agriculture alone cannot hold full rural employment. Therefore, diversification from agriculture sector is one of the important aspects that

Transcription

1 Chapter 1 INTRODUCTION Agriculture has largely been the main sector providing employment and income to majority of people living in rural areas of India. However, agriculture alone cannot hold full rural employment. Therefore, diversification from agriculture sector is one of the important aspects that need to be emphasized to achieve the objective of alleviation of rural poverty and unemployment. Sericulture is one such sector with immense rural employment potential. In India, sericulture has been identified as a major agro-based industry not just because of its employment potential but also for its profitability, low investment cost and high foreign exchange earnings (Bhargava et al., 1995). Over twenty lakhs people in rural sector of the country are employed in various activities of sericulture. In today s market oriented economy, if any industry has to sustain and succeed, it should meet the pre-requisites like quality, productivity, price and sericulture is no exception. Its contribution to the national economy is tremendous. Silk the queen of textiles and the end product of sericulture, is extremely important for consumer countries such as Italy, U.S.A, France, Germany and Japan as well as for producers countries such as China, India and Japan (Currie, 1991; Wu, 1991). The best quality silk of international grade is a proteinaceous fibre produced by the mulberry silkworm Bombyx mori L. (Kumar et al., 1994). More than 90 % silk in 1

2 India is produced from multivoltine bivoltine hybrids commonly known as cross breed. Cross breeding has been extensively used for silkworm improvement through exploitation of heterosis (Nagaraju et al., 1996). Exploitation of hybrid vigour has universally been accepted as the only way to improve silk productivity in sericulture industry (Sengupta et al., 1971). Large scale production of silk is possible only through utilization of hybrid vigour because hybrids in general are superior to parental breeds in their performance (Singh and Rao, 1996). No doubt, improvement in the efficiency of silk production in the major sericulture practicing countries including India has been due to exploitation of hybrid vigour (Narayanaswamy et al., 2002a). Hybrid vigour has been extensively utilized in the development of multivoltine and bivoltine silkworm breeds (Hirobe, 1968; Gamo, 1976; Krishnaswami, 1983; Datta, 1984; Noamani et al., 1990, Trag et al., 1992, Ravindra Singh et al., 1998, 2001a, b; Rao et al., 1998, 2001, 2002, 2003; Datta et al., 2000 a, b, 2001 a, b; Nazia Choudhary and Ravindra Singh, 2006a). The practical exploitation of hybrid vigour in silkworm was first made by Toyama in Japan in 1911 (Yokoyama, 1957). Thereafter, extensive research was carried out in Japan and several hybrids suitable for different regions and seasons were identified (Yokoyama, 1974). Commercial exploitation of hybrid vigour heralded a new era in sericulture that contributed substantially to the increased silk production in Japan (Toyama, 1905, 1906, Yokoyama, 1957, Petkov et al., 1984). Similarly, several productive hybrids were developed in other countries 2

3 such as China, USSR and South Korea to improve silk production (Strunnikov, 1971 and Abadhieva and Radka, 1980). Thus, the spread and vertical growth of the sericulture industry in the world can be attributed to the exploitation of heterosis (Yokoyama, 1957; Hirobe, 1985; Grekov and Petkov, 1989). In India, utilization of hybrid vigour started late during 1920s. Till then, pure indigenous multivoltine breeds were being utilized for rearing. Subsequently, exploitation of hybrids became popular. But the hybrids produced did not contribute to a rapid progress in cocoon productivity since the parental breeds involved in the preparation of hybrids were multivoltine breeds namely Pure Mysore and C.Nichi (Nanavathy, 1965). Systematic cross breeding of silkworm breeds were initiated during 1960s utilizing multivoltine Pure Mysore and various exotic bivoltine breeds viz. J 112, C 108, NN 6 D (Chandrashekaraiah and Jolly, 1983). Even though the hybrids were found superior to the female parents, expected goals of achieving desired objective of increasing productivity could not be achieved. Therefore, the need was felt to evolve new bivoltine silkworm breeds not only to suit tropical Indian conditions but also to utilize them for cross breeding. During 1970s, continuous efforts of the breeders at Central Sericultural Research and Training Institute, Mysore enabled them to isolate three bivoltine breeds namely NB 7, NB 4 D 2 and NB 18 utilizing Japanese hybrids Kinshu Showa and (Kokko Seihaku) (N 124 C 124 ) respectively. With the development of moderate productive breeds, crossbreeding with Pure Mysore took momentum and increased 3

4 cocoon productivity (Krishnaswami and Narasimhanna, 1974; Narasimhanna et al., 1976, Benchamin et al., 1983). The traditional hybrid PM C.Nichi was replaced by new multivoltine bivoltine hybrid i.e., PM NB 4 D 2, by more than 70% (Datta, 1984). But the silk produced remained low both in quality and quantity. Even though attempts were made to develop superior multivoltine bivoltine hybrids to improve the quality and quantity of the silk but the desired target could not be achieved. Realizing the main constraints poor yield and inferior silk quality, a need was felt for the development of promising hybrids with high silk content and quality, that would satisfy the requirement of authorized egg producing companies, sericulture farmers, filature factories and textile manufacturers. Thus, enormous efforts were made by various breeders to develop superior silkworm breeds / hybrids as a result of which quite a good number of multivoltine bivoltine hybrids have been developed such as MY 1 NB 18, P 2 D 1 NB 18, RD 1 NB 18, BL 23 NB 4 D 2, BL 24 NB 4 D 2, BL 43 NB 4 D 2 developed by the Central Sericultural Research and Training Institute, Mysore; MU 1 NB 4 D 2, MU 11 NB 4 D 2 developed by the University of Mysore, MH 1 NB 4 D 2, SL KMS NB 4 D 2 developed by Karnataka State Sericulture Research and Development Institute, Bangalore and APM 1 APS 8 developed by Andra Pradesh State Sericulture Research and Development Institute, Hindupur. Of late, gradable silk has been obtained from promising multivoltine bivoltine hybrids, Cauvery - BL 67 CSR 101 (Datta et al., 2001a; Rao et al, 2001a) and Jayalakshmi - ND 7 CSR 2 (Dandin et al., 2006). 4

5 Utilization of hybrid vigour has played a significant role in increasing the silk production, identifying better hybrid combination, combiners and maintenance of inbred lines in order to obtain heterotic hybrids for commercial exploitation. The ultimate results in silkworm breeding are judged by the excellency of characters of parental breeds that appear in F 1. Many silkworm breeds that the breeders develop with great caution may turn out to be the poor combiners in producing productive hybrids. Sometimes, characters of the parental silkworm breeds are very good but the performance of their hybrids is not so good. In order to identify better hybrid combination, their combining ability is analyzed. Combining ability studies are useful in assessing the ability of parents which when crossed would give rise to more desirable segregants (Pershad et al., 1986). It plays a significant role in the improvement of any genetic material. It helps the breeders in determining the nature of gene action involved in the expression of quantitative characters of economic importance (Rajalakshmi et al., 1997). Thus, combining ability analysis is the most widely used biometrical tool in identification of promising parents and hybrids and detecting relative magnitude of genetic variability both in plants (Sprague and Tatum, 1942; Arunachalam, 1974; Baker 1978; Kaushik et al., 1984, Ram et al., 1999; Sharma, 1999; Sood et al., 2000; Barth et al., 2003) and animals (Eisen et al., 1983; Crusio, 1987; Falconer and Mackay, 1996). Critical assessment of variability present in breeding through combining ability studies is necessary in giving way for 5

6 combining most of the desirable characters available in different breeds into a single breed / hybrid. It is the advent of biometrical genetics that has helped the breeders to identify the best parent / hybrid. Main biometrical approaches have been utilized from time to time for analyzing combining ability in silkworm such as (i) Hayman s approach (1953) developed by Jinks and Hayman (ii) Griffing s approach (1956) developed by Griffing who utilized diallel cross technique to understand combining ability of different breeds. In this method, set of all possible combinations among inbred lines, set of reciprocal crosses and F 1 crosses are made and (iii) Kempthorne s line tester approach (1957) developed by Kempthorne to understand heterosis, overdominance, general and specific combining ability. It is one of the most important methods to analyze the relative capacity of a number of female and male parents to produce desirable hybrids. Sprague and Tatum (1942) introduced the term general combining ability (GCA) and specific combining ability (SCA) that proved to be very useful in analyzing combining ability experiments. According to them, GCA is used to designate the average performance of a line in hybrid combination and SCA is used to designate those cases where some hybrids show relatively better performance than their expectations. The mean performance of the line in all its crosses when expressed as a deviation is GCA that is the main effect and SCA is the interaction of genes (Falconer and Mackay, 1996). GCA is heritable, can be fixed and generally parents with high GCA produce high hybrid vigour as it consists of additive and 6

7 additive additive type of gene interactions (Ravindra Singh et al., 2003). SCA is controlled by non-additive, dominant genes and other interactions (Bandyopadhyay, 1990). SCA is not heritable and therefore it cannot be used in breeding of pure lines and the hybrids with high SCA can be utilized for commercial exploitation (Ravindra Singh et al., 2003). Attempts were made to identify promising multivoltine silkworm breeds/ hybrids through combining ability analysis (Pershad et al., 1986; Kantaratanakul et al., 1987). Krishnaswami et al. (1964) have reported interaction of over-dominance and epistatic in the expression of economic characters in a diallel analysis involving five indigenous multivoltine breeds viz., Nistari, Pure Mysore, Bulupolu, Sarupat and Nistid. Studies on genetic variability in multivoltine silkworm breeds on the basis of analysis of quantitative and qualitative characters were carried out (Rao et al., 1991; Chatterjee et al., 1993). Sen et al. (1995) have indicated that additive and non-additive effects of genes facilitate selection for the amelioration of multivoltine silkworm breeds. Combining ability analysis has been extensively used in silkworm for the selection of promising multivoltine bivoltine F 1 hybrids. Pershad et al. (1986) have reported that both additive and non-additive gene actions are important in the inheritance of fecundity, larval duration, pupal duration, cocoon yield, cocoon shell percentage and filament length and environmental interaction is minimum in multivoltine bivoltine hybrids. Datta and Pershad (1988) observed that additive genes play more important role in the inheritance of fecundity, larval duration, pupation 7

8 rate, cocoon shell percentage and filament length and they further confirmed that multivoltine bivoltine hybrids are superior to bivoltine multivoltine hybrids to obtain maximum cocoon yield. Preponderance of additive gene action was found for cocoon weight and cocoon shell weight whereas importance of non-additive gene action was observed in the inheritance of larval duration, cocooning and pupation (Sattenahalli et al., 1989). Genetic variability in tropical races of silkworm has been observed (Murakami and Ohtsuki, 1989). Sarkar et al. (1991) recorded the importance of additive genetic variance for cocoon weight in a diallel cross analysis of cocoon weight in hybrids involving multivoltine and bivoltine silkworm breeds. Rao et al. (1998) have reported importance of both additive and non-additive gene actions in the inheritance of cocoon weight, cocoon shell weight, cocoon shell percentage, larval duration and survival potential and they further indicated that some hybrids like Moria Dong 34, Hua 204 Guangnong should be tried in place of Pure Mysore C. Nichi. Predominant role of additive gene action was recorded in the inheritance of fecundity, hatching percentage, larval duration, cocoon yield both by number and weight, cocoon shell weight and cocoon shell percentage (Ravindra Singh et al., 2001a) whereas preponderance of non-additive gene action was observed for fecundity, cocoon yield both by number and weight, cocoon weight, cocoon shell weight and cocoon shell percentage in a line tester analysis study in sexlimited breeds of the silkworm with coloured cocoons (Ravindra Singh et al., 2001b). Recently, importance of both additive and non-additive gene 8

9 actions in the inheritance of quantitative characters like fecundity, cocoon yield, cocoon weight, cocoon shell weight and cocoon shell percentage has been recorded (Datta et al., 2001a; Rao et al., 2002; Hadimani et al., 2003; Nazia Choudhary and Ravindra Singh, 2006a). Study on genetic divergence in bivoltine silkworm breeds was carried out to determine the magnitude of genetic diversity among breeds and to assess the importance of a set of quantitative characters (Jolly et al., 1989). Importance of both additive and non-additive gene actions in the inheritance of cocoon shell, cocoon shell percentage and larval duration and additive genetic variance for cocoon yield, cocoon weight, filament length and denier has been observed (Subba Rao and Sahai, 1989). Predominant role of additive gene action was recorded for larval duration, cocoon shell weight and filament length (Razdan et al., 1994) whereas Kumar et al. (1994) observed the importance of both additive and nonadditive gene actions for effective rate of rearing and filament length. Bhargava et al. (1995) have recorded predominant role of non-additive gene action for cocoon weight, cocoon shell weight, cocoon shell percentage, raw silk %, filament length, reelability and neatness. Similar predominant role of non-additive gene action in the inheritance of different characters has been studied by several workers (Rajalakshmi et al., 1997; Malik et al., 1999; Ravindra Singh et al., 2000; Rao et al., 2005). Contradictory results regarding relative importance of additive and non-additive gene actions may be due to genetic variability of different silkworm breeds utilized in the above studies. 9

10 In silkworm, for most of the economic characters including cocoon yield, cocoon weight, cocoon shell weight and cocoon shell percentage, nonadditive gene action was found predominant rather than additive gene action (Ravindra Singh et al., 2003). During the recent years, line tester analysis developed by Kempthorne (1957) is being extensively employed in mulberry silkworm (Rajalakshmi et al., 1997; Chauhan et al., 2000; Ravindra Singh et al., 2000, 2001a; b; 2003; 2005; Datta et al., 2001a; Narayanaswamy et al., 2001, 2003; Rao et al., 2002, Hadimani et al., 2003; Islam et al., 2004; Rao; 2001b, 2002, 2003; Kariappa and Rajan, 2005a; Nazia Choudahry and Ravindra Singh, 2006a) for analyzing combining ability. Genotype environment (G E) interaction is of major importance to the breeder in developing new breeds. According to Wright (1952), understanding the nature of genetic system underlying quantitative variability is of primary importance in the theory of evolution and in the importance of cultivated plants and animals. The concept of genotype environmental interaction influencing the level of heterosis has been well documented in both plants and animal species (Griffing and Zsiorios, 1971; Bains, 1976; Orozco, 1976; Talukdar and Bains, 1982; Islam et al., 1984; Ehiobu and Goddard, 1989; Manivel et al., 1999; Joshi et al., 2002; Moorthy et al., 2006; Narayanankutty et al., 2005; Strickberger, 2005). Adaptability is the property of genotype and an ideal variety with general adaptabilty is one with maximum yield potential under most favourable environment with maximum phenotypic stability 10

11 (Chakravorthy et al., 2005). Association of genes and environment in the phenotypic manifestation of quantitative characters has been studied in agriculture, horticulture, animal husbandry, fisheries, poultry as well as in sericulture. The choice of a breed depends not only on genotype itself but also on its performance under variable environment conditions. In silkworm, most of the economic characters are quantitative in nature and are greatly influenced by the environmental factors such as temperature, humidity, nutrition and rearing techniques (Kogure, 1933; Miyagawa and Sato, 1954; Legay, 1958; Takeuchi, 1959; Ueda and Lizuka, 1962; Suzuki et al., 1962; Yokoyama, 1963; Arai and Ito, 1967; Horie et al., 1967). It is important to measure the phenotypic expression of the major contributing characters of economic importance under different environmental conditions in order to understand the genetic endowment with regard to productivity. The improvement obtained by selection under favourable condition may not help in realizing the full potential when the selected strains are transferred to unfavourable conditions (Falconer, 1960). A stable variety over a wide range of environment with high potential is desirable for commercial exploitation (Eberhart and Russell, 1966; Mohanty and Prusti, 2001). Thus, in order to identify genotype with phenotypic stability under varying environmental conditions, investigation on G E interaction is a prerequisite. Studies on G E interaction have been reported in silkworm by several workers (Krishnaswami and Narasimhanna, 1974; Giridhar et al., 1990; Das et al., 1995; Kalpana and Reddy, 1998a). 11

12 Identification of promising silkworm breeds / hybrids on the basis of cumulative effect of several characters is important (Narayanaswamy et al., 2002 b). Among various evaluation methods used in silkworm, Multiple Trait Evaluation Index (E.I.) method of Mano et al. (1993) has been employed extensively in short listing of superior multivoltine bivoltine hybrids (Singh and Subba Rao, 1998; Vidyunmala et al., 1998; Narayanaswamy et al., 2002b; Mal Reddy et al., 2002; Nazia Choudhary and Ravindra Singh, 2006b), bivoltine breeds / hybrids (Rajalakshmi et al., 2000; Naseema Begum et al., 2000, Rao et al., 2001b; Ramesh Babu et al., 2002, 2004; Asma Maqbool et al., 2005), multivoltine breeds (Kariappa and Rajan, 2005b) and various multivoltine and bivoltine breeds (Nazia Choudhary and Ravindra Singh, 2006c). Cocoons as raw material have a significant bearing on the reeling industry since it constitutes 85-90% of the cost of production of raw silk (Sonwalkar, 1993). Of late, silkworm breeders are giving more emphasis on cocoon size uniformity due to its importance in silk industry in evolution, evaluation and identification of silkworm breeds / hybrids. Variations in cocoon size in different multivoltine breeds have been reported (Mukherjee et al., 2000) whereas cocoon size variations in F 1 hybrids of multivoltine bivoltine hybrids have been observed (Ravindra Singh et al., 1998, 2000, 2001 a, b, 2004; Rao et al., 2002, 2003, 2005; Umadevi and Rao, 2006). Concerted efforts made by the silkworm breeders have yielded quite a good number of multivoltine and bivoltine breeds / hybrids with well 12

13 defined genetic, physiological and morphological features. However, their full potential in terms of qualitative and quantitative characters has not been realized. The present study has been undertaken with the newly developed multivoltine and bivoltine silkworm breeds with the main objective to identify promising multivoltine and bivoltine breeds and their F 1 hybrids (multivoltine bivoltine) with improved qualitative and quantitative characters in different seasons and high magnitude of hybrid vigour, combining ability and less phenotypic variability. 13