The female-killing chromosome of the silkworm, Bombyx mori, was generated by translocation between the Z and W chromosomes.

Bombyx mori is a female-heterogametic organism (female, ZW; male, ZZ) that appears to have a putative feminizing gene (Fem) on the W chromosome. The paternally transmitted mutant W chromosome, Df(p ( Sa ) + ( p )W + ( od ))Fem, derived from the translocation-carrying W chromosome (p ( Sa ) + ( p )W + ( od )), is inert as femaleness determinant. Moreover, this Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome has been thought to have a female-killing factor because no female larvae having the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome are produced. Initially, to investigate whether the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome contains any region of the W chromosome or not, we analyzed the presence or absence of 12 W-specific RAPD markers. The Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome contained 3 of 12 W-specific RAPD markers. These results strongly indicate that the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome contains the region of the W chromosome. Moreover, by using phenotypic and molecular markers, we confirmed that the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome is connected with a partially deleted Z chromosome and that this fused chromosome behaves as a Z chromosome during male meiosis. Furthermore, we demonstrated that the ZZW-type triploid female having the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome is viable. Therefore, we concluded that the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome does not have a female-killing factor but that partial deletion of the Z chromosome causes the death of the ZW-type diploid female having the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome. Additionally, our results of detailed genetic analyses strongly indicate that the female-killing chromosome composed of the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome and deleted Z chromosome was generated by translocation between the Z chromosome and the translocation-carrying W chromosome, p ( Sa ) + ( p )W + ( od ).

Consequences of the AF-2 incident in Japan.

The discovery of the potent mutagenicity of AF-2, which was once used in Japan as a food preservative, has exerted a great influence not only on screening procedures for carcinogenic compounds but also on legislative approaches to mutagenic substances. It promoted the synthesis of exceedingly sensitive and reliable tester strains in Salmonella and supported the hypothesis of a common mechanism between mutagenicity and carcinogenicity. Thus preliminary screening for carcinogenic substances has become feasible using mutagenicity as an index. It also contributed greatly to the formulation of legislative measures for chemical substances which for the first time gave due attention to mutagenicity. Furthermore, the exposure of a large population to such a potent mutagen raised a question as to what extent the genetic constitution of the Japanese population might have been damaged. This suggested that urgent need for a system to monitor the total genetic damage to a human genome.