Prior to the establishment of the exclusive centre for sericultural germplasm, the silkworm germplasm activities were carried out at the three Central Sericultural Research & Training Institutes (CSR&TI) at Berhampore, Mysore and Pampore, few Regional Sericultural Research Stations (RSRS) at Jammu, Dehradun, Kalimpong and Coonoor; some of the Universities and state funded sericultural research institutes. After the establishment of the germplasm centre at Hosur, the entire base collection of silkworm germplasm was collected periodically from these institutes and universities and deposited as base collection in the national repository for further conservation, maintenance and utilization.
Collection: Eversince silkworm rearing was introduced into India, several silkworm races were brought from China, Japan, Russia, France, and Italy at different times and few of them survive and naturalised in India. Also several new races were evolved in different regions and maintained at local conditions. Therefore, the CSGRC, Hosur, has a regular programme to collect the available natural variants, geographical races, and wild species of Bombyx through exploration, survey, contacts and exchange. The centre has so far collected 357 silkworm germplasm from different geographic regions along with passport data.
2.1.1 Present status of silkworm germplasm maintenance
In India, the Central Sericultural Germplasm Resources Centre (CSGRC), at Hosur (12.45 °N, 77.51 °E; 942 m above MSL) maintains 357 silkworm germplasm accessions, out of which 63 are multivoltine and 294 are bivoltine silkworm accessions. The silkworm germplasm available in India are of three categories viz. i) indigenous, ii) introduced, and iii) evolved accessions. The Indian collection of silkworm germplasm has representation from 12 countries besides native Indian germplasm (Table-2). The silkworm germplasm available in India has more of evolved races and there is a low representation of geographical races and races with sex limited markers (Table-3), which indicate that there is great scope to enrich the silkworm germplasm from other geographical regions, particularly from Europe and Asia.
The present day silkworm genetic stocks maintained in India include collections from different regions of India, the indigenous and naturalised silkworm races (Table-4), and silkworm breeds evolved in India in different regions (Table-5). Among the evolved races in India, there are several mutants and sex-limited races of popular breeds in current use; which were mainly evolved for egg production purpose. By utilising these germplasm, the silkworm breeders in India have evolved high yielding silkworm races capable of yielding one kg of silk from 6 kg of cocoons. Also, some of the newly evolved bivoltine silkworm races and CSR hybrids have several desirable attributes like long silk filament length (more than 1200m) and high survival rate under tropical climate.
The silkworm germplasm maintained in the field gene bank at CSGRC, Hosur, in India exhibit wide variations for most of the heritable characters and therefore the germplasm collection is a valuable promising raw material for further crop improvement in sericulture (Table-6). High genetic variability in economic characters is a resource for breeding programme (Frankel and Brown, 1983). The evolved breeds, land races, mutants, sex limited races, breeders genetic stocks and geographical races constitute the serigenetic resources in India, which are maintained in the germplasm centre at Hosur in the ex-situ field gene bank. Apart from these genetic resources, there are wild relatives of Bombyx (collected from Kedarnath), B.mandarina, Ocinara sp., Theophila sp. etc. and these constitute the wild serigenetic resources of India. However, they are not maintained in ex-situ field gene bank.
2.1.2 Silkworm germplasm conservation strategy
Realising the pivotal role of sericultural germplasm resources for sustainable development of sericulture industry in India, the Government of India (GOI)/Central Silk Board (CSB) established Central Sericultural Germplasm Resources Centre (CSGRC) at Hosur in 1991 to act as a trustee of sericultural germplasm for easy accessions by the breeders and other users for their research programme to utilise these valuable germplasm towards mulberry and silkworm crop improvement. The CSGRC, Hosur is mandated to function as the nodal centre for planning and coordinating all activities relating to sericultural germplasm viz. collection, characterisation, evaluation, conservation, supply and exchange of mulberry and silkworm genetic resources in India. The centre has established a National silkworm gene bank for conservation and utilization of the genetic resources. Since 1991, the centre has collected mulberry and silkworm germplasm readily available in various sericultural research institutes and universities and also carried out exploration and survey mission particularly in the Himalayan ranges and Andaman Islands. As a result of concerted efforts, the centre has collected and conserved 357 silkworm germplasm resources of diverse origin in the field gene bank. The silkworm germplasm conserved at CSGRC, Hosur are maintained with an unique accession number allotted by the Centre for easy identification of the germplasm. The centre has also developed a system for registration of sericultural germplasm to protect the breeders rights/Intellectual Property Rights (I.P.R.).
2.1.3. Silkworm germplasm ex-situ conservation
The germplasm centre maintains nearly 63 multivoltine and 294 bivoltine silkworm accessions. The multivoltine silkworms are maintained in a 5 rearing cycle per year and the eggs are preserved at low temperature (5°C) for 30 days, whereas the bivoltine silkworm accessions are multiplied once in a year during favourable season, by consigning the eggs in 10 months preservation schedule at low temperature regimen. In order to maintain the genetic variability within the population, composite layings are prepared from 60 layings so that all the heritable variability within the population are maintained (Fig-4 & 5), since the genetic variation of the base population is very essential for genetic improvement in silkworm race evolution (Hozoo Lea, 1998).
Since the germplasm is a valuable genetic material, it is very essential to maintain the accessions as per the genetic characters listed in the passport data and preserve the genetic variability and the integrity of the race/breed. For this purpose, 60 layings are prepared; out of which 40 layings are preserved in one cold storage and the remaining 20 layings are preserved in another cold storage as a safety back-up. The 40 layings are further divided into two equal parts and consigned at two different schedules; and one set is kept as a buffer stock. From one set of the 40 layings two composite eggs are prepared at pin head stage of the egg and two batches of rearing is conducted; simulating two replications. In the rearing process, all the worms are maintained upto third instar and thereafter only 300 healthy worms are retained in multivoltine and 250 worms in bivoltine. From each replication, 200 to 250 healthy cocoons are selected for egg production for continuing the next rearing cycle and the details are given in fig-4 & 5 for multivoltine and bivoltine respectively. During the multiplication cycle adequate care is taken to maintain the original characters of the genotypes, as per the passport data.
2.1.4 Rearing schedule of silkworm germplasm
The multivoltine accessions are reared five times in a year and the eggs are preserved in the cold storage for 30 days at 5°C. One rearing cycle of multivoltine accessions takes 75 - 80 days including egg preservation. The bivoltine accessions are divided into three batches with nearly 100 accessions in each batch and grouped as I, II & III batch respectively and the rearing of these three batches are conducted in Jan-Feb, July-Aug and Aug-Sept. respectively, and the eggs are preserved under 10 months schedule in the cold storage. One rearing cycle of bivoltine accessions takes 360 - 370 days. (Table-7)
2.1.5 Net working of silkworm germplasm conservation
In India, sericulture is practiced in widely varied agro climatic conditions like hot and dry tropical climate, hot and humid tropical condition, sub tropical conditions and temperate climatic conditions and also under different cultivation practices viz. rainfed and irrigated, which require different silkworm races with desirable genetic potentials. In hot and dry as well as hot and humid climatic conditions in the tropical regions, bivoltine and univoltine silkworms do not survive and hence multivoltine silkworms and cross breeds (multivoltine female × bivoltine female) are reared in the tropical regions. Whereas in the subtropical and temperate regions, bivoltine hybrids are reared during the favourable seasons and hence India requires multivoltine as well as bivoltine silkworm races to suit different climatic conditions. For rainfed condition, hardy silkworm races are required.
Thus, the varied agroclimatic conditions and sericultural practices prevailing in different parts of India require 'season' and 'region' specific races / breeds / hybrids / cross breeds etc. Therefore, several silkworm breeding centres are established in different agro-climatic regions to evolve silkworm breeds suitable to the specific region and sericultural practice. Hence, the silkworm germplasm are maintained in a networking mode in different centres, representing different agro-climatic zones (Fig.6). This arrangement for silkworm conservation serves as a safety back-up and also facilitates silkworm breeding. The main centre at Hosur is the base collection and national repository and the regional centres are active collection centres. This arrangement is working extremely well and this also facilitates characterisation and evaluation of silkworm germplasm in the different agro-climatic condition and the data provide basic information on Gene × Environment interaction. Since silkworms are highly susceptible to diseases and abiotic stresses, the multiple conservation under networking plan ensures total safety of the silkworm germplasm and maintaining multiple populations enhances the chances of their long term survival, in terms of conservation.
The silkworm germplasm conservation strategy (Fig.7) adopted in India is a fail-safe device and ensures total safety of the silkworm germplasm and also promotes greater utilization of the silkworm germplasm for breeding and other studies. In addition to the existing field gene bank in the National repository in the main centre and active germplasm collections in the net working centres it is proposed to establish DNA bank for silkworm, so that total genomic DNA can be preserved for utilization to develop 'transgenic silkworm' and other molecular studies in B. mori. Similarly, the wild relatives of B. mori and species of Ocinara and Theophila belonging to the family Bombycidae are proposed to be conserved under in situ conditions, so that there could be gene-flow from the wild sericigenous insects to the Bombyx mori for widening its genetic diversity.
For sustainable development of sericulture and progressive increase in productivity and improvement in raw silk quality, we require continuous evolution of highly productive breeds of silkworm with desirable traits viz. short larval duration, high survival rate, higher fecundity, higher silk ratio, low denier, long silk filament length, higher reelability, low boil off loss, higher tenacity and cohesion etc. Also, silkworm races tolerant to adverse climatic conditions and resistant to diseases are required. To meet these challenges effective utilization of the available genetic diversity in silkworm is the only answer.
The silkworm germplasm available in Japan, China, India, Russia, South Korea, France and Italy constitute a great resource of silkworm breeding material, other countries like Brazil, Iran, Thailand, Indonesia, Vietnam, Bulgaria and Egypt also maintain some valuable silkworm germplasm. These silkworm germplasm exhibit wide genetic variations and therefore form a nucleus for evolution of silkworm breeds with specific characters viz. short larval duration breed, breeds tolerant to diseases, breeds with long silk filament length, breeds with high survival rate, breeds with high pupation rate, breeds with low denier silk filament etc.
2.2.1 At Global level
Agricultural crop improvement witnessed great progress after establishment of International Board for Plant Genetic Resources (IBPGR) in 1974 and the International Agricultural Research Centres (IARC) assembled diverse gene pool of wild germplasm along with cultivars, land races, breeders genetic stocks and supplied them to the needy countries for crop improvement programme under bilateral agreement mediated through Food and Agricultural Organization (FAO). Whereas, in sericulture crop improvement programme, such International Co-operation and periodical/need based exchange of silkworm germplasm is not taking place between countries due to various barriers and silkworm germplasm are still considered as exclusive possession of the nations.
Whereas, international co-operation in sericulture is very much limited, particularly, Japan has liberally assisted some countries like Brazil, Iran, India, Thailand, Indonesia and few other countries through Japan International Co-operation Agency (JICA) for development of sericulture in these countries, mainly with financial assistance, training in sericulture, and to a limited extent in silkworm breeding. However, exchange and supply of silkworm germplasm from Japan is very much limited. Similarly, South Korea, China and India extend training facilities in sericulture on specific request from developing countries and to a limited extent the silkworm germplasm have also been exchanged under bilateral agreements or mutual co-operation. For instance, India extended technical co-operation in sericulture to Sri Lanka under Indo-Colombo Plan and South Asian Association for Regional Co-operation (SAARC).
The Govt. of Switzerland has funded International Sericulture Training facility in tropical sericulture to developing countries through Swiss Development Co-operations (SDC) and the programme is conducted at Central Sericulture Research and Training Institute (CSR&TI), Mysore under the auspices of Central Silk board (CSB), Govt. of India (GOI) and some of the developing countries have received sericultural germplasm from India, apart from training in sericulture. However, these exchange of sericultural germplasm (Mulberry and Silkworm) at the global level among the countries is very much restricted, at time through diplomatic channel or personal contacts, and there is no need based and regular exchange of silkworm germplasm among the countries for silkworm breeding and other research purpose, as witnessed in the case of agriculture, horticulture, animal husbandry etc.
In fact, great progress has been made in food production particularly in wheat, rice, maize etc. in the developing countries, mainly because of the regular exchange of crop germplasm and agro-biodiversity through IARCs, viz., International Rice Research Institute (IRRI), Philippines, Centro International de Mejoramiento de Maiz Y Trigo, (CIMMYT), Elbatan, Mexico.
2.2.2 At National level
The very purpose of germplasm collection and conservation is for utilization to benefit the human society. The silkworm germplasm is the basic raw material for improving the quality of silk as well as increasing the productivity of silk; for this purpose elite and robust silkworm breeds resistant to diseases and tolerant to adverse climatic conditions are required, which can come mainly from silkworm breeding. The silkworm germplasm are therefore utilised to evolve breeds, rejuvenate the popular breeds and identification of potential donor from the available genetic resources through evaluation and selection. (Fig.8)
The CSGRC, Hosur is currently maintaining 357 silkworm germplasm (63 multivoltine and 294 bivoltine silkworm accessions). The silkworm germplasm are supplied to Research Institutes and Universities for crop improvement programme and the supply is regulated through Germplasm Supply Committee constituted for this purpose. Some of the silkworm germplasm are approved for supply to Research Institutes based on their indents submitted in prescribed format and the Director, CSGRC is authorised to supply those authorised silkworm germplasm. Indents which are outside this authorised silkworm accessions are screened out by the Germplasm Supply Committee and based on the merits of the indents and specific study, the silkworm germplasm supply is made. The request from other countries for supply of silkworm germplasm are decided by the Member Secretary, Central Silk Board, Govt. of India on merit basis. Thus, during the last seven years CSGRC, Hosur has supplied 445 silkworm collections to 45 indentors.
At the time of collection, the silkworm germplasm are collected with a passport data and also examined for diseases, pest etc. and if any pathogens or pest are identified, such materials are rejected and not transported to the germplasm centre. In the case of silkworm, the eggs obtained from the collection site are disinfected in the quarantine laboratory by way of surface sterilisation. This is the preliminary quarantine. Germplasm materials that are free from pest and diseases only are transported to the germplasm centre. On arrival at the germplasm centre, the silkworm accessions are first taken to the introduction laboratory, which is isolated, and the rearings are conducted carefully with periodical examination for disease at every stage, if any diseases are identified, such materials are screened out and rejected; this is the next stage of the quarantine and only those which are free from pathogens and disease or pest, such accessions transferred to the base collection. Thus, the silkworm germplasm accessions are collected with careful methodology to exclude exotic pests and diseases from entry into the germplasm centre. (Annexe-1)
Similarly, while supplying the silkworm accessions, the germplasm materials are thoroughly subjected to examination of pests and diseases and only those materials that are free from pests and pathogens are supplied along with quarantine certificate in case of silkworm. By this method, the introduction and spread of pests and pathogens of mulberry and silkworm are strictly avoided, and International standards of Phytosanitory and Quarantine measures are implemented at the time of collection, introduction and supply of silkworm germplasm.