Bombyx small RNAs: Genomic defense system against transposons in the silkworm, Bombyx mori

Shinpei Kawaoka, Nobumitsu Hayashi, Susumu Katsuma, Hirohisa Kishino, Yuji Kohara, Kazuei Mita, Toru Shimada

Research output: Contribution to journalArticlepeer-review

39 Citations (Scopus)


Selfish genetic elements called transposons can insert themselves at new locations in host genomes to modify gene structure and alter gene expression. Expansion of transposons can occur when novel transposition events are transmitted to subsequent generations after germline hopping. Therefore, organisms seem likely to have evolved defense mechanisms to silence transposons in the germline. Recently, small RNAs interacting with Piwi proteins (piwi-interacting RNAs: piRNAs) have been demonstrated to be involved in genomic defense mechanism against transposons. Here, we show that piRNA-like small RNAs are present abundantly in the Bombyx ovary. We cloned 38,493 kinds of Bombyx small RNA from the ovary and performed functional characterization. Bombyx small RNAs showed a unimodal length distribution with a peak at 28 nt and a strong bias for U at the 5′ end. We found that 12,869 kinds of Bombyx small RNAs were associated with transposons or repetitive sequences. We classified them as repeat-associated small interfering RNAs (rasiRNAs), a subclass of piRNAs. Notably, antisense rasiRNAs have a strong bias toward U at 5′ ends; in contrast, sense rasiRNAs have a strong bias toward A at nucleotide position 10, indicating that the piRNA amplification loop proposed in Drosophila is evolutionarily conserved in Bombyx. These results suggest that Bombyx small RNAs regulate transposon activity.

Original languageEnglish
Pages (from-to)1058-1065
Number of pages8
JournalInsect Biochemistry and Molecular Biology
Issue number12
Publication statusPublished - 2008 Dec
Externally publishedYes


  • Bombyx mori
  • Germ cells
  • piRNA
  • rasiRNA
  • Transposable elements

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Insect Science


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