Isolation and characterization of sex chromosome rearrangements generating male muscle dystrophy and female abnormal oogenesis in the silkworm, Bombyx mori

In deletion-mapping of W-specific RAPD (W-RAPD) markers and putative female determinant gene (Fem), we used X-ray irradiation to break the translocation-carrying W chromosome (W ^Ze ). We succeeded in obtaining a fragment of the W ^Ze chromosome designated as Ze ^W, having 3 of 12 W-RAPD markers (W-Bonsai, W-Yukemuri-S, W-Yukemuri-L). Inheritance of the Ze ^W fragment by males indicates that it does not include the Fem gene. On the basis of these results, we determined the relative positions of W-Yukemuri-S and W-Yukemuri-L, and we narrowed down the region where Fem gene is located. In addition to the Ze ^W fragment, the Z chromosome was also broken into a large fragment (Z₁) having the + ^sch (1-21.5) and a small fragment (Z₂) having the + ^od (1-49.6). Moreover, a new chromosomal fragment (Ze ^WZ₂) was generated by a fusion event between the Ze ^W and the Z₂ fragments. We analyzed the genetic behavior of the Z₁ fragment and the Ze ^WZ₂ fragment during male (Z/Z₁ Ze ^WZ₂) and female (Z₁ Ze ^WZ₂/W) meiosis using phenotypic markers. It was observed that the Z₁ fragment and the Z or the W chromosomes separate without fail. On the other hand, non-disjunction between the Ze ^WZ₂ fragment and the Z chromosome and also between the Ze ^WZ₂ fragment and the W chromosome occurred. Furthermore, the females (2A: Z/Ze ^WZ₂/W) and males (2A: Z/Z ₁) resulting from non-disjunction between the Ze ^WZ ₂ fragment and the W chromosome had phenotypic defects: namely, females exhibited abnormal oogenesis and males were flapless due to abnormal indirect flight muscle structure. These results suggest that Z₂ region of the Z chromosome contains dose-sensitive gene(s), which are involved in oogenesis and indirect flight muscle development.