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2. Function of Silkworm Germplasm Resources

2.1 Silkworm germplasm resource is the necessary material base for the sustainable development of sericulture

Sericulture is supported by the silkworm gene resource diversity. Sericulture advancement are largely based on the silkworm variety improvement, and breeding of new silkworm variety in turn based on the material supply of rich silkworm germplasm resource. Owing to the long history of sericulture practice and wide diversity of geographical conditions, there is a very rich resource of silkworm germplasm bank in China. At present there are over 600 strains preserved in the central germplasm maintenance center. As the progress in silkworm germplasm collection and investigation, many of the strains are used in silkworm breeding for commercial activity and played significant role in the advance of commercial silkworm varieties in China. For Example, the pure lines Huahe, 672, 652, 303, Su17, Dongfei, Su12 were selected from the germplasm bank to be used as the parent materials for the breeding of three way hybrid "Su3.Qiu3 × Su4" which had been distributed in Jiangsu and Sichuang province about 16million boxes from 1979 to1988, and it was estimated by a net increase of 7million yuan for its better performance. Other dominant hybrids such as Yunhan × Hua9, Su16 × Su17, Dongfei × Huahe, Jingsong × Haoyue, 306 × Huashi, Dong34 × Su12, are all bred by the utilization of purelines preserved in the germplasm bank, It was these hybrids that four times renovation was realized in silkworm varieties in China during the recent 40 years, and finally came to keep up with the Japanese level.

2.2 Sericulture industry is depending on the rich silkworm gene pool

Since the discovery of heterosis in silkworm breeding, hybrids were used almost all over the world in commercial cocoon production, which had greatly enhanced both in quantity and quality of cocoon yield per unit. Meanwhile, there is the tendency of similarity in main economic characters and close in genetic relationship, which resulted in the elimination of vast local varieties with significant ecological diversity and the lost of some specific genes. Therefore, ecological adaptability of the improved varieties were greatly decreased, and the risk of epidemic of certain disease increased, that is the genetic-erosion. One of the evidence is the decay of sericulture in Europe in 19th, that epidemic of the pebrine disease had ruined European sericulture due to the pebrine-sensitivity of silkworm varieties used during that period. Sustainable development of sericulture industry is largely depending on breeding new silkworm varieties with one or more distinctive character by the effective utilization of silkworm germplasm resources, those are the diversity of germplasm materials could always supplement the new genetic resource for the breedres.

2.3 To investigate silkworm gene bank will exploit the valuable gene mutations of silkworm

The utilization of silkworm gene mutation in commercial production is widely noticed by silkworm breeders. First of all, some of the mutations in economical characters can be directly utilized in silkworm breeding. The polyphagous mutation had been utilized to breed silkworm varieties adaptable to artificial diet; the trimolter mutation was utilized to breed trimolt silkworm varieties to produce especially fine silk; the non-glutinous mutation was used to produce natural loose eggs. Secondly, some of the morphological characters have multiple functions that also positively effect on the economical characters, therefore, it is possible to be directly utilized in new variety breeding. Experiment had showed that K gene had positive effect on feed efficiency and heterosis; PS and black pupa (bp) also showed the potential to increase feed efficiency; the introduction of non-scale mutation on the wing (nlw) can greatly improve the silkworm seed production condition. Thirdly, Sex-control can raise single sex larvae, such as to rear only male silkworm in commercial cocoon production. There are many practical methods developed to control the sex balance of silkworm under the joint effort of scientists from different countries, these are: androgenesis, parthenogenesis, balanced - lethal mutation, and control of incubation condition. It was discovered by Chinese scientists that the sex-linked chocolate gene mutation (sch) was recessive gene that is very sensitive to high temperature during incubation stage. When genotype ZW crossed with Z sch Z sch, the female embryos of their hybrid could be killed during the incubation stage, therefore, only male silkworm will be hatched.

2.4 Silkworm germplasm resource is the important material not only for basic research of sericultural science, but also for genetics and molecular biology research

Many genetic discoveries in silkworm are belong to the forefront of genetic research, and enriched the genetic theory development. It was estimated that silkworm genome consist of about 4.8 × 108 bp, its genetic information volume is about one sixth of human being. There are over 450 morphological, physiological and biochemical characters recorded at present, among them 300 (including multi-allele) had been located on 27 groups of the total 28 chromosomes. This is one of the most detailed linkage map in the agricultural biological resource. Silkworm is an excellent material as the experimental animal. It is well known fact that advantages of silkworm as genetic experimental materials. For example: the chocolate color mutation of newly hatched larvae, there are 6 mutations had been discovered in this charachter, that is common chocolate color (ch:13-9.6), light chocolate color(chP: 13-9.6), second recessive chocolate color(ch-2;18-0.0), dominant chocolate color(la: 9-22.1), sex-linked chocolate color(sch:1-21.5), maternal chocolate color(cm: 20-21.9), plus the artificial created system of sex-limited chocolate color(W.+sch/Zsch). Among these mutations, there are dominant and recessive inheritance, sex-linked and sex-limited inheritance, and maternal inheritance, in addition, some of them are distributed in same chromosome, some in different chromosomes (belonged to 4 linkage groups). Therefore, it is an excellent material for genetic experiment for student genetics courses.

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