Structural analysis of spontaneous Z-W translocations in the silkworm, Bombyx mori

In the silkworm, Bombyx mori, femaleness is determined by the presence of the W chromosome irrespective of the number of Z chromosomes (Hashimoto, 1933). During meiosis in females, all chromosomes, including sex chromosomes, form bivalents (Traut et al., 1999). However, the W chromosome is recombinationally isolated from the Z chromosome in B. mori, because meiosis is achiasmatic, and no recombination occurs in females (Sahara et al., 2012). In sericulture, male silkworms are more viable and productive than female silkworms (Strunnikov, 1995). The sex-linked balanced lethal (SLBL) strains make it possible to rear only male silkworms, thus improving the production and quality of silk. The construction of an SLBL strain requires two steps: (i) induction of translocation events between the W and Z chromosomes and (ii) induction of Z-linked recessive lethal mutations, which are compensated by the dominant allele translocated onto the W chromosome (Strunnikov, 1995). In Japan, one such SLBL strain “PLATINA BOY,” has been introduced for commercial use in the production of luxury fabrics (Ohnuma, 2007, 2010). Generally, translocations and mutations are induced by irradiation or exposure to a chemical mutagen. However, Ohnuma has developed a strategy for obtaining T(W;Z) chromosomes and Z-linked recessive lethal mutations arising from frequent spontaneous translocation events between the T(W;Z)+ chromosome and T(Z;2)Y females (Fig. 1a, b) (Ohnuma, 2000; Ohnuma and Takemura, 2006; Ohnuma et al., 2009). He effectively used the sch gene and sex-linked recessive lethal mutations, and screened translocations and mutations soon after hatching (Fig. 1a, b). In all, 25 types of aberrant chromosomes were identified (Ohnuma, 2000; Ohnuma and Takemura, 2006; Ohnuma et al., 2009; Ohnuma, 2010). Unfortunately, most of them, except for T(Z;W)l4, T(Z;W)l7, T(W;Z;2)Y-3, and T(W;Z;2)Y-4, were discarded without any analysis. However, the remaining four aberrant sex chromosomes are novel bioresources and it is of great interest to determine whether the breakpoints of the spontaneous translocation events reside within the W-chromosomal region or Z-chromosomal region of the T(W;Z)+ chromosome. Genome data is powerful tool to characterize chromosomal aberrations. In the Bombyx genome project, most areas of the Bombyx genome were assembled into large blocks of sequences (International Silkworm Genome Consortium, 2008). However, the W chromosome sequences were not included in the genome data, because the male genome was subjected to a whole-genome shotgun analysis to avoid the repetitive nature of the W chromosome (Abe et al., 2005a; Sahara et al., 2003). On the other hand, twelve W-chromosome specific sequences have been identified by screening arbitrary 10-mer primers, which yielded female-specific amplification products (Abe et al., 2005b). These sequences have been used to characterize the structural mutants of the W chromosome (Abe et al., 2005b; 2008; Fujii et al., 2006; 2007). In this study, the W-specific sequences were used to characterize four aberrant sex chromosomes arising from spontaneous translocations between T(W;Z)+ and T(Z;2)Y chromosomes. We obtained molecular evidence that the breakpoints of the translocation events reside within the W-chromosomal region of the T(W;Z)+ chromosome. We also described the relative location of the W-specific sequences on the W chromosome.