SUMMARY AND CONCLUSIONS

Transcription

1 SUMMARY AND CONCLUSIONS S SUMMARY ilk, the queen of fibres, is known for its luster and elegance and it is an integral part of Indian rich tradition and culture. In recent years, its utility in various scientific and technical fields is well established. Though India is second largest producer of mulberry silk, the demand for silk is on the rise and in domestic market; there is annual short fall of MT of raw silk (Dandin, 2006). The production of silk is an agrobased cottage industry Sericulture. It has made immense contribution for rural economic growth, poverty alleviation and urban employment in India and several developing countries. Sericulture is an activity comprising mulberry cultivation, silkworm rearing and silk reeling and weaving. Sustainability of sericulture depends upon largely on successful silkworm rearing leading to realization of cocoon crop. The silkworm rearing forms most sensitive activity involving risk in the form of larval and pupas mortality due to diseases. Silkworm breeders all over the world have made significant contribution by evolving silkworm breeds that are high in productivity of quality silk. However the higher productivity is generally linked to their susceptibility to diseases such nuclear polyhedrosis and sensitivity to environmental factors. In recent period, there are efforts to breed silkworm that are productive, resistant to diseases and low in sensitivity to environmental factors. The present study was one such study aimed at developing productive nuclear polyhedrosis resistant breed/hybrid tolerant environmental factor that could induce the disease in silkworm. The salient observations made in this study and conclusion drawn is presented chapter wise. Chapter 1 Twenty three productive multivoltine and twenty four bivoltine silkworm breeds were selected, based on the available data in the silkworm breeding laboratory, Central Sericultural Research and Training Institute, Mysore, for the screening them for resistance to Bombyx mori nuclear polyhedrosis virus (BmNPV). Among the multivoltine breeds (23) screened against BmNPV inoculua of BmNPV PIB/ml, eleven breeds viz., Sarupat, Nistari, Madagaskar, Pure Mysore, 226

2 BL37, BL36, Moria, 96C, MAR, BL65 and Mysore Princess were shortlisted for further screening. The eleven breeds recorded higher survival % and productivity traits and resistance evaluation Index (EI) value of 80.46% and 44 (Sarupat), 79.76% and 38 (Nistari), 72.87% and 52 (Madagaskar), 63.86% and 42 (Pure Mysore), 63.3% and 51 (BL37), 61.99% and 49 (BL36), 59.36% and 48 (Moria), 58.63% and 52 (96C), 58.31% and 37 (MAR), 53.87% and 47 (BL65) and 54.77% and 42 (Mysore Princess) respectively (Table 5). Among the bivoltine breeds (24) screened against BmNPV inoculum of BmNPV PIB/ml, eleven breeds viz., CSR18, CSR19, CSR2, S1, D7, D13, D15, 61N, 5HT, 8HT and CSR 2(SL) were shortlisted for further screening. The eleven breeds recorded higher survival % and productivity and resistance EI value of 39.61% and 51 (CSR18), 58.80% and 44 (CSR19), 39.70% and 53 (CSR2), 47.74% and 48 (S1), 52.69% and 46 (D7), 47.65% and 41(D13), 44.01% and 40 (D15), 36.70% and 51(61N), 40.78% and 40 (5HT), 45.69% and 50 (8HT), 44.42% and 42 (CSR 2(SL) respectively (Table 6). The selected multivoltine and bivoltine breeds were screened for resistance to Infectious dose ( BmNPV/PIB/ml) of BmNPV PIB inoculum, Induction of nuclear polyhedrosis by low temperature (5ºC for 24h) and Inoculation ( BmNPV PIB/ml) followed with induction (5ºC for 24h) of nuclear polyhedrosis. Among the multivoltine breeds, 100% survival was recorded by the breed Nistari against BmNPV inoculum dose of BmNPV PIB/ml It was followed by breeds with comparatively lower survival viz., Pure Mysore (97.33%), Moria (96.00%), Madagaskar (94.67%), Sarupat (94.33%), MAR (94%), Mysore Princess (91.00%), BL 36 (90.33%), BL37 (90.00%), 96C (84.67%) and BL 65 (80%) (Table 7). The screening of the same breeds with only low temperature (5ºC for 24h) treatment without any inoculation resulted in survival ranging from to 71.33% except in case of 96C the survival was only 2.33%. Moria recorded highest survival of 71.33% followed by Nistari (70.67%), Pure Mysore (69.00%), BL65 (59.00%), MP (58.33%), MAD (57.00%), Sarupat (56.67%), BL37 (50.00), MAR (52.00%) and BL36 (41.67%) (Table 7). 227

3 The same breeds screened with per os inoculation of BmNPV ( BmNPV PIB/ml) followed by exposure to low temperature (5 0 C for 24 hrs) resulted in comparatively lower survival and increased mortality due to nuclear polyhedrosis. The breed Nistari survived to the extent of 71.33% and it was followed by Mysore Princess (60.00%), BL36 (38.33%), Pure Mysore (38.00%), Sarupat (13.67%), Moria (10.76%) MAR (10.33%), Madagaskar (8.33%), BL65 (4.67%), BL37 (3.33%) and 96C (1.76%) (Table 7). Among the multivoltine breeds Nistari, Pure Mysore and Moria recorded high survival. Mysore princes and BL 36 recorded lower susceptibility to inoculation and induction. 96C, BL37, MAR, Sarupat and Madagaskar recorded higher susceptible to Inoculation, Induction and Inoculation and induction treatments. Out of these breeds, Nistari, Pure Mysore and Madagaskar were selected for their comparatively higher in resistance to treatments while BL36 was selected as it has Hua Moria genome. Moria is observed to be comparatively resistant to BmNPV. 96C was also selected as the breed parentage has CSR 5 genome and productive. Among the bivoltine, the breed 61N exhibited highest survival (75.00%) against inoculation BmNPV PIB inoculum of / ml and it was followed by CSR18 (74.00%), CSR19 (66.67%), S1 (58.00%), CSR202 (56.67%), D7 (37.67%), D13 (34.00%), D15 (27.33%), 5HT (24.67%), CSR2 (SL) (19.33%) and 8HT (14.33%) (Table 8). The screening of the same breeds with only low temperature (5ºC for 24h) treatment without any inoculation resulted in 100% survival in breeds D15, 5HT and 8HT. The survival was lower with breeds D7 (26.00%), D13 (26.67%), CSR202 (30.00%), CSR2- SL (36.00%), S1 (44.33%), CSR18 (47.67%), 61N (47.67%) and CSR19 (50.67%) (Table 8). The same breeds screened with per os inoculation of BmNPV ( BmNPV PIB/ml) followed by exposure to low temperature (5 0 C for 24 hrs) resulted in significantly lower survival. None of the larvae of the silkworm breeds CSR 202, D13 and 8HT survived while 0.33% of larvae of CSR2 (SL), 1.67% of D15, 3.00% of SI, 3.33% of 5HT, 4.00% of CSR19, 6.33% of D7, 11.67% of CSR18 and 24% of 61N only survived (Table 8). 228

4 Among the bivoltine breeds, 61N, CSR18 and CSR19 recorded comparatively higher survival against BmNPV Inoculum, Induction, and Inoculation and induction treatments. The breed S1 and CSR 4 recorded higher survival against Inoculation and induction by low temperature. Hence these breeds were selected for further screening to develop breed resistant to nuclear polyhedrosis. Chapter 2 Multivoltine silkworm breeds viz, Nistari, Pure Mysore, 96C, BL36 and Madagaskar and bivoltine breeds viz., 61N, CSR19, S1, CSR4 and CSR18 shortlisted as result of studies under chapter 1, were further screened to identify breeds for enhancement of their resistance to BmNPV. Screening of the breeds was based on their requirement of BmNPV inoculum concentration for obtaining 30, 50 and 80% mortality due to nuclear polyhedrosis under normal conditions. The screening also considered the BmNPV PIB requirement under low temperature induction conditions for obtaining 30, 50 and 80% mortality due to nuclear polyhedrosis. From the result of screening, it is observed that multivoltine silkworm breeds viz., Pure Mysore, BL36 and 96 C were comparatively most resistant to BmNPV PIB inoculation than Nistari and Madagaskar but 96C appears to be become most susceptible to BmNPV PIB inoculation combined with treatment for induction of nuclear polyhedrosis. Nistari which is comparatively higher in susceptibility is observed to be more resistant to inoculation combined with induction of nuclear polyhedrosis. The breed Pure Mysore under inoculated and Inoculation and induction condition for 30, 50 and 80% mortality required and ; and ; and and BmNPV PIB/ml. BL 36 under inoculated and Inoculated + induction condition for 30, 50 and 80% mortality required and ; and and and BmNPV/ml while 96C the LC30, 50, and 80 concentration for 30, 50 and 80% mortality under Inoculation and Inoculated combined with induction condition were and , and , and and BmNPV PIB/ ml respectively. The productive breed, 96C is significantly lower in its resistance to nuclear polyhedrosis compared to Pure Mysore, BL36 and even Nistari but its productive traits offer advantages for breeders if its resistance is improved. The breed could become an important breeding resource material for breeding disease resistant lines. Hence Pure 229

5 Mysore, BL36 and 96C were selected for enhancement of their resistance to nuclear polyhedrosis (Table 1&2). It is observed from the result of the screening of the shortlisted bivoltine breeds, that the breeds 61N, CSR19 and S1 required higher dosage of inoculum than CSR18 and CSR4 to cause LC30, 50, and 80% mortality due to nuclear polyhedrosis under the Inoculation condition as well as Inoculation and induction condition. Hence these breeds were selected for enhancement of resistance to BmNPV. The LC30, 50, and 80 dosages for 61N, CSR19 and S1 under inoculation condition were , and ; , and ; and , and BmNPV PIB/ml respectively. Under inoculation and induction condition the LC30, 50 and 80 concentration for the three breeds viz., 61N, CSR19 and S1 were , and ; , and ; and , and BmNPV PIB/ ml respectively (Table 3&4). To enhance the resistance, the LC60 and LC70 concentrations of BmNPV PIB to cause 60 and 70% mortality under Inoculation and Inoculation and induction condition was also determined in multivoltine and bivoltine breeds. The LC60 and 70 concentrations of BmNPV PIB for multivoltine breeds viz., Nistari, Madagaskar Pure Mysore, BL36 and 96C, under BmNPV PIB inoculation condition were and ; and ; and ; and and and BmNPV PIB/ml respectively. Under Inoculation and induction condition, the LC60 and 70 concentrations of BmNPV PIB inoculum required for 60 and 70% mortality by Nistari, Pure Mysore, BL36, 96C and Madagaskar were and ; and ; and ; and and BmNPV PIB/ml respectively (Table 1&2). The LC60 and 70 concentrations of BmNPV PIB for bivoltine breeds viz., 61N, CSR19, S1, CSR4 and CSR18 under BmNPV PIB Inoculation condition, were and ; and ; and ; and and and respectively. Under Inoculation and induction condition the LC60 and 70 concentrations of BmNPV PIB inoculum required for 60 and 70% mortality by 61N, CSR19, S1, CSR4 and CSR18 for 60 and 70% mortality were and ; and ; and ; and and and respectively (Table 3&4). 230

6 Inheritance studies Since the expression of any particular trait in silkworm depends upon the genetic make up and the prevailing environmental condition. Hence the understanding the mode of inheritance of traits is most essential for success in evolving a breed with specific trait. The results of the studies on the mode of inheritance (Tables 7-11) have shown that the resistance to BmNPV is controlled by four dominant genes, N1, N2, N3 and N4 and each is contributing about 25% tolerance. The survival percentage of breeds Pure Mysore, BL36, 96C, CSR18 and 61N subsequent to NPV inoculation was ranging from 50 to 63%, while in the NPV susceptible breeds viz., BL61 and CSR2 the survival was observed to be 29.8% and 16.3% respectively. But in the F1s and RF1s, the survival was found to be much higher than the NPV tolerant parents, which was ranging from 70 to 75%. The results of the survival data of parents and F1s have clearly shown the possibility that resistance to BmNPV could have been controlled by four dominant genes, N1, N2, N3 and N4 and each is contributing about 25% resistance (tolerance). Pure Mysore, BL36, 96C, CSR18 and 61N has genetic constitution of N1, N2, n3, and n4 having two dominant genes N1 and N2 and two recessive genes n3 and n4. The two dominant genes offer about 50% tolerance. The NPV susceptible parents, BL61 and CSR2 have the gene combinations of n1, n2, N3 and n4, with only one dominant gene contributing to about 25% tolerance. Based on this assumption, expected values were estimated in the F1, F2 and BC progeny in the crosses and were compared with the observed values. The expected survival in the F1s and RF1s was estimated to be 75% and likewise, in the population of F1 progeny back crossed with the resistant breed (BCT) and its reciprocals, TBC, RBCT and TRBC, the expected survival % is estimated to be 87.5, as 50% of the BC population have the genotype of the F1s and the remaining 50% resemble the resistant breeds. In case of F1 progeny back crossed with susceptible breed BL61, (BCS) and its reciprocals, SBC, RBCS and SRBC, the BC progeny is expected to comprise of 50 % population resembling F1 and the remaining 50% are of the susceptible parental genotypes and thus the expected survival is estimated to be 62.5%. Further, it was observed that there was no marked difference in the survival of the straight crosses of 231

7 F1, F2 and BC and its reciprocals, which clearly showed that there is no maternal inheritance or sex-linkage. Chi square ( 2 ) test was employed and the 2 value less than 3.84 was considered as non-significant (NS) at 5% level of significance, which denotes that the observed values is in agreement with the expected values. Values more than 3.84 were treated as significant, indicating large deviations from the expected values, leading to rejection of hypothesis. In F1s raised between the resistant breeds Pure Mysore, BL36, 96C, CSR18 and 61N and the susceptible breeds BL61 and CSR2, the survival % was found to be in PM BL61 cross (Table 7), 74.5 in BL36 BL61 cross (Table 8), in 96C BL61 cross (Table 9), in CSR18 CSR2 (Table 10) and in 61N CSR2 cross, respectively. These observed values did not deviate from the expected survival % of 75, showing non-significant (NS) Chi square ( 2 ) values, thus agreeing with the hypothesis, except in case of F1 of CSR18 CSR2 (Table 10) and RF1 of 61N CSR2 (Table 11), where the 2 values were significant. The population of F1 progeny back crossed with the resistant breed (BCT) the expected survival % is 87.5, as 50% of the BC population has the genotype of the F1s and the remaining 50% resemble the resistant breeds. The observed survival percentage of BCT works out to 85.83%, 86.33%, 86.33%, 85.67% and 86.33% respectively in the five crosses. The observed survival values are found to show insignificant deviation from the expected values, except in case of RBCT of (CSR2 61N) 61N (Table 11), where the 2 values were significant. In the population of F1 progeny back crossed with the resistant breed (BCT), the expected survival % is 87.5, as 50% of the BC populations have the genotype of the F1s and the remaining 50% resemble the resistant breeds. The observed survival percentage of BCT works out to 85.83%, 86.33%, 86.33%, 85.67% and 86.33% respectively in the five crosses. The observed survival values are found to show insignificant deviation from the expected values, except in case of RBCT of (CSR2 61N) 61N (Table 11), where the 2 values were significant. In case of F1 progeny back crossed with susceptible breed BL61, (BCS), the BC progeny is expected to comprise of 50 % population resembling F1 and the remaining 232

8 50% are of the susceptible parental genotypes and thus the expected survival is estimated to be 62.5%. In the five crosses, the survival % of the BCT progeny was observed to be 66.16%, 62.50%, 61.00%, 61.67% & 62.00% in the crosses involving PM BL61, BL36 BL61, 96C BL61, CSR18 CSR2 and 61N CSR2 respectively (Table 7-11). In general it was observed that the most of the 2 values of the F1, F2 and BC progeny in all the five crosses were less than 3.84 indicating insignificant deviations between the observed and the expected values, however only in few cases significant differences were noticed between the observed and expected numbers of surviving progeny. Enhancement of resistance to BmNPV Enhancement of resistance in silkworm of particular breed whose resistance is to be enhanced is attempted by feeding the silkworm with the pathogenic microorganisms and the progeny from the surviving population fed, in successive generation with the pathogenic microorganism in higher dosage than the previous generation. In the breed Pure Mysore, under the inoculated condition, the survival percent increased from 54% with LC50 dosage ( ) of BmNPV inoculum increased to 84.5% with LC70 dosage ( ) of BmNPV inoculum. The mortality due to nuclear polyhedrosis reduced from 46% in the first generation with LC50 dosage ( ) to 15.5% with LC70 dosage ( ). The breed under inoculated condition combined with the disease induction treatment, the survival percent increased from 47.3% with LC50 dosage ( ) of BmNPV inoculum increased to 70.8% with LC70 dosage ( ) of BmNPV inoculum. The mortality due to nuclear polyhedrosis reduced from 52.7% in the first generation with LC50 dosage ( ) to 29.2% with LC70 dosage ( ) (Table 12 & 15). In the breed BL36, under the inoculated condition, the survival percent increased from 50.3% with LC50 dosage ( ) of BmNPV inoculum increased to 84.7% with LC70 dosage ( ) of BmNPV inoculum. The mortality due to nuclear polyhedrosis reduced from 49.7% in the first generation with LC50 dosage ( ) to 15.3% with LC70 dosage ( ).The breed under inoculated condition combined with disease induction treatment the survival percent increased from 50.0% with LC50 dosage ( ) of BmNPV inoculum increased to 71.3% with LC70 dosage ( ) of 233

9 BmNPV inoculum. The mortality due to nuclear polyhedrosis reduced from 50.0% in the first generation with LC50 dosage ( ) to 28.7%% with LC70 dosage ( ) (Table 13 & 16). In the breed 96 C, under the inoculated condition, the survival percent increased from 48.0% with LC50 dosage ( ) of BmNPV inoculum increased to 72.5% with LC70 dosage ( ) of BmNPV inoculum. The mortality due to nuclear polyhedrosis reduced from 52.0% in the first generation with LC50 dosage ( ) to 27.5% with LC70 dosage ( ). The breed under inoculated condition combined with disease induction treatment the survival percent increased from 47.8% with LC50 dosage ( ) of BmNPV inoculum increased to 69.3% with LC80 dosage ( ) of BmNPV inoculum. The mortality due to nuclear polyhedrosis reduced from 52.2% in the first generation with LC50 dosage ( ) to 30.7% with LC80 dosage ( ) (Table 14 & 17). In the breed CSR18, under the inoculated condition, the survival percent increased from 50.0% with LC50 dosage ( ) of BmNPV inoculum increased to 74.5% with LC70 dosage ( ) of BmNPV inoculum. The mortality due to nuclear polyhedrosis reduced from 50.0% in the first generation with LC50 dosage ( ) to 25.5% with LC70 dosage ( ). The breed under inoculated condition combined with disease induction treatment the survival percent increased from 48.7% with LC50 dosage ( ) of BmNPV inoculum increased to 67.3% with LC80 dosage ( ) of BmNPV inoculum. The mortality due to nuclear polyhedrosis reduced from 51.3% in the first generation with LC50 dosage ( ) to 32.7% with LC80 dosage ( ) (Table 18 & 20).. In the breed 61N, under the inoculated condition, the survival percent increased from 50.2% with LC50 dosage ( ) of BmNPV inoculum increased to 80.5% with LC70 dosage ( ) of BmNPV inoculum. The mortality due to nuclear polyhedrosis reduced from 49.8% in the first generation with LC50 dosage ( ) to 19.5% with LC70 dosage ( ). The breed under inoculated condition combined with disease induction treatment the survival percent increased from 50.2% with LC50 dosage ( ) of BmNPV inoculum increased to 71.2% with LC80 dosage ( ) of BmNPV inoculum. The mortality due to nuclear polyhedrosis reduced from 49.8% in 234

10 the first generation with LC50 dosage ( ) to 28.8% with LC80 dosage ( ) (Table 19 & 21). From the studies leading to enhancement of resistance three multivoltine lines Pure Mysore(NPV), BL36 (NPV) and 96C (NPV) and two bivoltine lines CSR18 (NPV) and 61N (NPV) were developed with survival of 84.5 and 70.8% Pure Mysore(NPV); 84.7 and 71.3% (TSS2); 72.5 and 69.3% (96C); 74.5 and 67.3% (TSS4) and 80.5 and 71.2%(61N) against LC70 (Pure Mysore and BL36) and LC80 (96C, CSR18 and 61N) concentration of inoculum and Inoculum and induction treatments respectively. Chapter 3 Consistent efforts of silkworm breeders, to improve the quality and the quantity of silk by conventional breeding methods, resulted in synthesis of large number of pure breeds exhibiting distinct genetic and morphological features. However, the ultimate objective of silkworm breeding is not only to synthesis new genotypes but also to identify sustainable silkworm hybrids for commercial exploitation at farmer s level. The breeder selects the hybrids on the basis of combining ability of parents. In the present study, the combining ability of the five newly developed lines from Pure Mysore (NPV), BL36 (NPV), 96C (NPV), CSR18 (NPV) and 61N (NPV) were crossed with popular and productive silkworm breeds such as CSR2, CSR3, CSR4, CSR6 and CSR16 as testers to find out the genetic worth of newly developed lines and to identify new hybrid combinations through line tester analysis. The performance of parental breeds and their hybrids were analyzed for ten traits of economic importance by employing the statistical procedure of Line Tester analysis (Kempthorne, 1957), evaluation Index (Mano, 1993) and heterosis studies (Harada, 1961). In the present investigation, analysis of variance for 10 economic traits viz., Fecundity, Yield / 10,000 larvae by weight, Pupation rate, Cocoon weight, Cocoon shell weight, Cocoon shell ratio, Filament length, Raw silk percentage, Reelability and Neatness, of 5 5 parental breeds, 25 hybrids and Parents vs. Hybrids indicate a great deal of difference among themselves in the phenotypic expression of most of the characters reflecting genotypic variability (Tables 14 to 49). Highly significant (p<0.01) variations were observed between the Parents, Hybrids, Parents vs. Hybrids in almost all the traits which indicate the presence of considerable degree of heterosis. Also 235

11 observed are highly significant values (p<0.01) in all the treatment conditions viz., control (Table 14), BmNPV inoculation (Table 22) and Inoculation followed by induction (low temperature) (Table 30). The high variability noticed for all important economic traits in the hybrids can be attributed to genetic diversity present among the lines utilized in hybrid preparation. This indicates that the lines have acquired resistance / tolerance character for survival against BmNPV in inoculated and inoculated and induced conditions (Table 23 and 31). This improved health condition of the lines must have contributed significantly to their superiority for characters such as fecundity, cocoon yield, cocoon weight, cocoon shell weight, shell ratio, filament length, raw silk, reelability and neatness even in adverse conditions of rearing. In case of Testers, percent contribution was more for filament length. The data on Line Testers indicates superior contribution for survival rate, cocoon yield, cocoon weight, shell weight, filament length, reelability and neatness in the inoculated conditions. Their superiority even in adverse conditions of rearing. In the inoculated and induced conditions the superior contribution of line Testers were observed for survival rate, cocoon yield, shell weight, reelability and neatness indicated the more hybrid vigour in the hybrids. Among the polyvoltine lines, Pure Mysore (NPV), BL36 (NPV) and 96C (NPV) are poor in their GCA effects under control conditions (Table 16). These lines against BmNPV inoculation, Pure Mysore (NPV) was found good general combiners exhibiting significant (p<0.01) positive GCA effects for single characters and followed by 96C (NPV) (Table 24). These lines on inoculation and followed by induction also exhibits poor GCA effects (Table 32), demonstrating major role of additive and non additive genes in the inheritance of these characters. The newly evolved bivoltine lines (CSR18 (NPV) and 61N (NPV)) possess greater general combining ability for most of the traits evaluated. CSR18 (NPV) was found good general combiner exhibiting significant (p<0.01) GCA effects for 8 traits out of 10 traits evaluated. It was followed by 61N (NPV) for 7 traits under control conditions (Table 16). 236

12 TSS4 also exhibited good general combining ability with significant (p<0.01) positive GCA effects for 8 traits out of 10 traits evaluated, against BmNPV inoculation. It was followed by 61N (NPV) for 5 traits (Table 24). These lines, 61N (NPV) and CSR18 (NPV) were found good general combiners exhibiting significant (p<0.01) positive GCA effects for maximum characters evaluated against BmNPV inoculation and induction which indicates their merit in combining with popular testers and giving good source of general combining ability (Table 32). Among the multivoltine hybrids under control conditions, Pure Mysore (NPV) CSR3 were found good specific combiner for 7 traits out of 10 traits evaluated indicating its genetic worth and specifically combining with popular testers (Table 18). Pure Mysore (NPV) CSR3 were also found good specific combiner for most of the traits evaluated against BmNPV inoculation (Table 26) as well as Induction and exhibited significant SCA effects for 8 traits, out of 10 characters evaluated (Table 34). The overall results indicates that the hybrid Pure Mysore (NPV) CSR3 have shown positive SCA effects for maximum economic traits in all the treatments with good performance and superiority is selected as best hybrid. The bivoltine hybrids CSR18 (NPV) CSR16 showed a better significant positive SCA effects for 8 traits out of 10 characters evaluated under the control and inoculated conditions; where as under Inoculation and induction, 61N(NPV) CSR16 has recorded positive significant SCA effects for 6 traits out of 10 traits evaluated showing its superiority. The bivoltine hybrids 61N (NPV) CSR16 also exhibited significant SCA effects for 6 traits out of 10 characters against BmNPV inoculation followed by induction. Two hybrids CSR18 (NPV) CSR2 and 61N (NPV) CSR6 showed significant SCA effects for 4 traits. However, the hybrids CSR18 (NPV) CSR16 and 61N(NPV) CSR16 have shown positive SCA effects for maximum economic traits and also exhibited good performance and superiority when analyzed by evaluation index method. In the present study, GCA of lines and SCA of hybrids were analyzed under inoculation and low temperature induction conditions to identify the best combiners and short listing of promising hybrids under unfavourable conditions as the improvement obtained by selection under favourable conditions may not help in 237

13 realizing the full potential of the hybrid, when the selected strains are preferred to be used in unfavourable conditions (Falconer, 1960). It is observed that the lines and hybrids (Tables 51 and 52) have performed well under the adverse conditions. Among the lines, Pure Mysore (NPV), CSR18 (NPV) and 61N (NPV) performed well for majority of the traits evaluated. The hybrids Pure Mysore (NPV) CSR3, CSR18 (NPV) CSR16, 61N (NPV) CSR16 have shown significant positive SCA effects (p<0.01) for majority of the traits confirming their superiority over parents. Parents with higher GCA values produce high heterosis. Similarly, the hybrid 61N (NPV) CSR16 and Pure Mysore (NPV) CSR3 show positive SCA effects in adverse climatic conditions for majority of the quantitative characters evaluated (Table 31 and 32) indicates its superiority. Evaluation index is an important criterion and has been widely used to select silkworm hybrids and in recent period the multiple traits evaluation indices have been used to select silkworm breeds / hybrids by silkworm breeders. The bivoltine hybrid CSR18 (NPV) CSR16 recorded highest index values under control conditions and also exhibited higher average evaluation index value in inoculated conditions (Table 41 and 45). The hybrid 61N (NPV) CSR16 recorded high average evaluation index values (Table 46) under BmNPV inoculation followed by induction (low temperature). The Multi x Bivoltine hybrid Pure Mysore (NPV) CSR3 was found promising under control conditions. This hybrid also exhibited superiority under BmNPV inoculation and low temperature conditions. Hence, the present study finally adjudicated one multi x bivoltine hybrid Pure Mysore (NPV) CSR3 and two bivoltine hybrids CSR18 (NPV) CSR16, 61N (NPV) CSR16, as best hybrids for commercial exploitation and will be recommended for race authorization test at National level (Tables 40, 44 and 48). CONCLUSION Sustainability of sericulture depends upon successful realization of cocoon crop, the ultimate goal of rearing silkworm. It is one the most critical activity in sericulture and involves risk of cocoon crop loss due to silkworm disease. Silkworm breeders all over the world have made significant contribution by breeding silkworm breeds/hybrids that is high in productivity of quality silk. However the higher productivity is generally linked to their 238

14 resistance to diseases such nuclear polyhedrosis and adaptability to environmental factors. In the present study an attempt was made to develop breeds / hybrids, productive and resistant to nuclear polyhedrosis. This study precisely aimed to identify multivoltine and bivoltine breeds and hybrids that are endowed with traits for superior qualitative and higher quantitative production, better tolerant to adverse environmental conditions as well as resistance to the disease nuclear polyhedrosis. In this study, 23 multivoltine and 24 bivoltine silkworm breeds from silkworm germplasm of CSRTI., Mysore were screened for various traits such as productivity, resistance to nuclear polyhedrosis under normal, inoculated and inoculation and low temperature nuclear polyhedrisis induction conditions. Three multivoltine silkworm breeds viz., Pure Mysore, BL36 and 96C and 2 bivoltine breeds CSR 18 and 61N viz., were selected and the mode of inherence of resistance trait was established. The selected multivoltine and bivoltine breeds were subjected to experimentation to enhance their resistance to nuclear polyhedrosis by treating them with BmNPV PIB inoculum or BmNPV inoculum and disease induction with low temperature as disease inducer for 7 generations. In each generation the survived progeny were collected and exposed to higher dose of inoculum for 7 generations. It was observed that by the end of seventh generation, there was increase in survival of the multivoltine breeds by 21 34% and bivoltine breeds by 21 30% over their respective LC50 concentration, against inoculum of LC70 or LC80 BmNPV concentration of respective breeds. The new lines developed were coded as Pure Mysore (NPV), BL36 (NPV), 96C (NPV), CSR18 (NPV) and 61N (NPV). In order to find out the genetic worthy of the newly evolved lines viz., Pure Mysore (NPV), BL36 (NPV), 96C (NPV), CSR18 (NPV) and 61N (NPV) to identify promising hybrid combinations, the new lines were crossed with popular and productive silkworm breeds such as CSR2, CSR3, CSR4, CSR6 and CSR16 as testers following the principle of Line tester analysis. The performance of parental breeds and their hybrids were analyzed for ten traits of economic importance by employing the statistical procedure of Line Tester analysis, Evaluation Index and heterosis studies. Based on the result of these studies finally adjudicated one single multi bivoltine hybrid Pure Mysore (NPV) CSR3, two promising bivoltine hybrids viz., CSR18 (NPV) CSR16 and 61N (NPV) CSR16 were short listed for commercial exploitation and will be recommended for race 239

15 authorization test at National level. The better performance of the hybrid was judged under control conditions, against BmNPV inoculation, followed by BmNPV inoculation and low temperature induction. The F1 hybrids have shown their superiority in survival rate, resistance to nuclear polyhedrosis, cocoon yield, Shell weight, shell percentage, raw silk %, reelability and neatness under normal as well as adverse conditions. These hybrids also showed superiority for fecundity, pupation rate (control), survival rate (inoculation/induction) cocoon yield, shell weight, shell percentage, filament length, raw silk, reelability and neatness (all treatments). 240