Genetic Analysis Of Quantitative Trait Loci Affecting Silk Quality In Silkworm (Bombyx Mori)

Silk quality is the most important economic characteristic of silkworms (Bombyx mori).It is controlled by multiple loci. The silk quality of the silkworm strains Jingsong and Lan10have significantly diverged. A backcross population (BC1) was bred using Jingsong andLan10as parents to identify the quantitative trait loci (QTLs). In this research, a geneticlinkage map of the silkworm was constructed using the BC1mapping population, whichcontained87sequence-tagged site markers,28simple sequence repeat markers, and10single nucleotide polymorphism markers. A linkage map was constructed from the data,which consisted of125markers, spanning1602.2cM in total length, distributed over theexpected28linkage groups. Eight QTLs were detected for silk length, whole cocoon weight,pupae weight, silk weight, and cocoon shell ratio. The8QTLs were distributed in3linkagegroups (linkage groups1,14, and23) based on the constructed linkage map. In addition,five QTLs were located on the first chromosome, and the LOD values were the highest, andthree of them are between the same marks in linkage group1.This research provides an excellent foundation for map-based cloning of major genesthat control the production of silk-related products．Compared with previous research onQTLs for cocoon quality, the locations of the QTLs in the chromosomes could not beidentified because of the difficulties in duplicating their AFLP or RAPD results. In thisresearch, the exact location of the STS, SSR, and SNP markers were designed from thesilkworm fine genome sequence, and these markers can be duplicated in any other silkwormstrains. The results are consistent with the results of Zhan et al.(2009). First, the detectedQTLs were distributed mainly in groups1and23. Second, the LODs of the QTLs in group1are extremely high. These results indicate that the data are accurate and can be duplicated.Both results showed the existence of one or more QTLs that affect silk and cocoon qualitieson chromosome1. On the other hand, in the findings by Zhan indicated two QTLs forcocoon shell ratio in groups18and19; however, no other QTLs were detected. This couldbe attributed to the insufficient markers in those groups, and the low marker density isinsufficient to detect any QTLs. Although we constructed a linkage map that covers all ofthe28chromosomes for silkworm, the marker density in this research was still low, maybesome QTLs that controlled silk and cocoon qualities were not detected. More SSR andSTS/SNP markers will be designed in future studies to finely map the QTLs to allowmap-based cloning to better understand the mechanism of silk production by silkworms. We believe that these results have laid an important foundation for the map-basedcloning of QTLs and marker-assisted selection for improving the silk quality ofeconomically important silkworm strains.