1. Introduction

China, 'the cradle of sericulture' achieved stupendous progress in sericulture development over the last five decades since liberation in 1949 and occupies the prime position in the overall global silk production. Mulberry sericulture as a viable rural occupation and as a potential constituent of economy has been well realized through Government's pragmatic and practical policies, realistic planning, concerted and intensive Research and Development efforts and meticulous implementation.

Sericutlure in China can be traced back to 5000 years ago, the first written record about silkworm varieties were made in 1637 in the book "Tian Gong Kai Wu" by Song Yinxing. Improvement of silkworm varieties has been always the basic elements to improve the sericulture productivity, selection of better silkworm varieties and their multiplication and maintenance had been practiced consciously or unconsciously for long a history, however, systematically collection and investigation on silkworm varieties just started from 19^th century. An overview on the history of the germplasm maintenance in China is briefed as following.

• Before 19^th century: Xu Guangqi had written "Encyclopaedia on Agriculture" in 1639, in which recorded: Northern silkworm are mainly trimolters, and southern silkworm are mainly tetrmolters, which reared in spring, summer and autumn. In the book "A Bibliography on Hu Silkworm" written by Wang Rizhen in 1874 and Keep Note for Farmers by Chen Kaisi in 1896 had detail description on silkworm varieties and rearing techniques.
  
  
• In 1897, first Sericulture Institute established in Hangzhou City, Zhejiang Province, mainly engaged in the collection and investigation of local silkworm varieties. Until 1904, there are total 1277(including some different resources or name but the same variety) types of silkworm varieties collected in China, among them, 760 are univoltine, 449 are bivoltine, 41 multivoltine, and 27 yellow cocoon.
  
  
• Before 20^s of 20^th century, mainly the collection and investigation of local varieties, and select the better varieties for commercial cocoon production.
  
  
• Between 20s to 1949, more research institutes and egg production farms established, purified local varieties were mainly used for cocoon production. In 1926, first hybrid "Zhugui × Cishu" were produced and distributed to farmer's rearing. 1n 1933, pure local varieties were forbidden distributing for cocoon production in Zhejiang Province, only the appointed hybrids (total four combinations) could be used.
  
  
• After the liberation of China: Institution development was further strengthened, National Sericulture Research Institute was set up in 1951. Soon after, Department of Sericulture (total 7) was established in all major Agriculture University, and regional Sericulture Research Institute was set up in all main sericulture provinces. Tremendous improvement and progress in silkworm varieties had been achieved, that is, improved varieties instead of local varieties were 100% used for cocoon production; supersilkgeneous varieties replaced the media varieties; varieties with highly resistance to adverse conditions substitute for general varieties; some varieties with special characteristics were bred; systematical research on germplasm bank were carried out in morphological, physiological, biochemical and molecular characteristics by individual, population, cell and molecular level; silkworm new variety identification net and authorization committee was established. Up to 2000, there are 76 new hybrids applied for national identification of silkworm new varieties, their performances refer to Annex 1.

In China, sericulture, basically the agro-based cash crop is practiced in over 18 provinces where the climate and rural backdrop vary considerably. The phased development of sericulture is oriented towards specific direction given in the successive ten 5 year plans of the country and made it as a sustainable enterprise. In 1984, the national Sericulture Research Institute had awarded one million yuan fund to carry out the research on Silkworm Gene Analysis, Establishment and Enrich of Silkworm Germpalsm Resource, and Application of Mutation Genes as a key project by National Natural Science Foundation in the 7^th 5-year plan.

After that, the next national five-year plan had awarded the second key project for Research on Silkworm Gene Mutation and Linkage Analysis to Southwest University. Therefore, the major silkworm germpalsm resources are maintained in the National Sericulture Research Institute (with the most complete germplasm bank in China) and Southwest University with emphasizing on gene mutations and molecular mapping analysis, though there are other universities and provincial institutes maintained their own breeder's stock and some special materials. Since the economical reform and especially the privatization of state owned co-operations, some private owned companies such as Sichaung Huachen Group, also started involving the collection, maintenance and investigation on silkworm germplasm resources.