| Domestic silkworm is an importantly economic insect, most economic traits of which show quantitative heredity, and cocoon quality is one of them. Hereditary of quantitative traits shows continuous variations, mainly due to the piling effect of polygenes, and polygenes controlling one quantitative trait should be studied as a unit through mathematics and statistics. However, the traditional methods limit the efficiency of utilizing desired quantitative traits greatly for being unable to identify single gene or chromosome segment controlling quantitative traits, and more difficult to make clear the locations of quantitative trait loci (QTL) on chromosomes and their relations with other genes. It is advisable to construct a high quality and density linkage map containing all the major genes which have great effect on the quantitative traits and hence to make clear their locations on chromosomes, single effect and mutual effect in order to monitor and to use the target genes more effectively.It is more and more difficult for geneticists to improve the present varieties through traditional theory and methods. Therefore it is very meaningful to realize molecular breeding through combining quantitative genetics with molecular genetics and developing genetic linkage map based on morphological characters into a molecular linkage map with a high density and thus to monitor the important economic characters of domestic silkworm on the basis of molecular level. What we have carried out in the dissertation is mainly to construct a molecular linkage map with a high density using amplified fragment length polymorphism (AFLP), locate and analyze the quantitative traits of domestic silkworm such as cocoon quality and pupal weight, evaluate the direct selection for the regular pattern of DNA polymorphism of different inbred lines and their hybrid offsprings through the directed differentiation selection for the cocoon quality of domestic silkworm, and to breed a new excellent race of domestic silkworm using in both spring and autumn on the basis of combing traditional breeding with the techniques of DNA molecular marker.1. Constructing a molecular linkage map with a high density using AFLPTwo presently used varieties 872 and Xianghui, which had broad genetic background, were used to construct a map as parents. F2 generation was self crossed from FI generation, which derived Irom the parent 872 and Xianghui, and 91 individuals were selected randomly from the F2 generation as a group to constructing a map.1.1 Total 1559 polymorphic bands were amplified using 66 AFLP primer combinations and 23.6 polymorphic bands were amplified by a pair of primer combination on average. Of all the 1559 polymorphic bands, there were 692 polymorphic bands conforming to the ratio of 3:1 after X2 detection, accounting 44.39% of all. The detected fragments ranged from 100bp to 1000bp. Of the 692 effective polymorphic loci, 550 loci were used to construct a and b subgroup, accounting 79.48% of the effective loci and 35.28% of all the polymorphic loci. The other 142 polymorphic loci didn't link each other and thus couldn't be used to construct the linkage map. The 302 polymorphic loci from the female parent Xianghui and the other 390 polymorphic loci from the male parent 872 were divided into 21 and 28 linkage groups respectively. 21 linkage groups from Xianghui were named as a subgroup and 28 linkage groups from 872 b subgroup.1.2 In a subgroup, 233 polymorphic loci linked each other while the other 69 polymorphic loci didn't; The number of markers ranged from^ to 43 in the individual linkage group of a subgroup, the average map distance of which was from 2.96cM to 13.08cM. The average distance between 2 loci was 8.81cM while the length of linkage group was from 22.3cM to 424.3cM. Of the 21 linkage groups in a subgroup, 19 had an average distance less than 10cM while 2 had an average distance between 10cM to 20cM. The average length of 21 linkage groups was 89cM (Table.2 ) and on each linkage group there were 11.1 loci on average.
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