Linkage and mapping analyses of the normal marking gene +P in the silkworm (Bombyx mori) using SSR markers

In the silkworm, Bombyx mori, normal markings are mainly controlled by the +P gene, which is located on the second chromosome. Due to a lack of crossing over in females, reciprocal backcrossed F1 (BC1) progenies were used for linkage analysis and mapping of the +P gene based on an SSR linkage map using silkworm strains P50 and H9, which are normal marking and sex-limited marking, respectively. The +P gene was found to be linked to 3 SSR markers. Using a reciprocal BC1M cross, we constructed a linkage map of 22.5 cM, with +P mapped at 11.3 cM and the nearest SSR marker S0206 at a distance of 3.0 cM. Based on a fine genome map of domesticated silkworms, Kaikoblast analysis showed that the physical distance between the nearest markers (containing the +P gene) is 995 kb. Further analysis showed that BGIBMGA009689, BGIBMGA009688, and BGIBMGA009687 are closer to +P, and that BGIBMGA009689 is closest to +P, with a physical distance of 19.1 kb.