SSR Marker Mapping and Linkage Analysis of ch-2,the Second Recessive Gene of Chocolate Larvae in the Silkworm,Bombyx mori

The second recessive gene of chocolate larvae (ch-2) is one of the body color mutations in newly hatched larvae of the silkworm,Bombyx mori.This mutation has practical values for being used as a specific genetic system.In this research,silkworm variety P50 of which newly hatched larvae have the normal black body color and silkworm variety k04 with the second recessive gene of chocolate larvae (ch-2) were used as parental strains to prepare reciprocal populations.After back-crossing,two back-crossed populations,(k04 × P50) × k04 and k04 × (k04 × P50) ,were obtained and designated as BC1F and BC1M,respectively.Owing to a lack of crossing over between chromosomes Z and W of silkworm females,an already established silkworm SSR molecular marker linkage map was used for mapping and linkage analysis to ch-2 gene.After screening 20 polymorphic markers on the 18th linkage group of Bombyx mori,ch-2 gene was found to link with 7 SSR markers.Based on the existing microsatellite sequences that do not display polymorphism on the 18th linkage group,other SSR sites on the Scaffold were searched and were used for designing primers.As a result,2 new polymorphic SSR markers were obtained.All larvae with black body color in BC1 F population showed the same heterozygous profile with F1(k04 × P50) hybrids,and all larvae with chocolate body color in BC1F showed the same homozygous profile with parental strain k04,demonstrating that silkworm ch-2 gene is located on the 18th linkage group.An SSR molecular marker genetic linkage map was constructed by utilizing the other backcrossed population,BC1M.This linkage map has a genetic distance of 70.7 cM,with ch-2 mapped at 69.6 cM.The genetic distance between ch-2 gene and two nearest SSR markers,S1814 and S1819,is 7.9 cM and 1.1 cM,respectively.