Genomic Southern blot hybridization of two sex-limited silkworm strains, W3a and W73, which show the female specific larval markers, crescent mark and star spot, were carried out to identify the areas of attached second chromosome by using RFLP probes of the ESTcDNA clones. When the two clones, e61 and m113 on the second linkage group were used, W3a showed female specific bands. On the other hand, even if these clones were used, W73 showed no difference in the band pattern between females and males. When other clones on the second linkage group were used, both strains showed no difference in the band pattern between females and males. This result indicated that the area of e61-m113 was included in the attached chromosome of W3a. The difference of the area of the attached chromosome between these strains suggested that the length of the W3's attached chromosome was longer than W73's.

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This research focused on the genetic analyses of cocoon filament fineness and its use for breeding silkworm races that have characteristics of cocoon filament fineness. Based on two kinds of Chinese parent silkworms (Bombyx mori), the results of the experiment showed that cocoon filament size had a positive interaction on cocoon shell weight and the ratio of cocoon shell weight, but it had a negative interaction on cocoon filament length. Moreover, the hybrid vigor rate was high in association with crop yield, cocoon shell weight, and cocoon filament mass.
However, examination against other characteristics showed that hybrid vigor was weakly associated with cocoon filament size, and had moderate association with cocoon weight, percentage of cocoon shell weight, and cocoon filament length. The curves plotted for estimated and measured cocoon filament lengths and sizes of F₁, F₂ and BF₁ showed similarities. This result suggests that it is possible to make predictions about several hybrids based on estimations drawn from the parent silkworm specimens. In addition, it was clarified that the size curve of cocoon filaments reflected an average of the parent silkworms. Therefore, this breeding strategy for silkworm races, of the thin cocoon filament, can meet the recent demands posed by various sides of the silk industry. This is because silk companies, sericulture farmers and egg production companies primarily base their selection preferences on cocoon filament fineness and cocoon filament length, followed secondarily by hybrid vigor or cocoon crop yield, etc.

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Based on the strategy for silkworm breeding in the former report (Shirota and Aso under consideration), the parental silkworm races of N12, N54, C32, and C66 have been under evaluation for nine years. The newly developed silkworm race, N12EN54~C32EC66 (Matsuoka-Hime), has excellent characteristics for cocoon filament fineness and has been offered to the silk industry in Japan.

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