Abstract: Heterosis and combining ability studies of the isolated variable multivoltine races and developed bivoltine races with high yield capacity and their eighteen combinations were evaluated in two commercial rearing seasons in Bangladesh. Appropriate statistical methods were employed to interpret data on the hybrid performance, general and specific combining ability of the hybrids. Analysis of variance for combining ability revealed larger mean squares. Here high δ2gca/δ2sca values for most of the characters reflected predominant additive gene action. Similarly, the variance of components of specific combining ability (δ2sca) of multivoltine×bivoltine were higher than those of general combining ability (δ2gca) for most of the characters in both the seasons indicating preponderance of non-additive gene action inheritance of all these traits. Among the combinations BSR-BN(B1)×BSRB-BB1 and BSR-3×BSRB-BB2 exhibited highly productive performance which could be recommended for commercial cocoon production in Bangladesh.
INTRODUCTION
In recent times among the various genetic principles and breeding techniques, utilization of heterosis is one of the major breakthroughs which contributed to the success of modern silkworm breeding. Hybrids obtained from genetically diverse inbred lines are usually vigorous, productive and sturdier than either the parents. As a result, introduction of hybrids, instead of pure strains is the target to increase production. As a result, preparation of suitable hybrids, selection of parents and knowledge on the nature as well as magnitude of gene action is of paramount importance to the breeders to have a sound breeding program. Combining ability has been used for the assessment of relative breeding potential of parental materials along with the genetic components involved in the inheritance of commercially important traits of B. mori (Kempthorne, 1957; Strunnikov, 1986; Subba Rao and Sahai, 1989; Satenahalli et al., 1989a, b, 1990a, b; Sarkar et al., 1991). In Bangladesh heterosis breeding in silkworm has not been properly exploited due to inadequate information on the magnitude of heterosis of quantitative traits (Mahfuzar et al., 1992; Rahman et al., 1996; Rahman and Jahan, 1997; Ali and Kamal, 1998; Haque et al., 2007). In the present study, an attempt was made to find out the magnitude of heterosis and combining ability in silkworm for characters contributing to yield.
MATERIALS AND METHODS
Materials: A line×tester crossing design was followed where 6 lines of multivoltine (BSR-BN(M)-P1, BSR-BN(B1)-P2, BSR-BN(P)-P3, BSR-IDC-P4, BSR-3-P5, BSR-10-P6 and 3 testers of bivoltine (BSRB-BB1-P7, BSRB-BB2-P8 and BSRB-03(sl)-P9) silkworm B. mori L. were utilized for the experiment.
Methods: A total of 27 hybrid combinations and parents were developed through line×tester analysis technique (Kempthorne, 1957). The experiment was conducted in three replications in the two very important commercial rearing seasons viz. Agrahyoni (October-November’12-S1) and Chaita (February-March’13-S2). The experimental rearing was conducted in a randomized block design and standard rearing techniques suggested by Krishnaswami (1978) were followed. Cocoons were harvested as per the schedule without any interruption. Observations on single cocoon weight, shell weight and Shell Ratio percentage (SR%) were carefully recorded. General and Specific Combining Ability (GCA and SCA) and their variations were calculated according to Kempthorne (1957). Heterosis over both mid-parent (MP) and better-parent (BP) were calculated for all the 18 crosses in both the seasons by using the following equation:
The combining ability analysis was based on the procedure developed by Kempthorne (1957). In the first step, analysis of variance for different seasons were estimated to test the significance of differences among the genotypes, including crosses and parents (treatment). The treatment SS was further partitioned into various components like parents, crosses and parent vs. crosses. These components were calculated as follows:
and:
| Cij | = | Observation i×jth cross |
| Pij | = | Observation for the parents |
| r | = | No. of replications |
The SS (parents vs. crosses) was calculated by a generalized formula, as follows:
| n | = | No. of observations added in a sum |
| c | = | Coefficient of contrast |
| a | = | No. of groups |
| y | = | Sum of entries in a contrasted set |
Different components for line×tester ANOVA were obtained from the pooled data on line×tester. The estimates of general and specific combining abilities were calculated using the following equation:
and:
| t | = | No. of testers |
| l | = | No. of lines |
| r | = | No. of replications |
General and Specific Combining Ability (GCA and SCA) were calculated as:
and:
| MSe | = | Error mean sum of square |
RESULTS AND DISCUSSION
Results revealed that for cocoon weight and shell weight significant positive heterosis was expressed over better-parent in S1, where for Shell Ratio percentage (SR%) only three crosses showed significant positive values (Table 1). The percentage of heterosis for single cocoon weight and shell weight over their respective better-parents ranged from 5.38-31.79 and 4.65-24.05 in S1 while for SR%, the value ranged from 2.89-8.26 in the same season. Similarly, 17 crosses for single cocoon weight, 14 crosses for shell weight and none for SR%, exhibited significant positive values in S2 season. The percentage of heterosis for single cocoon weight and shell weight over the same parent, ranged from 5.08-19.75 and 2.88-12.25.
| Table 1: | Heterosis in percentage over better-parent and mid-parent for three economic parameters in silkworm Bombyx mori L |
| SR: Shell ratio, *,**Significant at 5 and 1% level, respectively | |
| Table 2: | Analysis of variance for combining ability of cocoon characters in silkworm Bombyx mori L. |
| *,**Significant at 5 and 1% level, respectively, tested against the error mean square, SR: Shell ratio, df: Degrees of freedom | |
Results indicated that almost all the crosses for single cocoon weight, shell weight and SR% showed significant positive heterosis over their mid-parents in both S1 and S2 seasons. The percentage of heterosis over their mid-parents ranged from 10.89-38.61, 7.35-33.15 and 2.43-12.84 in S1 and 10.39-29.69, 10.57-28.35 and 6.31-12.32 in S2 season, respectively for single cocoon weight, shell weight and SR% (Table 1).
The analysis of variance for combining ability (Table 2) revealed that the mean squares due to genotypes was highly significant, indicating much diversity among the present population. The significant results due to parent and crosses in all the characters exhibited the presence of sufficient diversity within parents and their hybrids. The lines and testers for all the characters, in both the seasons expressed high significance. Similarly, the other sources like parent vs. crosses and line×testers also exhibited high significance in both the seasons, indicating heterotic response.
Variance due to specific combining abilities for single cocoon weight, shell weight in both the seasons for all the characters highlighted predominant roles of non-additive gene action. Experimental data further expressed very high δgca/δsca in both the seasons, clearly indicating preponderance of additive gene action (Table 3).
The present investigation further indicated that the BSR-BN (B1) among all the lines showed significant positive GCA in both the rearing seasons for all the economic characters under consideration (Table 4) while BSR-BN(M) for cocoon weight and shell weight in S2 followed by BSR-10 for SR% in S1 were significant. Similarly, among the testers BSRB-BB2 expressed significant positive GCA in both the seasons for cocoon and shell weight as well as for SR% in S2 season, followed by similar trend in BSRB-BB1 for SR% in S2.
| Table 3: | Components of variance for different characters in two rearing seasons |
| SR: Shell ratio | |
| Table 4: | Effect of general combining ability (GCA) of parents for three parameters in multivoltine×bivoltine silkworm, B. mori L. |
| SR: Shell ratio, *,**Significant at 5 and 1% level, respectively | |
The experimental results on Specific Combining Ability (SCA) exhibited both positive and negative effects in different characters where three crosses resulted in significant positive values in most of the characters in both seasons (Table 5) while 2 crosses showed significant positive values for two characters in the same S1 season and the other for two characters in two separate seasons. Similarly, another 3 crosses exhibited significant positive values for single character, only in one season. Among the most encouraging combinations, only one cross (P2×P7) showed significant positive values for all the characters in both the seasons, followed by 3 crosses (P1×P7, P5×P8 and P6×P8) which exhibited positive values for the same two characters like cocoon and shell weight in both the seasons.
Heterosis has been exploited by crossing the inbred lines of genetic diversity, geographical origin and voltinism in silkworm, B. mori L. In a hybridization programme, identification of parents is a prerequisite for maximum exploitation of heterosis. To achieve this goal, it is necessary to understand the combining ability of parents, which help in the selection of suitable parents for hybridization. This is because, high yielding parents may not necessarily transfer their superiority to the progenies in the hybrid crosses. Selection of suitable parents and information on nature and magnitude on gene action of traits of economic importance determine the success of any crop improvement programme.
Under the multivoltine×bivoltine experiment, heterosis has been observed in a series of crosses of B. mori L. for different characters. Their range of heterosis varied from season to season. In this study a reasonable amount of heterosis was seen in their respective crosses, where BSR-BN (B1) followed by BSR-BN (M) and BSR-10 were involved as lines and BB1 followed by BB2 as testers. Similar findings have been reported by many silkworm breeders (Subba Rao and Sahai, 1989; Satenahalli et al., 1989a, b; 1990a, b). Experimental results further indicated that, the high δ2gca/δ2sca values in S1 for most of the characters showed predominantly an additive gene action, while the variance of component of specific combining ability (δsca) of multivoltine×bivoltine were higher than that of general combining ability (δgca) for most of the characters in S1 and S2 expressing preponderance of non-additive gene action in inheritance of all these traits. Non-additive gene actions governing the expression of some quantitative traits have been reported by some breeders in B. mori (Mahfuzar et al., 1992; Rahman et al., 1996).
| Table 5: | Specific combining ability effects for three parameters in multivoltine×bivoltine silkworm, Bombyx mori L. |
| SR: Shell ratio, *,**Significant at 5 and 1% level, respectively | |
Additive and non-additive gene actions of cocoon yield contributing characters in B. mori L. have been observed by many silkworm breeders (Goel et al., 2010; Nagalakshmamma et al., 2010; Singh and Nirupama, 2012; Singh and Gangopadhyay, 2013). Predominant role of additive gene actions have been reported for cocoon weight (Islam et al., 2005; Haque et al., 2007), cocoon weight and shell weight (Mahfuzar et al., 1992), cocoon weight, shell weight and SR% (Rahman and Jahan, 1997; Ali and Kamal, 1998). Results clearly indicate that the combinations BSR-BN (B1)×BSRB-BB1 and BSR-3×BSRB-BB2 for all the characters, followed by BSR-BN (M)×BSRB-BB1 and BSR-10×BSRB-BB2 for two characters in both the seasons exhibited encouraging results that may be released for further field trials. On satisfactory performance in the field, these combinations may be utilized for commercial cocoon production to increase the productivity of silk in Bangladesh.