Construction And Analysis Of A Multi-marker Linkage Map For Silkworm, Bombyx Mori

Construction of a highly dense molecular linkage map can provide theoretical basis for maker-assisted silkworm selection. Chromosomes of female silkworm are complete linked, conditions in which the commonly used software Mapmaker for linkage mapping had not fully considered. Aiming at solving this problem,Wu published an article named"Constructing linkage maps with achiasmatic gametogenesis"and designed a new mapping software named F₂lnkgSilk, which, however,hasn't been used for silkworm linkage map construction. Besides, there have been few studies so far on multi-marker silkworm linkage mapping using the same segregating population, which makes it difficult to integrate different types of molecular linkage map. Based on these, this study was carried out as followed:Firstly, two populations were used for linkage analysis in comparison between Mapmaker and F₂lnkgsilk: one F₂ segregating population including 91 individuals with 300 AFLP makers and another F₂ segregating groups including 69 individuals with 470 RAPD makers. The results showed that the number of groups acquired from F₂lnkgsilk was slightly more than from Mapmaker, and the total genetic distance significantly longer using F₂lnkgsilk than using Mapmaker, the average mapping distance showing a similar pattern. But the number of two-marker groups was similar using these two softwares. As to individual linkage group, the total genetic distance and its average one were increased correspondingly using F₂lnkgsilk than Mapmaker. In some cases mapping distance of more than 50cM appeared, reflecting partial defective in F₂lnkgsilk.Secondly, a linkage map containing both RAPD primers and SSR primers was constructed using P50 and C108 as parental strains, and their 92 F₂ individuals as mapping population. The results were as followed:(1)A total of 386 polymorphism loci were obtained between two parental strains using 101 RAPD primers, averaging 3.8 loci per primer. 303 loci (137 in P50 and 166 in C108) had a Mendel segregation ratio of 3 to 1 according toχ2 test, taking 78.5% of all the polymorphism loci. A total of 62(64.0%) SSR primer sets out of 98 gave results showing polymorphism sites, among which 56 (88.9%) had a Mendel segregation ratio of 1 to 2 to 1 according toχ2 test.(2)A linkage map was constructed using Mapmaker with 359 effective markers. As a result, a total of 33 linkage groups were set up with 143 linkage markers, making 39.8% of all the effective loci and 31.9% of all the polymorphism loci.(3)Among the 33 linkage groups, group 2 was the biggest one of 24 markers compared to only two markers of the smallest ones. As to covering distance the largest linkage group was 705.7cM, and the shortest one only 1.1 cM. The maximum distance was 89.1 cM between two markers, and the minimum was 1.1 cM.(4)The 33 linkage groups covered about 2940.9 cM of the genomics of silkworm,Bombyx mori.. The average genetic length of each linkage group was 89.1 cM. In this paper 33 linkage groups were constructed, however Bombyx mori. has only 28 chromosomes, suggesting some of the groups were linked together.Finally, comparison was made between the multi-marker linkage map constructed above and a published linkage map using SSR markers by Miao et al.(2005) which was in accordance with 28 chromosomes of silkworm. The multi-marker linkage map contained 28 groups by RAPD markers, and 12 groups by SSR markers. The number of RAPD-SSR linkage group was seven, and 5 groups by SSR markers showed no linkage with RAPD markers.Increases in marker types and density for this multi-marker linkage map may be applied in QTL studies of economic traits and for integration process of different kinds of molecular linkage maps for silkworm. These studies will provide important theories for technology construction and application of marker-assisted selection.