression of Fem piRNA resulted in male rather than female splice variants of the sex-determining gene Bombyx mori doublesex (Bmdsx). The authors found that this was because Fem cuts up the mRNA of Masculinizer (Masc), which encodes for the protein that controls male-specific splicing of Bmdsx. Therefore, in the absence of Fem, Masc is produced and male development follows.
However, switching silkworm sex is not simply a matter of targeting Fem. Although silencing Fem at the embryonic stage resulted in the male splice variant of Bmdsx at 72 hours post-silencing, hatched embryos reverted to the female splice variant, possibly because the treatment only had a transient effect.
Nonetheless, these results are an important contribution to our understanding of sex determination in insects and are a promising first step towards being able to manipulate sex in insects and develop novel pest control strategies.
The article can be found at: Kiuchi et al. (2014) A single female-specific piRNA is the primary determiner of sex in the silkworm.
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Copyright: Asian Scientist Magazine; Photo: Kiuchi et al./Nature Publishing Group.
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