Transgenic Expression of the piRNA-Resistant Masculinizer Gene Induces Female-Specific Lethality and Partial Female-to-Male Sex Reversal in the Silkworm, Bombyx mori

摘要

In Bombyx mori ( B . mori ), Fem piRNA originates from the W chromosome and is responsible for femaleness. The Fem piRNA-PIWI complex targets and cleaves mRNAs transcribed from the Masc gene. Masc encodes a novel CCCH type zinc-finger protein and is required for male-specific splicing of B . mori doublesex ( Bmdsx ) transcripts. In the present study, several silkworm strains carrying a transgene, which encodes a Fem piRNA-resistant Masc mRNA ( Masc-R ), were generated. Forced expression of the Masc-R transgene caused female-specific lethality during the larval stages. One of the Masc-R strains weakly expressed Masc-R in various tissues. Females heterozygous for the transgene expressed male-specific isoform of the Bombyx homolog of insulin-like growth factor II mRNA-binding protein ( Imp ~( M )) and Bmdsx . All examined females showed a lower inducibility of vitellogenin synthesis and exhibited abnormalities in the ovaries. Testis-like tissues were observed in abnormal ovaries and, notably, the tissues contained considerable numbers of sperm bundles. Homozygous expression of the transgene resulted in formation of the male-specific abdominal segment in adult females and caused partial male differentiation in female genitalia. These results strongly suggest that Masc is an important regulatory gene of maleness in B . mori . Author Summary In the silkworm, Bombyx mori , a W-chromosome-linked gene Feminizer ( Fem ) determines femaleness. Fem transcript yields a piRNA ( Fem piRNA) and Fem - piRNA-PIWI complex targets and cleaves mRNAs transcribed from the Masculinizer ( Masc ). Masc is required for male-specific expression of Bmdsx , which is an important regulatory gene for sexual differentiation, and therefore, Masc is considered to be essential for maleness. However, there has been no direct evidence that Masc indeed causes maleness in sexually dimorphic structures. To clarify this point, we established silkworm strains carrying a transgene that expresses Fem -piRNA-resistant Masc gene ( Masc-R ). Transgenic expression of the Masc-R induced male mode of expressions of downstream sex-determining genes in females. Notably, ovaries in these females exhibited testis-like structures that contained sperm bundles. Homozygous expression of the Masc-R caused formation of the male-specific abdominal segment in adult females and induced partial male differentiation in female genitalia. Thus, Masc can induce maleness at the morphological level and is sufficient for spermatogenesis. This is the first report to our knowledge on a gene that can masculinize a wide variety of sexual characteristics in lepidopteran insects.