Mottled mosaic strains in the silkworm, induced by X-ray irradiation, contain chromosomal fragments carrying the larval body marking genes that are lost occasionally during larval development. In one of the mosaic strains, mottled zebra (Zem), the somatic loss of the chromosomal fragment is presumed to cause the mosaic pattern, but the fragment has not yet been identified. Here, we showed that Zem individuals have an extra small chromosomal fragment (Ze fragment) using genetical and cytological methods. The rate of loss of the Ze fragment, calculated based on the data of segregation distortion, is higher than one from a different mottled strain, mottled striped (pSm). Fluorescent in situ hybridization with theBombyxtelomeric sequence (TTAGG)n as a probe demonstrated that the Ze fragment has a telomeric repeats at only one chromosomal end, although the fragment of pSm (pS fragment) has the repeats at both ends. These data show that the broken ends of chromosomal fragments generated due to X-ray irradiation could be basically healed by de novo addition of the telomeric repeats and the structural difference of telomere may be related to the stability of chromosomal fragments.
