Linkage and mapping analyses of the densonucleosis non-susceptible gene nsd-Z in the silkworm Bombyx mori using SSR markers.

In the silkworm Bombyx mori, non-susceptibility to the Zhenjiang (China) strain of the densonucleosis virus (DNV-Z) is controlled by the recessive gene nsd-Z (non-susceptible to DNV-Z), which is located on chromosome 15. Owing to a lack of crossing over in females, reciprocal backcrossed F1 (BC1) progeny were used for linkage analysis and mapping of the nsd-Z gene using silkworm strains Js and L10, which are classified as being highly susceptible and non-susceptible to DNV-Z, respectively. BC1 larvae were inoculated with the DNV-Z virus at the first instar, and DNA was extracted from the individual surviving pupae and analyzed for simple sequence repeat (SSR) markers. The nsd-Z gene was found to be linked to 7 SSR markers, as all the surviving larvae in the BC1female (F1female x L10male) showed the homozygous profile of strain L10, and the sick larvae in the BC1female (F1female x L10male) showed the heterozygous profile of Js x L10 F1 hybrids. Using a reciprocal BC1male (L101female x F1male) cross, we constructed a linkage map of 80.6 cM, with nsd-Z mapped at 30 cM and the closest SSR marker at a distance of 4.4 cM.