The Application Of Microsatellite Markers In Silkworm (Bombyx Mori)

Molecular markers are very important in functional genome researches and it has beensuccessfully used in silkworm (Bombyx mori L.). Although SSR markers (or microsatellites)have been increasingly used and shown advantages over other markers, they have not beenwidely used in silkworm. In order to pursue the application of SSR markers in theeconomically important insect, Bombyx mori, firstly, robustic SSR markers were obtained bythe flow chart of large insert genome DNA library construction, Southern blotting, sequencingand primer designing, then these markers were used to study the genetic diversity amongsilkworm germplasms, construct SSR linkage map, locate the nonsusceptible to (Zhenjiangstrain) densonucleosis virus gene (nsd-Z) and breed new silkworm strains using markerassisted selection method. The results are shown as follows.1. The genetic relationship among 31 different types of silkworm strains was studiedusing 26 SSR markers. All of the markers were polymorphic and unambiguously separatedsilkworm strains from each other. A total of 188 alleles were verified with a mean value of7.2 alleles/loci (range 2-17). The average heterozygosity value for each SSR locus rangedfrom 0 to 0.60, and the highest was 0.96. The average polymorphism index content (PIC)was 0.66 (range 0.12-0.89). UPGMA cluster analysis of Nei's genetic distance groupedsilkworm strains based on their origin. Principal components analysis (PCA) for SSR datasupports their UPGMA clustering.2. A backcross population (BC1) was derived from two parentals, Jingsong, a widelyused strains in silk production and Lan10, which possesses short silk length. The SSRtechnique was employed for mapping the QTLs. The QTLs for the whole cocoon weight,cocoon shell weight, ratio of cocoon shell, weight of pupae were analyzed. Three QTLs forwhole cocoon weight, pupa weight and cocoon shell weight were mapped on linkage group 1and 23, respectively, and two QTLs were mapped to a same locus. Two QTLs for cocoonshell ratio were mapped on linkage group 18 and 19, respectively.3. Due to a lack of crossing over in females, reciprocal backcrossed F₁ (BC₁) progenywere used for linkage analysis and mapping of the nsd-Z gene using silkworm strains Js andL10, which are classified as being highly susceptible and non-susceptible to DNV-Z,respectively. The nsd-Z gene was found to be linked to seven SSR markers, using BC1Fpopulation. Using a reciprocal BC1M cross, we constructed a linkage map of 80.6 cM withnsd-Z mapped at 30 cM, with the closest SSR marker at a distance of 4.4 cM.4. Marker-assisted selection of nonsusceptible to DNV-Z silkworm strains wereexercised in predicting DNV-resistant type in backcrossed generations. One silkworm strainnonsusceptible to DNV-Z had been bred using this method and its economic characters weresimilar to the commercial silkworm strains. These PCR-based markers will be useful insilkworm breeding programs screening for this trait in segregating populations.