Genetical studies on the N-methyl-N-nitrosourea induced non-molting b“ mutation in Bombyx mori”

Newly laid silkworm eggs of a inbred normal strain were immersed in 0.5mM N-methyl-N-nitrosourea solution for 60 minutes. The eggs were at an age of 90±15 minutes when the treatment was started. In the M₃ progenies after the treatment, a considerable number of dwarf larva was found on the second day of the first larval instar. All dwarf larvae continued to eat mulberry leaves slightly increasing body-size for about ten days, and then died without entering into molt. Duration of the first larval instar is 3–4 days for normal development of this insect. Genetic analysis revealed that the dwarf and non-molting lethal character was controlled by an induced recessive mutation. The mutant was named “non-molting b” with gene symbol nm-b. Linkage experiment showed that nm-b was linked with plain (p) and Yellow-blood (Y) which had been mapped at 0.0 and 25.6 position, respectively, on ganetic map of the second linkage group. Precise localization of nm-b gene was made by a three-point experiment involving p^M, Y and nm-b. The recombination value between p^Mand Y was calculated to be 25.68%, nm-b and Y 0.55%, and p^Mand Y 26.23%, Hence the arrangement of these three gene loci on the chromosome is in. the order of p-nm-b-Y. Taking correction factor into account, the locus of of nm-b is determined as 25.1 position on the second linkage group.