of magnitude. Further, index scoring method (Arunachalam and Bandyopadhyay, 1984) obtained on thirty one breeds also, brings in similar and comparable

Transcription

1 Discussion The breeding of silkworm since long has been aimed towards evolving of superior and hardy breeds either by means of selection alone or by combining outcrossing or backcrossing with selection in the subsequent generations. The final aim of the breeder is primarily to evolve a breed which can give rise to stabilized crops and secondly to improve both quantity and quality of silk (Tazima, 1984). The breeding of silkworm races probably dates back to the beginning of the history of silkworm rearing, but it has made great progress rather recently (Hirobe, 1968). Sericulturally advanced countries like Japan has achieved remarkable progress by executing systematic breeding plans for the development of productive races. In silkworms, studies carried out for various characters have shown that the characters could be changed to suit the breeders choice, since selection for one trait has correlation with genetic change of other characters. The correlation for few traits is negative and for some it is positive (Tsuchiya and Kurashima, 1959 and 1960; Ohi et al., 1970; Gamo and Ichiba, 1971; Gamo, 1976). Therefore, during the course of breeding of new breeds, the breeder has to be aware of the response of certain characters in selection and its correlated changes with other economic traits. Inbreeding of hybrids to stabilize silkworm breeds which bred true is well documented (Osawa and Harada, 1944; Harada, 1952, 1953 and 1956; Hirobe, 1956, 1967 and 1968; Yokoyama, 1956, 1959, 1976 and 1979 and Gamo, 1976). Similarly, Kovalov (1970) is of the opinion that improvement of silkworm races is possible by out-breeding with exotic races and improvement of cocoon quality by repeated backcrossing (Tazima, 1964). Thirty one bivoltine breeds of different geographical origin and possessing important genetic potential on various economic traits in varying magnitude were screened for evaluation at Satellite Silkworm Breeding Station, Coonoor, twice, during a favourable season, Spring (February-April) and during a Winter (November- January) season, in order to obtain fair and precise assessment of various traits. The mean values and the Co-efficient of variation on the economically important traits, indicates these breed s genetic potential as well as the variability and thereby their utility in the breeding study proposed. Employing the multiple trait index on these nine economically important traits, fourteen breeds had scored >50 as index values on the cumulative mean obtained. These selected breeds represented heterogeneous mixture of all desired traits available among them in varying degree 165

2 of magnitude. Further, index scoring method (Arunachalam and Bandyopadhyay, 1984) obtained on thirty one breeds also, brings in similar and comparable evaluation. The selection of breeding resource material helps the breeder to successfully amalgamate desired traits. Appropriate experimental design, selection methods employed in fixing the major traits contributing to the improved cocoon yield leads to the success of any breeding programme. Besides, understanding the genetic diversity of parental strains to be utilized in the breeding programme by their systematic evaluation, critical assessment of their quantitative nature which is greatly influenced by the environmental factors such as temperature, light, relative humidity, nutrition and rearing techniques which paves the way for the breeder for effective utilization (Kogure, 1933; Legay, 1958; Ueda and Lizuka, 1962; Suzuki et al., 1962; Yokoyama, 1963; Arai and Ito, 1967; Horie et al., 1967; Naseema Begum et al., 2001 and Sudhakar Rao, et al., 2002). Twelve selected breeds which were possessing desired targeted traits on larval duration, fitness, productivity and fibre quality, were subjected for amalgamation, in a selective fashion. Oval breeds were crossed with ovals and dumbbells were with dumbbells as also the breeds of plain larvae with plain and breeds of marked with marked larvae. Priority was given to top performers in the evaluation studies and selective crossing was made keeping the objective and giving representation to different groups. Thus, seventeen combinations were prepared considering the above parameters (Table-6). The population in filial generation1 was kept sufficiently broad by deriving from composite population obtained from 25 layings and three replicates of 250 larvae were kept in each combinations. The mean performance of these seventeen combinations on fifteen traits covering pre-cocoon, cocoon and post-cocoon parameters, indicated the genetic potential of various combinations made and the scope for selection and improvement in reaching the targeted traits. To continue breeding programme and to obtain new progeny for each line the selected cocoons were subjected for pupal sex separation. Line wise sex separation was carried out and kept for moth emergence. On the day of emergence female and male moths were applied for mating (inbreeding) for 3½ to 4 hours. Each pair individually picked and depaired and the gravid female moths were allowed for oviposition in semi dark room. After egg layings the moths were collected and subjected to individual mother moth examination to ensure for disease freeness. 166

3 Twenty five to thirty disease free laying were prepared from each breed and treated with HCL acid within 24 hrs of oviposition to break the diapause following the hot acid treatment methods of Yokoyama (1963). Layings were incubated at 25±1 C temperature and 70-80% relative humidity in incubators with 16 hrs light and 8hrs darkness. By utilizing rich egg laying showing good hatching percentage composite layings were prepared from F1 to F5 and from F6 to F12 individuals layings with good fecundity as well as good hatching percentage were wrapped in tissue paper then black boxed to synchronize the development of embryo once after eggs turned to blue colour on eighth day of incubation to ensure healthy and uniform hatching on expected day of brushing Employing multiple trait evaluation index, on nine traits, viz., fecundity, fifth age, total larval duration, pupation rate, yield per larvae by number, cocoon weight, shell weight, shell percentage, silk productivity per day of seventeen filial generation 1, ten combinations were short-listed. After further screening of shortlisted combinations, four oval types and six dumbbell types were short-listed for further studies. While allowing for random gene recombination in filial generation 2 and also, in filial generation 3, mass culture was resorted and deriving from composite population obtained from 25 layings with three replicates of 250 larvae kept in each combinations. The mean performance of filial generation 2 had indicated slight decline in most of the trait values Further, it was observed that Coefficient of variation also, came down from previous generation, considerably indicating that selected combinations were more or less falls in the similar groups only. While allowing for random gene recombination in filial generation 3, again mass culture was resorted and derived from composite population obtained from 25 layings with three replicates of 250 larvae kept in each combinations. The mean performance of filial generation 3 had indicated further decline. However, it was observed that Co-efficient of variation had increased from previous generation, considerably indicating that selected combinations were distinct from each other. At filial generation 4, to breed for shorter larval duration, the productivity index as suggested by Gamo, (1976) was followed along with separation of early spinners hand in hand. However, this reduction had a bearing as there was considerable reduction seen on cocoon weight, cocoon shell weight, shell percentage from previous generation. By filial generation 6, the targeted value of 22 days larval duration was achieved more or less in all ten breeding lines, however, the shell 167

4 percentage stood slightly less. Therefore, in the subsequent filial generations of 8, 9 and 10, efforts were made to improve the shell percentage and silk productivity by continuing the selection pressure on silk productivity, but discontinued the separation of early spinners. The mean performance of ten breeding lines at filial generation 7 indicates that the larval duration remained in targeted level but the cocoon weight, cocoon shell weight and shell percentage were still less than the desired level. However, few breeding lines had attained good improvement in many traits. Filial generation 8 of ten breeding lines showed improvement in cocoon weight, shell weight in particular, whereas other trait values were maintained to that of previous generation. Filial generation 9 of ten breeding lines showed slight increase in larval duration and improvement in shell percentage but reduction in cocoon weight and shell weight. At filial generation 10 of ten breeding lines, the larval duration remained with improvement in cocoon weight in particular, to that of previous generation. Performance of ten breeding lines from filial generation 1 to 10 had clearly brings in the measures of breeding strategies adopted and the results obtained thereon. Target traits of larval duration was brought down from 24 days to 22 days in ten breeding lines, while balancing the productivity merits around optimum level (Shell percentage-22). It can be seen from these data that random recombination of genes allowed at filial generation 1 to 3, had invariably resulted in reduction of productivity merits in the segregating populations of all ten breeding lines. Efforts to improve productivity while applying selection pressure for shorter larval duration, from filial generation 4 to filial generation 6, had resulted in considerable improvement invariably in all ten breeding lines. Further, stabilization of breeding lines when attempted from filial generation 7 to filial generation 10, the insignificant difference observed in many trait values pertaining to these generations, in most of breeding lines showed fair amount of achievement in fixation of breeding lines. The hybridization as such in both plants and animals has been in practice even earlier to the studies of Koelreuter in 1763 (cf. Hayes, 1952). Heterozygosis is the end result of such hybridization of cross breeding. Hayes (1952) is of the opinion that such crossing produces heterozygotes in all characters exhibiting superiority over their parents. Toyama in 1905 (Yokoyama, 1959) discovered a marked superiority in the F1 hybrids for economic traits in silkworm, Bombyx mori and he was the first to introduce F1 hybrids instead of pure strains in the Japanese 168

5 sericulture industry. It has been pointed out that the F1 hybrids in silkworm, Bombyx mori in several respects are superior to their pure line parents (Singh and Hirobe, 1964). Such superior expression often referred to as hybrid vigour has been called heterosis by Shull in 1914 (Shull, 1949). To achieve desired goals, cross breeding is widely used in commercial animal production as a means of exploiting heterosis (Sang, 1956 and Bowman, 1959). Such heterotic manifestations in silkworm has been well documented by many researchers in Japan of which the works of Katsumata (1948) and Harada (1949) are noteworthy. They have proved the superiority of the F1 hybrids over their parents with regard to economically important traits such as hatchability, larval duration, cocoon weight and shell weight etc., Evaluation of hybrid vigour with reference to expression of economic characters in the crosses is an important task that enables to understand the manifestation of hybrid vigour independently for each character and in conjugation with others. Generally, hybrid vigour is manifested when two genetically distinct breeds are crossed (Falconer, 1981). In the present study, in order to know the degree of manifestation of hybrid vigour in twenty seven hybrids, the heterosis values over mid and better parents were computed for each character to adjudicate the best hybrid. The results revealed that some hybrids expressed positive heterosis for various economic characters in temperate or hill conditions. Four oval breeds, viz., SLD1, SLD2, SLD3, SLD4 and six dumbbell breeds, SLD5, SLD6, SLD7, SLD8, SLD9, SLD10, evolved from the breeding study were crossed in all possible combinations and thereby, forty-eight new hybrids were prepared to undertake hybrid evaluation for commercial exploitation. Mean performance of forty-eight new hybrid combinations measured in two rearings on fourteen economically important parameters were presented along with control hybrid, CSR2 x CSR4. The mean values on important traits indicates the genetic potential available in new hybrid combinations. Further, realization of the targeted trait of 22 days larval duration along with shell percentage of 22, had indicated the fair success obtained in the breeding study. Multiple trait evaluation index employed on these forty-eight new hybrid combinations revealed that twenty combinations had scored >50 as mean index values measured on fourteen traits. Further, there was a clear indication of reciprocal effect as the straight combinations as much as 16 out of total

6 combinations had performed well by scoring > 50 index value, whereas only 4 out of total 24 combinations as reciprocals could score > 50 as index value. While considering the individual traits, 22 combinations had scored >50 index value for fecundity, 26 combinations had scored >50 index value for fifth age larval duration, 23 combinations had scored > 50 index value for total larval duration, 25 combinations had scored > 50 index value for pupation rate, 23 combinations had scored >50 for cocoon yield by weight for 10,000 larvae, 19 combinations had scored >50 for cocoon weight, 23 combinations had scored >50 for cocoon shell weight, 20 combinations had scored >50 for cocoon shell percentage, 19 combinations had scored >50 for average filament length, 26 combinations had scored > 50 for cocoon filament size, 22 combinations had scored >50 for raw silk content, 25 combinations had scored >50 for renditta, 23 combinations had scored for reelability and 35 combinations had scored >50 for neatness, out of 49 combinations (including control hybrid). Forty-eight new hybrid combinations after multiple trait index scoring were short-listed to twenty combinations which had scored >50, which is about 41% of total combinations made for the study. This on the one hand, shows that a good number of combinations had in possession of various economic traits at a desirable level and on the other hand the extent of phonotypic expression had more or less in nearby spheres, though their parental resources are from distinct groups. Alternately, one can say that the objective of obtaining hybrid combinations possessing genetic constellation as desired by way of target perceived in the breeding programme and the degree of success obtained through adopting various breeding manipulation strategies, has been largely satisfactory as denoted by the performance scored by above 41% combinations. Clearly the best performers is SLD 4 x SLD 8, wherein the compatibility expressed in most of the economically important traits in both straight as well as reciprocals are significantly higher. According to Kovolov (1970), the improvement of indigenous races could be achieved through hybridization utilizing exotic races. Further, Harada (1956) opined that new silkworm breeds could only be evolved through hybridization followed by selection. Hybridization studies involving distinct group of parents in the present study, had resulted in the development of new breeds with relatively shorter larval duration and possessing productivity traits In silkworms, studies carried out for various characters have shown that the characters 170

7 could be changed to suit the breeders choice, since selection for one trait has correlation with genetic change of other characters. The correlation for few traits is negative and for some it is positive (Tsuchiya and Kurashima, 1959 and 1960; Ohi et al., 1970; Gamo and Ichiba, 1971; Gamo, 1976). Most of the economic traits in silkworm are of polygenic nature and are under the influence of environment (Yokoyama, 1979). By adopting appropriate selection pressure on early spinners and monitoring the possession of productivity traits in such segregated population, polygenes of desired genetic constellation are obtained in the new breeds developed. The final aim of the breeder is primarily to evolve a breed which can give rise to stabilized crops and secondly to improve both quantity and quality of silk (Tazima, 1984). Evaluation studies carried out covering fourteen economically important traits on pre-cocoon, cocoon and post-cocoon areas in the present study is certainly, aimed in this direction. The new superior hybrid combinations developed from this study viz., SLD 4 x SLD 8, SLD 1 x SLD 8, SLD 2 x SLD 8, SLD 4 x SLD 6 and SLD 4 x SLD 9, are clearly directed towards this goal and are expected to fare under not only sub-tropical conditions as they are derived with relatively shorter larval duration but also, under plain conditions as they are in better position to overcome poor yield levels. Double hybrids, are possessing a significant advantage of inherent genetic plasticity because of more than two parental resources involved in the genetic architecture and therefore, new breeds evolved were subjected for a concise study, to utilize this advantage. By utilizing the 10 breeding lines, four oval and six dumbbell, 21 foundation crosses, six oval crosses and eleven dumbbell crosses, were made in a selective fashion by avoiding reciprocals. Performance of these 21 new crosses was evaluated along with two controls, the parental crosses of ruling double hybrid, Krishnaraja (Table-31). The mean performance of new foundation crosses on fecundity (499), fifth age (6.08), total larval duration (22.02), pupation rate (93.28), yield per larvae (16.765), cocoon weight (1.672), shell weight (0.368), shell percentage (22.00), filament length (1041), filament size (2.74), raw silk (17.6), renditta (5.79), reelability (82.6), neatness (92), silk productivity (5.81) indicates the available genetic potential in parental crosses. Analysis of performance on above economic traits, through multiple trait evaluation index, resulted in six short-listed foundation crosses, three oval, viz., SLD1 x SLD2 (63.37), SLD2 x 171

8 SLD4 (63.89), SLD3 x SLD4 (64.30) and three dumbbell SLD5 x SLD9 (62.74), SLD8 x SLD9 (63.27), SLD9 x SLD10 (62.81) foundation crosses. By utilizing the short listed six foundation crosses, nine new double hybrids were prepared in a selective crossing by avoiding reciprocals and repetitive crosses which were subjected for evaluation along with Krishnaraja. The mean performance of new double hybrids on fecundity (558), fifth age (6.11), total larval duration (22.09), pupation rate (93.85), yield per larvae (17.936), cocoon weight (1.865), shell weight (0.411), shell percentage (22.00), filament length (1032), filament size (2.84), raw silk (17.7), renditta (5.66), reelability (82.4), neatness (92), silk productivity (6.38) indicates the genetic potential available in them. Analysis of performance on various economic traits, through multiple trait evaluation index, resulted in identification of one promising hybrid, (SLD2 x SLD4) x (SLD8 x SLD9). Manifestation of heterosis in 48 new single hybrids was studied on various economic parameters, to evaluate their hybrid vigour imported. The mean values of heterosis over mid parents, indicated positive heterosis on the traits of fecundity (3.15), pupation rate (0.30), yield per larvae (21.60), cocoon weight (16.43), shell weight (18.70), shell percentage (1.64) and reelability (3.65). The magnitude of such heterosis was more pronounced in yield per larvae (21.60), cocoon weight (16.43) and shell weight (1.64), while it was less in other traits. Further, a negative heterosis was observed in larval duration (-1.41) which is desirable but in raw silk (-1.75) and neatness (-0.15) which are not desirable. Further, the heterosis magnitude on traits such as yield per larvae (32.67), cocoon weight (28.50), shell weight (33.24) in SLD1 x SLD6 was high indicating the existence of maximum genetic distance between the parents, SLD1 and SLD6. However, in the top ranking candidates evaluated through multiple trait index, viz., SLD4 x SLD8, the extent of heterosis was fecundity (0), pupation rate (1.28), yield per larvae (23.65), cocoon weight (16.26), shell weight (17.71), shell percentage (1.57), filament length (0.43), raw silk (4.01), reelability (2.92), neatness (0), larval duration (-2.20), renditta (-3.93) which were desirable along with filament size (15.32) which was not desirable and in SLD1 x SLD8, it was fecundity (2.05), pupation rate (2.08), yield per larvae (28.18), cocoon weight (15.75), shell weight (20.73), shell percentage (4.23), filament length (12.95), raw silk (1.64), reelability (1.99), neatness (0), larval duration (-3.70), renditta (-1.64) which were desirable along with filament size (9.13) which was not desirable and in SLD2 x SLD8, it was fecundity (1.8),, yield per 172

9 10000 larvae (18.89), cocoon weight (12.50), shell weight (17.92), shell percentage (4.71), filament length (2.11),raw silk (1.48), reelability (4.01), neatness (0.53), larval duration (-1.84), renditta (-1.46) which were desirable along with pupation rate (0.97) filament size (9.60) which was not desirable and in SLD4 x SLD6, it was fecundity (5.31), pupation rate (0.05), yield per larvae (24.95), cocoon weight (20.76), shell weight (20.77), shell percentage (0.18), filament length (2.92), raw silk (1.53), reelability (0.59), neatness (0.53), larval duration (-5.42), renditta (-1.76) which were desirable along with filament size (24.39) which was not desirable and in SLD4 x SLD9, it was it was yield per larvae (20.65), cocoon weight (15.92), shell weight (15.50), shell percentage (0.09), filament length (5.93), raw silk (1.14), reelability (2.99), neatness (0.53), larval duration (-0.55), renditta (-1.10) which were desirable along with fecundity (-5.24), pupation rate (-0.14), filament size (16.67) which was not desirable, indicates that their candidature as superior combination stands justified. Further, the extent of manifestation of heterosis was not found to be high in most of the individual traits in these hybrid combinations indicates that the genetic distance of parental breeds involved in these crosses were not as high as in some other combinations wherein maximum heterosis had manifested. But it also, indicates that having attained desirable heterosis in positive as well as negative traits, the genetic distance existing among these breeds were optimum and the difference was not so wide but belongs to more or less in nearby spheres. Manifestation of heterobeltiosis in 48 new single hybrids was studied on various economic parameters, to evaluate their hybrid vigour imported. The mean values of heterobeltiosis, indicated positive and desirable heterobeltiosis only on the traits of yield per larvae (19.77), cocoon weight (13.91), shell weight (14.39), reelability (2.39) and larval duration (0.21), while for remaining traits such as fecundity (-1.44), pupation rate (0.79), shell percentage (-0.32), filament length ), raw silk (-4.80), neatness (-0.59), renditta (5.29) and filament size (22.93), only negative and undesirable heterobeltiosis was observed. However, in the top ranking candidates evaluated through multiple trait index, viz., SLD4 x SLD8, the extent of heterobeltiosis was pupation rate (0.99), yield per larvae (22.63), cocoon weight (15.66), shell weight (17.41), shell percentage (0.83), filament length (0.09), raw silk (3.46), reelability (2.33), neatness (0), larval duration (-2.20), renditta (-3.38) which were desirable along with fecundity (-5.05), filament size (20.17) which was not desirable and in SLD1 x SLD8, it was fecundity (1.48), pupation rate (1.87), 173

10 yield per larvae (26.58), cocoon weight (14.24), shell weight (14.43), shell percentage (0.17), filament length (8.28), reelability (0.79), neatness (0), larval duration (-2.80), which were desirable along with raw silk (-1.38), filament size (10.50) renditta (1.32) which was not desirable and in SLD2 x SLD8, it was yield per larvae (18.82), cocoon weight (11.73), shell weight (12.94), shell percentage (0.96), filament length (0.26), reelability (1.56), neatness (0), larval duration (-1.29), which were desirable along with fecundity (-1.91), pupation rate (-0.29), raw silk (- 1.59), filament size (15.13), renditta (1.69) which was not desirable and in SLD4 x SLD6, it was yield per larvae (19.48), cocoon weight (14.87), shell weight (10.22), neatness (0), larval duration (-3.44), which were desirable along with fecundity (-3.2), pupation rate (-0.71), shell percentage (-4.13), raw silk (-3.82), filament length (-5.38), reelability (-1.16), filament size (30.21), renditta (3.90) which was not desirable and in SLD 4 x SLD9, it was it was yield per larvae (17.69), cocoon weight (12.46), shell weight (11.47), filament length (0.78), raw silk (0.11), Reelability (0), neatness (0), larval duration (-0.37), renditta (-0.19) which were desirable along with fecundity (-11.78), pupation rate (-0.55), shell percentage (- 0.83), filament size (22.13) which was not desirable, indicates the existence of epistasis or over-dominance in many traits and argues well for finding placement in selection. Manifestation of heterosis in nine new double hybrids was studied on various economic parameters, to evaluate their hybrid vigour imported. The mean values of heterosis over mid parents, indicated positive and desirable heterosis only on the traits of fecundity ( 6.48) and cocoon weight (3.60), while for the traits of pupation rate (-1.86), yield per larvae (-1.02), shell weight (-0.87), shell percentage (- 7.92), filament length (-7.92), filament size (12.35), raw silk (-3.84) and reelability (- 3.11), neatness (-1.87) negative or undesirable heterosis was observed. However, in case of the double hybrid, (SLD2 x SLD4) x (SLD8 x SLD9), elected by employing multiple trait index, the manifestation of heterosis was fecundity (9.91), pupation rate (1.31), yield per larvae (12.44), cocoon weight (10.67), shell weight (10.26), shell percentage (0.13), raw silk (2.17), reelability (0.58), neatness (0.53), larval duration (-0.49), renditta (-0.38) which were desirable along with filament size (1.98) which is undesirable indicates that the genetic distance between the foundation parents was sufficiently enough to bring in such heterosis. On the contrary, negative and undesirable heterosis seen in other combinations indicates 174

11 that the genetic distance between those foundation crosses were very less thereby the desirable manifestation is limited to few traits. Further, it was also to be noted that a good amount of heterosis was already manifested and exploited while obtaining the foundation parents and therefore, the further manifestation was limited to few traits. Manifestation of heterobeltiosis in nine new double hybrids was studied on various economic parameters, to evaluate their hybrid vigour imported. The mean values of heterobeltiosis, indicated positive and desirable heterobeltiosis only on the traits of fecundity (1.95) and cocoon weight (2.88), while for the traits of pupation rate (-2.44), yield per larvae (-2.00), shell weight (-2.10), shell percentage (- 5.14), filament length (-9.21), raw silk (-4.52), filament size (13.19), reelability (-3.39), negative or undesirable heterobeltiosis was observed. In the double hybrid, (SLD2 x SLD4) x (SLD8 x SLD9), the manifestation of heterobeltiosis was fecundity (3.89), pupation rate (0.35), yield per larvae (12.07), cocoon weight (9.70), shell weight (8.71), raw silk (0.53), reelability (0), neatness (0), larval duration (-0.39), renditta (0) which were desirable along with shell percentage (-0.94), filament length (-3.02), filament size (1.98) which are undesirable, indicates the existence of genetic distance though it was not pronounced but minimal, among the foundation parents. Expression of hybrid vigour for only certain traits in some hybrids can be related to the impact of the selection of particular gene complexes during the course of breeding of the particular breeds. The variable degree of manifestation of hybrid vigour and over dominance recorded in the hybrids under study form the basis to identify the promising hybrids. In addition, the uniformity in the expression of characters in the hybrids compared to parental breeds as evidenced by the present study for all major metric traits also form the basis for the identification of superior hybrids. The phenotypic consequences with reference to metric traits analysed in these hybrids are observed to be greatly dependent upon the genetic loci that are combined to gather by the parents. The manifestation of hybrid vigour was explained by over dominance hypothesis by Shull (1949) and epistasis theory by Sheridion (1980, 1981). As evidenced by the manifestation of hybrid vigour for various traits in the hybrids under study, it is possible that the theories put forwarded are not mutually exclusive as it could be due to cumulative effects of several factors responsible for the manifestation of hybrid vigour in varying proportions which support the views of Bowman (1959). 175

12 Although, the genesis of hybrid vigour is debated for long, questioning it to be a result of the heterozygocity or its origination from other causes, the fact that the hybrids perform much better than the parental breeds implies that certain groups of favourable alleles combine together in the hybrid which interact with the environment and produce desirable levels of hybrid vigour and confer the superiority of hybrids over parental breeds. Among the 48 hybrids derived and studied, no single hybrid expressed consistency in hybrid vigour for all the traits. Most of the hybrids manifested desired hybrid vigour for pupation rate, cocoon yield/10,000 larvae, Cocoon weight, Cocoon shell weight, cocoon shell percentage, filament length, raw silk percentage, neatness and over dominance for pupation rate, cocoon yield/10,000 larvae, Cocoon weight, Cocoon shell weight. The present results indicating variable magnitude of hybrid for different combinations clearly establish the superiority of hybrids and corroborate the observations of Harada (1961) for cocoon shell weight, followed by cocoon weight and survival rate, Subba Rao and Sahai (1989) for cocoon yield followed by cocoon weight, Yokoyama (1974), Gerkov and Petkov (1989), Bhargava et al., (1992), Ahsan (1992), Yamamoto et al., (1994), Veeraiah (1999) and Angadi (2007), Nirmal Kumar, et al., (2010 and 2011) for hybrid vigour, who attributed the manifestation of hybrid vigour to variable combinations of genes that are brought together in the hybrids, which in turn interact with another in relation to the prevailing environmental conditions to which the hybrids are exposed during the growth and development. The percentage improvement of the new breeds and single and double hybrids over the control breeds and hybrids was evaluated. Among new oval breeds, SLD 4 exhibited pronounced and desirable improvement over the control, CSR2 in many traits, while other three oval breeds exhibited such improvement in less number of traits only. Among new dumbbell breeds, SLD8 exhibited pronounced and desirable improvement over the control, CSR4 in many traits in varying magnitude, while other four dumbbell breeds exhibited such improvement in fewer number of traits only. New single hybrid, SLD4 x SLD8 exhibited desirable improvement over the control, CSR2 x CSR4, in many traits studied in varying magnitude. New double hybrid, (SLD2 x SLD4) x (SLD8 x SLD9) exhibited desirable improvement over the control, (CSR2 x CSR27) x (CSR6 x CSR26), in many traits studied in varying magnitude. 176

13 The silk productivity estimates as suggested by Gamo, (1976) was calculated for all 48 new single hybrids The silk productivity per day expressed in centigrams in selected hybrid SLD4 x SLD8 was higher, when compared to the control hybrid, CSR2 x CSR4. Study confirms the superiority of promising hybrids identified through evaluation index method to a larger extent. Similar silk productivity estimates were calculated on nine new double hybrids. The silk productivity per day in selected double hybrid, (SLD2 x SLD4) x (SLD8 x SLD9) was higher, when compared to the control hybrid, (CSR2 x CSR27) x (CSR6 x CSR26). Measurement of cocoon uniformity in forty-eight new single hybrids indicates placements for many promising hybrids in accordance with set standards of size deviation in terms of SD (Standard Deviation) and CV (Co-efficient of Variation). Selected hybrid, SLD 4 x SLD8, falls well within the optimum limits, standard deviation in cocoon index and co-efficient of variation. Similar measurement of cocoon uniformity in nine new double hybrids were recorded and in case of new double hybrid, (SLD2 x SLD4) x (SLD8 x SLD9), the standard deviation in cocoon index and the co-efficient of variation confirms the adherence of set norms. Studies on grainage parameters such as egg yield per kg cocoons, egg number per gram and fecundity conversion of three oval, SLD1, SLD2, SLD4 indicates that SLD4 exhibits better yield potential, viz., egg yield per kg of cocoons and fecundity than other breeds over control breeds, CSR2 and in case of three dumbbell breeds, SLD6, SLD8, SLD9, and SLD8 exhibits better yield potential, viz., egg yield per kg of cocoons and fecundity than other breeds over control breeds CSR4 and values were well within set standards. Similar studies on grainage parameters such as egg yield per kg cocoons, egg number per gram, and fecundity conversion of three oval foundation crosses, SLD1x SLD2, SLD2 x SLD4, SLD3 x SLD4 indicates that SLD2 x SLD4 exhibits better yield potential, viz., egg yield per kg of cocoons and fecundity than other breeds over control, CSR2 x CSR27 and among three dumbbell breeds, SLD5 x SLD9, SLD8 x SLD9, SLD9x SLD10 indicates that SLD8 x SLD9 exhibits better yield potential, viz, egg yield per kg of cocoons and fecundity than other breeds over control, CSR6 x CSR26 and values were well within set standards. The present study has resulted in the development of six new breeds, SLD1, SLD2, SLD4, SLD6, SLD8, and SLD9; of which SLD4 and SLD8, are marginally 177

14 superior to CSR2 and CSR4 in productivity merits and relatively shorter larval duration; One new single hybrid, SLD4 x SLD8 with shorter larval duration and better productivity traits than the control, CSR2 x CSR4; Six new foundation crosses, SLD1 x SLD2, SLD2 x SLD4, SLD3 x SLD4, SLD5 x SLD9, SLD8 x SLD9 and SLD9 x SLD10; of which SLD2 x SLD4 and SLD8 x SLD9, are marginally superior to CSR2 x CSR27 and CSR6 x CSR26, in productivity merits and shorter larval duration; One new double hybrid ( SLD2 x SLD4 ) (SLD8 x SLD9) with better productivity traits and shorter larval duration than the control, ( CSR2 x CSR27 ) x (CSR6 x CSR26). Improvement of breeds necessarily means selection of desirable genes in appropriate combinations, which contribute to the overall genetic worth of the population. With respect to the economic value, the focus should be on all the genes affecting the traits thereby contributing to the viability and productivity. Therefore, selection of hybrid combinations emphasize the need to organize the genetic material in a way that help to improve the manifestation of commercially important traits (Nirmal Kumar, 1995). This can be achieved precisely by adopting a strategy by setting up a common index giving adequate weightage to all the component traits manifested among an array of hybrids. The rationale for judging the utility of multiple trait evaluation index in the present study is based on major metric traits, which are considered to be economically important. However, asymmetry is found in most of the traits as evidenced by indices of the individual component traits that the index value obtained individually for each trait can fail but the overall index values help in adjudicating the performance of the hybrids derived. In view of this, all the major traits have been considered together to obtain the aggregate index value, since exclusion of any one trait can result in negative situation. 178

15 SUMMARY The objective of this breeding study, was to amalgamate desirable traits available with a group of potential bivoltine genetic resources possessing attributes on fitness, productivity, fibre quality and shorter larval duration through appropriate selection of breeding resource materials, adopting specific breeding and selection strategies and by undertaking systematic hybrid evaluation so as to develop and identify superior bivoltine hybrid combinations possessing the desired targeted traits on productivity (shell percentage >22) along with relatively shorter larval duration (around 22 days). Thirty one bivoltine silkworm breeds available at Satellite Silkworm Breeding Station, Coonoor, of which, fourteen breeds, viz., C108, C120, Dong 306, NN6D, CJ3P, SPC1, SH2, J2P, JC2P, JZH PO, NB1, G177,A120 and A121, spin Chinese type or oval shaped cocoons and seventeen breeds, viz., N4, NJ1, SN1, SPJ1, SPJ2, 36PC, J1M, J2M, JA1, JB2, J122, JZH MC, M2, G146, 14M, European and B120, spin Japanese type or peanut shaped cocoons were utilized for the new breeding study. Nine important traits viz., larval duration, yield per larvae by number, yield per larvae by weight, cocoon weight, cocoon shell weight, shell percentage, filament length, filament size and raw silk content, were considered for evaluation Employing the multiple trait index suggested by Mano, et al (1992 and 1993) along with modified method developed by the investigator group on measurement of negative traits like larval duration and filament size and by index scoring method (Arunachalam and Bandyopadhyay, 1984), twelve breeds, five oval or Chinese type breeds viz., A120, A121, SPC1, CJ3P, J2P and seven dumbbell or Japanese type breeds, viz., B120, JA1, 36PC, European, G146, M2 and SPJ2, were selected representing heterogeneous mixture of all desired traits available in them at varying degree of magnitude. Twelve selected breeds were subjected for amalgamation, in a selective fashion and in line with their attributes, oval breeds with ovals and dumbbells with dumbbells as also the breeds of plain larvae with plain and breeds of marked with marked larvae. Priority was given to top performers in the evaluation studies and 179

16 selective crossing was made keeping the objective and giving representation to different groups. Employing multiple trait evaluation index on seventeen new combinations, on nine traits, viz., fecundity, fifth age, total larval duration, pupation rate, yield per larvae by number, cocoon weight, shell weight, shell percentage, silk productivity per day of seventeen filial generation 1, ten combinations were shortlisted (Table-7). They are, JA1 x SPJ2 (57.70), J2P x CJ3P (56.81), A121 x CJ3P (56.58), CJ3P x A120 (55.95), B120 x JA1 (55.42), SPC1 x A121 (55.08), G146 x 36 PC (50.80), M2 x B120 (50.14), G146 x M2 (47.86) and M2 x 36 PC (44.64), which had obtained index values more compared to the control, CSR2 x CSR4 (41.63). Mass recombination of genes, while allowed from filial generation 1 to filial generation 3, had invariably resulted in reduction of productivity merits in the segregating populations of all ten breeding lines. Efforts to improve productivity while applying selection pressure for shorter larval duration (Gamo, 1976), from filial generation 4 to filial generation 6, had resulted in considerable improvement invariably in all ten breeding lines. Further, stabilization of breeding lines when attempted from filial generation 7 to filial generation 10, the insignificant difference observed in many trait values pertaining to these generations, in most of breeding lines showed fair amount of achievement in fixation of breeding lines (Table- 17 to Table-26). Four oval breeds, viz., SLD1, SLD2, SLD3, SLD4 and six dumbbell breeds, SLD5, SLD6, SLD7, SLD8, SLD9, SLD10, evolved from the breeding study were crossed in all possible combinations and forty-eight new hybrids obtained were subjected for hybrid evaluation. Multiple trait evaluation index employed on these forty-eight new hybrid combinations revealed that twenty combinations had scored >50 as mean index values measured on fourteen traits recorded (Table-29). They are, SLD 1 x SLD 5, SLD 1 x SLD 6, SLD 1 x SLD 8, SLD 1 x SLD 9, SLD 2 x SLD 6, SLD 2 x SLD 7, SLD 2 x SLD 8, SLD 2 x SLD 9, SLD 2 x SLD 10, SLD 3 x SLD 10, SLD 3 x SLD 10, SLD 4 x SLD 6, SLD 4 x SLD 7, SLD 4 x SLD 8, SLD 4 x SLD 9, SLD 4 x SLD 10, SLD 6 x SLD 4, SLD 8 x SLD 1, SLD 8 x SLD 2 and SLD 8 x SLD 4. The highest index score of was observed with SLD 4 x SLD 8, followed by with SLD 1 x SLD 8. The least index score of was observed with SLD 9 x SLD

17 The best performers are SLD 4 x SLD 8 and SLD 1 x SLD 8, wherein the compatibility expressed in most of the economically important traits in both straight as well as reciprocals are significantly higher. Further, it can well be derived that SLD 2 x SLD 8 and SLD 4 x SLD 6, were also, in the run for the potentially good combiners as evidenced by their fairly good expression of various economic traits. A concise double hybrid study, utilizing 10 new breeds, four oval and six dumbbell, with selective crossing resulting 21 foundation crosses, six oval crosses and eleven dumbbell crosses, were attempted. Analysis of performance on economic traits, through multiple trait evaluation index, resulted in six short-listed foundation crosses, three oval, viz., SLD1 x SLD2 (63.37), SLD2 x SLD4 (63.89), SLD3 x SLD4 (64.30) and three dumbbell SLD5 x SLD9 (62.74), SLD8 x SLD9 (63.27), SLD9 x SLD10 (62.81) foundation crosses (Table-32). Nine new double hybrids obtained in a selective crossing by avoiding reciprocals and repetitive crosses which were subjected for evaluation along with Krishnaraja (Table-33). Analysis of performance on various economic traits, through multiple trait evaluation index (Table-34), resulted in identification of one promising hybrid, (SLD2 x SLD4) x (SLD8 x SLD9). In the assessment of manifestation of heterosis with 48 new single hybrids studied on various economic parameters, the mean values of heterosis over mid parents, indicated positive heterosis on the traits of fecundity (3.15), pupation rate (0.30), yield per larvae (21.60), cocoon weight (16.43), shell weight (18.70), shell percentage (1.64) and reelability (3.65) (Table-35). The magnitude of such heterosis was more pronounced in yield per larvae (21.60), cocoon weight (16.43) and shell weight (1.64), while it was less in other traits. Further, a negative heterosis was observed in larval duration (-1.41) which is desirable but in raw silk (- 1.75) and neatness (-0.15) which are not desirable. In the selected hybrid combinations, SLD4 x SLD8, SLD1 x SLD8, SLD2 x SLD8, SLD4 x SLD6, SLD 4 x SLD9, the extent of manifestation of heterosis was not found to be high in most of the individual traits indicating that the genetic distance of parental breeds involved in these crosses were not as high as in some other combinations wherein maximum heterosis had manifested. But it also, indicates that having attained desirable heterosis in positive as well as negative traits, the genetic distance existing among 181

18 these breeds were optimum and the difference was not so wide but belongs to more or less in nearby spheres. In the assessment of manifestation of heterobeltiosis with 48 new single hybrids studied on various economic parameters, the mean values of heterobeltiosis (Table-36), indicated positive and desirable heterobeltiosis only on the traits of yield per larvae (19.77), cocoon weight (13.91), shell weight (14.39), reelability (2.39) and larval duration (0.21), while for remaining traits such as fecundity (-1.44), pupation rate (0.79), shell percentage (-0.32), filament length ), raw silk (- 4.80), neatness (-0.59), renditta (5.29) and filament size (22.93), only negative and undesirable heterobeltiosis was observed. However, the manifestation of heteobeltiosis in the top ranking candidates evaluated through multiple trait index, viz, SLD4 x SLD8, SLD1 x SLD8, SLD2 x SLD8, SLD4 x SLD6, SLD 4 x SLD9, indicates the existence of epistasis or over-dominance in many traits and argues well for finding placement in selection. In the assessment studies of manifestation of heterosis with nine new double hybrids studied on various economic parameters, the mean values of heterosis over mid parents (Table-37), indicated positive and desirable heterosis only on the traits of fecundity ( 6.48) and cocoon weight (3.60), while for the traits of pupation rate (- 1.86), yield per larvae (-1.02), shell weight (-0.87), shell percentage (-7.92), filament length (-7.92), filament size (12.35), raw silk (-3.84) and reelability (-3.11), neatness (-1.87) negative or undesirable heterosis was observed. However, in case of the double hybrid, (SLD2 x SLD4) x (SLD8 x SLD9), elected by employing multiple trait index, the manifestation of heterosis indicates that the genetic distance between the foundation parents was sufficiently enough to bring in fair amount of heterosis. In the assessment of manifestation of heterobeltiosis with nine new double hybrids studied on various economic parameters, the mean values of heterobeltiosis (Table-38), indicated positive and desirable heterobeltiosis only on the traits of fecundity (1.95) and cocoon weight (2.88), while for the traits of pupation rate (-2.44), yield per larvae (-2.00), shell weight (-2.10), shell percentage (-5.14), filament length (-9.21), raw silk (-4.52), filament size (13.19), reelability (-3.39), negative or undesirable heterobeltiosis was observed. In the double hybrid, (SLD2 x SLD4) x (SLD8 x SLD9), the manifestation of heterobeltiosis indicates the existence of 182

19 genetic distance though it was not pronounced but minimal, among the foundation parents. Among new oval breeds, SLD 4 exhibited pronounced and desirable improvement over the control, CSR2 in many traits (Table-39), viz., fecundity (8.79), pupation rate (4.28), yield per larvae (2.36), shell percentage (0.83), filament length (12.48), raw silk (5.79), reelability (4.87), larval duration (-4.21), silk productivity per day (11.78), while other three oval breeds exhibited such improvement in less number of traits only. Among new dumbbell breeds, SLD8 exhibited pronounced and desirable improvement over the control, CSR4 in many traits (Table-40) in varying magnitude, viz, fecundity (7.22), pupation rate (3.91), yield per larvae (1.73), cocoon weight (0.80), shell weight (6.91), shell percentage (5.48), filament length (15.07), raw silk (8.41), silk productivity per day (26.41), while other four dumbbell breeds exhibited such improvement in fewer number of traits only. New single hybrid, SLD4 x SLD8 exhibited desirable improvement over the control, CSR2 x CSR4, in many traits (Table-41) studied in varying magnitude, viz., fecundity (4.41), larval duration (-4.47), pupation rate (1.26), yield per larvae, shell weight (5.35), shell percentage (3.29), raw silk (8.29), silk productivity per day (19.46). New double hybrid, (SLD2 x SLD4) x (SLD8 x SLD9) exhibited desirable improvement over the control, (CSR2 x CSR27) x (CSR6 x CSR26), in many traits (Table-42) studied in varying magnitude, viz., fecundity (3.88), larval duration (-7.66), pupation rate (5.54), yield per larvae (10.39), shell weight (11.32), shell percentage (3.26), filament length (6.63), raw silk (4.40). The silk productivity estimates as suggested by Gamo, (1976) was calculated for all 48 new single hybrids (Table-43) confirms the superiority of promising hybrids identified through evaluation index method to a larger extent and mean values revealed that forty three candidates (except five) in the study were in possession of better values than control hybrid, CSR2 x CSR4. The silk productivity per day in selected double hybrid, (SLD2 x SLD4) x (SLD8 x SLD9) (7.59) was higher, when compared to the control hybrid, (CSR2 x CSR27) x (CSR6 x CSR26) (6.38). Further, the mean on silk productivity per day of 183

20 nine double hybrids (6.38) indicates the new hybrids are possessing higher productivity compared to the control, (CSR2 x CSR27) x (CSR6 x CSR26). Measurement of cocoon uniformity in forty-eight new single hybrids (Table-45) indicates placements for many promising hybrids in accordance with set standards of size deviation in terms of SD (Standard Deviation) and CV (Co-efficient of Variation). Selected hybrid, SLD 4 x SLD8, falls well within the optimum limits, standard deviation in cocoon index (6.28) and co-efficient of variation (3.39). In other promising hybrids, SLD1 x SLD8, the SD (7.18) and CV (3.82), SLD2 x SLD8, the SD (7.46) and CV (4.32), SLD 4 x SLD6, the SD (7.76) and CV (3.99), SLD 4 x SLD9, the SD (8.20) and CV (4.54) indicates their adherence to the standard limits. Similar measurement of cocoon uniformity in nine new double hybrids were recorded (Table-46) and in case of new double hybrid, (SLD2 x SLD4) x (SLD8 x SLD9), the standard deviation in cocoon index (7.76) and the co-efficient of variation (3.99) confirms the adherence of set norms. Studies on grainage parameters such as egg yield per kg cocoons, egg number per gram and fecundity conversion of three oval, SLD1, SLD2, SLD4 indicates (Table-47) that SLD4 exhibits better yield potential, viz., egg yield per kg of cocoons (80.230) and fecundity (552) than other breeds over control breeds, CSR2 and in case of three dumbbell breeds, SLD6, SLD8, SLD9, and SLD8 exhibits better yield potential, viz., egg yield per kg of cocoons (77.280) and fecundity (514) than other breeds over control breeds CSR4 and values were well within set standards. Similar studies on grainage parameters such as egg yield per kg cocoons, egg number per gram, and fecundity conversion of three oval foundation crosses, SLD1x SLD2, SLD2 x SLD4, SLD3 x SLD4 indicates (Table-48) that SLD2 x SLD4 exhibits better yield potential, viz., egg yield per kg of cocoons (82.218) and fecundity (596) than other breeds over control, CSR2 x CSR27 and among three dumbbell breeds, SLD5 x SLD9, SLD8 x SLD9, SLD9x SLD10 indicates that SLD8 x SLD9 exhibits better yield potential, viz, egg yield per kg of cocoons (78.340) and fecundity (547) than other breeds over control, CSR6 x CSR26 and values were well within set standards. 184