Instability of w-translocation of dark purple serosa color gene (W₂⁺) in the silkworm, bombyx mori

In stability of W-translocation of the wild type dark purple serosa color(black egg color) gene(W₂⁺) has been examined from two successive generations of the silkworm, Bombyx mori : strain SLE-1. The translocation stability index (SI) ranged from 67.3 to 97.7% with a mean value of 85.1 and standard deviation of 6.0, according to the data from 72 batches raised in Spring 1982. Only 6 batches(8.3%)out of 72, had SI over 95.0%. Ninety batches were abtained from the crosses between black egg female moths from 4 batches with SI over 90% in Spring 1982 and white egg male moths from the strain JHW-1, and raised in Spring 1983. The SI values of 1983 ranged from 57.8 to 100% with mean of 85.9 and standard deviation of 12.9, excluding a single batch with exceptionally low SI value -17.8%, The exceptionally low SI can partly be interpreted as a result of detachment of translocated chromosomal segment during the mitosis of gonadal or oogonial cells. Only 8 batches(8.9 %) out of 90 had SI values over 95.0%, Selection for two generations did not increase the average SI level. In a single batch(R87-89), all black eggs were all female and all white eggs all male, which may indicate that W-translocation has been stabilized in this particular family. The family R87-89 could be recommended to be used in the succeeding breeding program to improve economic characteristics, and eventually to be utilized in the male-silkworm rearing. However the W-translocation in R87-89 should be ascertained in the future experiments. In addition to the detachment and reversion hypothesis, hypotheses of somatic instability of translocation, chromosomal imbalance in translocation heterozygotes, and transposable controlling elements were also considered to explain the instability of W-translocation and outliers with exceptional segregation ratios. Estimation of SI values has also been proposed as a measure of the extent of instability in the interpretation of W-translocation instability by other hypotheses. To improve the SI level and to study the possible such gene action as controlling elements, pedigree methods has been suggested, accompanied by successive γ-irradiation.