The art of silk production is called sericulture that comprises cultivation of mulberry, silkworm rearing and post cocoon activities leading to production of silk yarn. Sericulture provides gainful employment, economic development and improvement in the quality of life to the people in rural area and therefore it plays an important role in anti poverty programme and prevents migration of rural people to urban area in search of employment. Hence several developing nations like China, India, Brazil, Thailand, Vietnam, Indonesia, Egypt, Iran, Sri Lanka, Philippines, Bangladesh, Nepal, Myanmar, Turkey, Papua New Guinea, Mexico, Uzbekistan and some of the African and Latin American countries have taken up sericulture to provide employment to the people in rural area.
Apart from silk, there are several other bye-products from sericulture. The mulberry fruits are rich in minerals and vitamins and from the roots, barks and mulberry leaves several ayurvedic and herbal medicines are prepared. Some of the woody mulberry trees provide timber which are resistant to termites and the timber is used for making sports items, toys etc. The mulberry branches after silkworm feeding are generally dried and used as fuel particularly in the villages. The foliage of mulberry is used as a fodder for cattle. The mulberry trees are also planted in the embarkment area for protection of the soil to prevent soil erosion, and mulberry trees are planted as avenue trees. The silkworm pupae are rich in oil content and pupal oil is used in cosmetic industry and the remaining pupal cake is a rich source of protein suitable for poultry and fisheries. In some tribal population, the people eat eri pupa as a source of protein and nourishment. The silkworm litter is used for bio-gas production and used as a fuel for cooking in the rural area. Thus sericulture not only provides silk for fashionable clothings, it also provides several very useful bye products to the human society. Therefore, sericulture development provides opportunities to improve the living standards of people in the rural area in developing countries.
The present global silk production is fluctuating around 70, 000 to 90, 000 M.T. and the demand for silk is annually increasing by 5%. With the increase in population and also with the increased demand for fashionable clothing items due to fast changing fashion designs in developed countries, the demand for silk is bound to increase even more. For increasing the silk production we require highly productive mulberry varieties and silkworm races and also silkworm races tolerant to adverse climatic conditions and diseases which can come mainly from the sericultural germplasm resources and also from the wild relatives of Bombyx available in the natural habitats.
Though accurate data are not available on the silkworm germplasm in different countries of the world, an approximate information indicate that there are 4310 silkworm germplasm accessions available in different countries (Table 1). There is every likelihood that some of these silkworm accessions are duplicated; for instance the silkworm germplasm from China, Japan, France, Russia and India might be represented in the germplasm collection of other countries since these are the principal source of sericultural germplasm and also several countries might have exchanged some silkworm germplasm for silkworm breeding and hence a proper documentation on the availability of silkworm germplasm in different countries is very much required.
A very recent compilation of silkworm genetic stocks indicate that there are around 3000 genotypes of Bombyx mori at the global level, which includes mutants, parthenoclones, polyploids and geographical races (Nagaraju et. al 2001). In fact much of the genetic diversity of Bombyx mori is derived from the inbred lines of land races and elite stocks evolved by the silkworm breeders and also from hybridisation of different geographical races; mainly the Japanese, Chinese, European and tropical races, which are distinct for several economic characters. The geographical races also possess several heritable characters for a variety of morphological, biochemical and quantitative characters. Among the four geographical races, the bivoltine and univoltine races of temperate origin and multivoltine races of tropical origin differ widely and exhibit contrasting characters. The bivoltine and univoltine races produce high quantity of good quality silk, whereas the multivoltine races are hardy, tolerant to pathogen load and thereby resistant to diseases compared to the bivoltines but produce low amount of poor quality silk. Thus, these geographical races are very valuable genetic stocks for further improvement of silkworm races and evolution of superior breeds of B. mori.
Apart from a rich biodiversity of geographical races, there are also a large number of mutants. The silkworm genetic stocks include more than 500 mutants for a variety of characters viz., serosal colours; larval and adult integument colours; skin markings and body shapes; cocoon colours and shapes; physiological traits such as diapause, number of larval moults and timing of larval maturity; food habits and biochemical features such as digestive amylase, blood and egg esterases, larval integument esterase, alkaline and acid phosphatases; haemolymph proteins; silk production and fibroin secretion; homeoproteins and body plan determination etc. and the various mutants, gene locus and phenotype were documented recently (Nagaraju et. al, 2001).
Apart from the geographical races and mutants there is a large genetic stock of B.mori evolved by the breeders mostly utilising the geographical races and mutants of larval, pupal and cocoon colour variants of sex limited races, particularly in Peoples Republic of China, Japan, India and erstwhile United Soviet Socialist Russia (USSR) and some of these breeds are commercially exploited in these countries for silkworm rearing to produce raw silk and the remaining breeds are maintained in the silkworm germplasm of these countries as breeders genetic stocks and they are utilised as the genetic material in the silkworm breeding programmes for evolution of more superior and elite races.
Thus, the geographical races, mutants and the elite breeders stock constitute the major portion of the present day silkworm germplasm at the global level apart from the parthenoclones, triploid, polyploids and wild relatives of Bombyx and Bombycidae (Fig-1).
The domesticated silkworm species, Bombyx mori L. evolved almost 4600 years ago from the wild species, Bombyx mandarina Moore, which is a native of China and Palaearctic region (Hampson, 1892; Hirobe, 1968). The eggs of silkworm, B. mori were first introduced from China into Japan and Korea in the first century and subsequently into Middle Eastern and European countries and later into the neighbouring countries around China in the sixth century. The historical background of silkworm entry into India is still a mystery; and the historical evidence indicates that a flourishing silk trade was practising between India and Rome/Greece during Kaniska period (56 B C).
This is the authentic historical record of silk production and trade in India, which indicates the early history of Indian sericulture. The rich tradition of silk and silk use are evident from ancient sacred literature like the Rigveda, the Ramayana, and the Mahabharatha, which are more than 2000 years old, but the information about indigenous silkworm races and their stock maintenance are not well documented. Silkworm rearing was prevalent in Kashmir and North Eastern states during sixteenth century, the Moghul period where the univoltine and multivoltine silkworms were respectively reared and the Tippu Sultan introduced silkworm rearing in south India in 1875. During eighteenth century, the British rule in India, quite a few univoltine and bivoltine races were imported from Italy, France, Russia and China, and the races were bred and maintained by the farmers (Krishna Rao, 1997); and there was no systematic maintenance of the silkworm germplasm and hence only few races survived under Indian climatic condition.
At present only few old indigenous races are surviving viz. Barapolu, Chotapolu, Nistari, Sarupat, and Moria, whereas the indigenous univoltine Kashmiri races are almost extinct. Systematic silkworm stock maintenance and breeding started in the early nineteenth century. Prior to 1922, only pure races were reared and hybrid silkworms were introduced later, Pure Mysore × C. Nichi was probably the first hybrid in Karnataka and exploitation of hybrids in West Bengal and Kashmir came much later during 1956 and 1959 respectively (Thangavelu, 1997). Silkworm genetic stock maintenance started during 1940 in an organised way at Sericultural Research Station, Berhampore in West Bengal and subsequently temperate silkworm germplasm stocks were established at Univoltine Silkworm Seed Station, Pampore in Kashmir and multivoltine and bivoltine silkworm stocks were established at Central Sericultural Research Institute, Mysore in Karnataka and Coonoor in Tamil Nadu.
During the recent years, biodiversity conservation programmes have drawn the attention of many countries including developing nations, because of the genetic erosion due to indiscriminate use of bio resources and damage to the environment, destruction of forest, human interference in eco-system, upsetting the equilibrium of the biosphere. The Convention on Biological Diversity (CBD) organised by United Nations Conference on Environment and Development (UNCED) at Rio de Jeneiro Earth Summit in 1992 made an awakening call to draw the global attention for conservation of biodiversity. Since then the biodiversity conservation and gene bank maintenance have gained greater momentum since the germplasm resources are considered as "Common Heritage of Mankind" and "Sovereign Right of Nations". The issues related to access the genetic resources and its sustainable use, benefit sharing, farmers rights are being deliberated at various national and international fora.
Realising the importance of biodiversity conservation for sustainable development of agriculture, the Consultative Group on International Agricultural Research (CGIAR) established the International Board for Plant Genetic Resources (IBPGR) in 1974 at Rome with a global network of genetic resources centres, mainly for conservation of natural genetic resources including the wild species to promote crop improvement programmes and increase the food production. The role of wild relatives and wild species in agricultural crop improvement are well known (Rana, 1995). Similarly, there is an urgent need for seribiodiversity conservation, particularly the wild relatives of Bombyx and Bombycidae.
Improvement in silkworm race heavily depended on the geographical races of B. mori and the wild relatives of Bombyx were not explored, unlike in agriculture. Whereas in agricultural, horticultural and sericultural crop improvement programme the wild species of several crop plants have contributed very valuable genes for resistance to diseases and pests and tolerance to adverse agroclimatic conditions (Jackson and Ford-Lloyd, 1990) and similar exploitation of genes from wild relatives of B.mori have not been reported.
The genus Bombyx Hubner (1818) has two species, Bombyx mori L. and Bombyx mandarina Moore. Apart from the genus Bombyx there are eleven other genera in the family Bombycidae Hubner; 1) Genus - Theophila Moore (1867), 2) Genus - Ocinara (Walker 1856), 3) Genus - Mustilia (Walker 1865), 4) Genus - Gunda (Walker 1862), 5) Genus Penicillifera (Walker) 6) Genus - Ernolatia (Moore) 7) Genus - Norasuma Moore 8) Genus - Trilocha Dieri, 9) Genus - Prismosticta (Swinhoe), 10) Genus - Andraca (Walker), and 11) Genus - Ectrocta (Hampson). Among these genera, Theophila and Ocinara are very close to the genus Bombyx. The wild sericigenous species of Bombyx, Theophila and Ocinara are naturally distributed in the Himalayan ranges of Indo-China range and also in Andaman Islands in India, besides, Jawa, Sumatra, Borneo and Malaya Peninsular (Barlow, 1982). The wild species of these genera have not been explored for transferring the useful genes to confer resistance to diseases and tolerance to adverse agro-climatic conditions into the domesticated species, B.mori. The useful genes from the wild relatives of B. mori may be cloned and these cloned genes may be transferred into the germ cells of the silkworm to develop transgeneic silkworm. Hence, there is an urgent need to collect and conserve the wild species of Bombyx, Theophila and Ocinera and study their genetics for possible use in the breeding programme of B.mori and widen the genetic base as well.
Indian gene centre is harbouring great faunal diversity and nearly 11.9% of the world flora are present in India and hence recognised as one among the twelve mega biodiversity rich centres of the world. Floristically India is very rich, harbouring three mega centres of endemnism i.e. Western and Eastern Himalayas and Western Ghats. It is a treasure house of several diverse sericigenous flora and fauna. Wild species of Bombyx and other genera of Bombycidae do exist in the great Himalayan ranges and Andaman islands, under natural habitat and therefore the Indian gene centre possesses a rich seri-genetic resources.
Eggs and cocoons of a wild silkworm belonging to Bombycidae were collected from wild mulberry tree Morus serrata near Kedarnath (30.47 °N, 79.02 °E) at an altitude of 800 meter above MSL (Tikader 2001). The eggs were incubated and rearing was conducted on the mulberry plants at Central Sericultural Germplasm Resources Centre (CSGRC), Hosur and the produced cocoons and eggs are very similar to B. mori (Fig-2). It is a potential and interesting genetic material with several unique characters, utilising such wild relatives of Bombyx, it is quite possible to create additional seribiodiversity and widen the genetic base of B. mori.
Biodiversity is the result of evolution that is a continuous phenomenon induced by natural selection pressure and the population of organisms evolve through adaptation to the biotic and abiotic stress. Ever since B.mori was domesticated, the species does not survive in the wild state in natural condition and also does not survive without human care and hence natural selection induced genetic diversity in B.mori is rather very limited to voltinism. Hence, it is very essential to conserve and utilise the wild relatives of Bombyx mori to broaden its genetic diversity, apart from the geographical races, mutants, sex-limited races, evolved breeds and breeders genetic stocks. The wild relatives of Bombyx are very vulnerable and the vulnerability at different spatial and temporal scales are not known. The design of biodiversity network in sericulture involving the complementarity of wild relatives and domesticated B. mori is also not well established. Therefore, conservation of wild as well as domesticated seribiodiversity resources is very essential for sustainable development of sericulture (Fig-3) since loss of genetic resources of domesticated and wild relatives of Bombyx species along with their unique genes may disadvantage future generation.