title: Guardian small RNAs and sex determination.
abstract: The W chromosome of the silkworm Bombyx mori has been known to determine femaleness for more than 80 years. However, the feminizing gene has not been molecularly identified, because the B. mori W chromosome is almost fully occupied by a large number of transposable elements. The W chromosome-derived feminizing factor of B. mori was recently shown to be a female-specific PIWI-interacting RNA (piRNA). piRNAs are small RNAs that potentially repress invading "non-self" elements (e.g., transposons and virus-like elements) by associating with PIWI proteins. Our results revealed that female-specific piRNA precursors, which we named Fem, are transcribed from the sex-determining region of the W chromosome at the early embryonic stage and are processed into a single mature piRNA (Fem piRNA). Fem piRNA forms a complex with Siwi (silkworm Piwi), which cleaves a protein-coding mRNA transcribed from the Z chromosome. RNA interference of this Z-linked gene, which we named Masc, revealed that this gene encodes a protein required for masculinization and dosage compensation. Fem and Masc both participate in the ping-pong cycle of the piRNA amplification loop by associating with the 2 B. mori PIWI proteins Siwi and BmAgo3 (silkworm Ago3), respectively, indicating that the piRNA-mediated interaction between the 2 sex chromosomes is the primary signal for the B. mori sex determination cascade. Fem is a non-transposable repetitive sequence on the W chromosome, whereas Masc is a single-copy protein-coding gene. It is of great interest how the piRNA system recognizes "self "Masc mRNA as "non-self" RNA.
authors.affiliation: a Department of Agricultural and Environmental Biology; Graduate School of Agricultural and Life Sciences ; The University of Tokyo ; Bunkyo-ku , Tokyo , Japan.
authors.forename: Susumu
authors.initials: S
authors.lastname: Katsuma
authors.forename: Munetaka
authors.initials: M
authors.lastname: Kawamoto
authors.forename: Takashi
authors.initials: T
authors.lastname: Kiuchi
mesh_terms: D000818:Animals; D012831:Bombyx / Q000196:embryology / Q000235:genetics*; D005260:Female; D034741:RNA, Small Interfering / Q000235:genetics*; D012730:Sex Chromosomes / Q000235:genetics*; D019849:Sex Determination Processes / Q000235:genetics*
chemical_list: D034741:RNA, Small Interfering
keywords: PIWI proteins; amplification cycles; piRNA; ping-pong; sex determination; silkworm