Abstract: The genetic manifestation of hybrid vigor among newly developed silkworm hybrid combinations over the parents was analyzed for the identification of superior cross breeds. Ten homozygous inbred polyvoltine breeds as Lines viz., APMG1, APMG2, APMG3, APMG4, APMW1, APMW2, APMW3, APMW4, APMW5 and APMW6 and three bivoltine breeds as Tester viz., APS8, APS12 and APS45 were used for the study. Adopting the LinexTester method, thirty hybrid combinations were prepared and reared at standard conditions. The data was measured on the nine important genetic traits viz., fecundity, yield per 10,000 larvae, pupation%, cocoon weight, shell weight, shell ratio%, filament length, reliability and neatness%. The data was analyzed for their Mid Parent Heterosis (MPH) and Better Parent Heterosis (BPH), six hybrid combinations viz., APMG1xAPS8, APMG1xAPS45, APMG3xAPS12, APMW1xAPS8, APMW2xAPS8 and APMW4xAPS45 were shown as significant heterotic combinations over mid parents for all the economical traits studied. The hybrid combination, APMW2xAPS45 with seven traits and APMG1xAPS8, APMG1xAPS12, APMG3xAPS12, APMW1xAPS8 and APMW1xAPS8 exhibited positive heterosis for six out of nine traits over better parent heterosis. Further, based on the evaluation index the study sturdily demonstrate that two new hybrid combinations viz., APMW1xAPS8 (59.58) and APMG1xAPS8 (58.68) were adjudicated as superior heterotic hybrid combinations and recommended for large scale laboratory trial.
INTRODUCTION
The silkworm, Bombyx mori L. is an important economic insect and also a tool to convert mulberry leaf protein into silk. Industrial and commercial use of silk, the historical and economic importance of production and its application in all over the world finely contributed to the silkworm promotion as a powerful laboratory model for the basic research in biology (Ramesh-Babu et al., 2009). The success of silkworm breeding depends on the ability of the breeder to assemble and recombine the genetic variability to isolate the potential combiner from the genetic resource material based on the expression of the various qualitative and quantitative traits over generations. In silkworm, majority of the characters that contribute to the yield of silk are under the control of polygenic nature. Developing of potential hybrid required for the field has become a very difficult task to silkworm breeders. In spite of continuous efforts for the development of sericulture through various conventional silkworm breeding programs; still there is a demand for productive superior hybrids to fulfill the needs of sericulture industry. In consideration of the crop stability and adaptability to fluctuating environmental conditions, development of productively and qualitatively superior cross breed varieties is necessary.
The purpose of hybrid preparation is to produce a heterotic effect rather than to provide genetic variation and also to provide the productive hybrid for commercial exploitation. In the tropical countries like Indian sericulture, the hybrid comprises female of polyvoltine with male of bivoltine are successfully exploited commercially as a cross breed. As a result, nearly 90% of the total silk produced is derived from the polyvoltine cross breeds (polyvoltinexbivoltine) in India (Umadevi et al., 2005). In this context, some of the silkworm breeders have made successful attempts in the identification of new productively superior cross breed varieties (Datta, 1984; Nagaraju et al., 1996; Rao et al., 2004; Lakshmi et al., 2008). So, there is an immediate need to identify productively superior silkworm hybrids for reliable crops and for sustainability of sericulture industry in the country.
The various attempts were made earlier by the silkworm breeders in manifestation of hybrid vigor by adopting the combining ability studies or LinexTester analysis methods (Bhargava et al., 1993; Datta et al., 2001; Rao et al., 2004). The silkworm breeder has to give due consideration on the performance of all quantitative and qualitative parameters of hybrid combinations while evaluating the silkworm hybrids for its commercial exploitation. Keeping the objectives in view, the present study was aimed to identify the potential cross breeds (polyxbivoltine) based on their performance, heterosis and evaluation index methods.
MATERIALS AND METHODS
Parental Silkworm Breeds
For the present study ten polyvoltine breeds as Line viz., APMG1, APMG2,
APMG3, APMG4, APMW1, APMW2, APMW3, APMW4, APMW5, APMW6 and three bivoltine breeds
as Tester viz., APS8, APS12 and APS45 were drawn and the experiment was
carried out during January, 2007 to February, 2008 in the Silkworm Breeding
and Molecular Genetics Laboratory, Andhra Pradesh State Sericulture Research
and Development Institute (APSSRDI), Hindupur, India. By crossing the polyvoltine
female and bivoltine male parents, thirty silkworm hybrid combinations were
prepared. The mother moth examination for hybrid as well as parental layings
was carried out to confirm the pebrine free infection.
Silkworm Rearing
The disease free layings of parents and hybrid combinations were incubated
in a well disinfected rearing house after surface disinfection with the 2% formalin
solution. After hatching, the larvae were brushed on the freshly chopped mulberry
leaf and reared under standard rearing conditions. The chawkie silkworm larvae
(young silkworm larvae up to 3rd instar) were reared at the temperature of 26-28°C
with a Relative Humidity (RH) of 85-90%. After resuming from the 3rd moult,
300 larvae were retained in each bed with three replications for all hybrid
combinations and parental breeds. The late age rearing was maintained at 24-26°C
with a relative humidity of 65-75% as suggested by Datta (1992).
The data pertaining to the nine important genetic traits viz., fecundity, yield
per 10,000 larvae, pupation%, cocoon weight, shell weight, shell ratio%, filament
length, reliability and neatness% were pooled and analyzed to asses the hybrid
performance. The analysis of hybrid combinations was carried out on the mean
values of parental breeds with their hybrid combinations. The genetic manifestation
of hybrid vigor was carried out as percentage of increase among hybrid combinations
(F1) over the mid parent and better parent performance on the genetic
traits with assistance of statistical analysis.
Statistical Methods Adopted for Hybrid Vigor Manifestation
Mid Parent Heterosis (MPH) and Better Parent Heterosis (BPH) are calibrated
as per the procedure adopted by Bhargava et al. (1993).
The percent of MPH and BPH with respect to a particular trait was calculated
as below:
Mid parent heterosis (MPH) = 100 (A-B)/B
Better parent heterosis (BPH) = 100 (A-C)/C
Where:
| A | = | Actual performance of the hybrid |
| B | = | Mean performance of the female and male parents |
| C | = | Performance of better parent |
Multiple Evaluation Index
The promising hybrid combinations were identified based on the average values
of multiple Evaluation Index (EI) method (Mano et al.,
1993). The hybrid combinations were adjudicated as promising based on the
average values obtained for the genetic traits on multiple evaluation index
values were calculated with the assistance of the following formula.
Where:
| A | = | Value obtained for a trait for the hybrid |
| B | = | Overall mean of particular trait |
| C | = | Standard deviation |
| 10 | = | Standard unit |
| 50 | = | Fixed value |
RESULTS AND DISCUSSION
Performance of the Polyvoltine (Line)
The rearing performance on the nine genetical traits for parental breeds
pertaining to the ten polyvoltine (Line) and three bivoltine (Tester), utilized
for the development of superior cross breed. Among the polyvoltine parents,
highest fecundity (number of eggs per brood) recorded for the APMW5 (501) and
lowest was in APMW2 (478) with the average of 489 eggs per brood. The cocoon
yield per 10,000 larvae by weight varied between 12.699 kg (APMW6) to 14.000
kg (APMW3) with an average yield of 13.539 kg. The average pupation rate recorded
was 93.53% with the maximum of 95.33% (APMW1) and minimum of 88.90% (APMW6).
Average of 1.418 g was recorded for single cocoon weight with the highest of
1.445 g (APMW3) and lowest of 1.395 g (APMW4). With regard to shell weight,
the average was recorded 0.239 g with the maximum of 0.255 g (APMW3) and minimum
of 0.228 g (APMW1). Maximum filament length was recorded in APMW3 (825 mts.)
and minimum in APMW1 (711). The average reliability was observed 74.1% with
the highest of 77% (APMG3) and lowest of 71% (APMW1). Highest neatness was recorded
in APMG4 (82%) and lowest in APMW5 (70%) with an average of 77% (Table
1).
Performance of the Bivoltine (Tester)
Among the bivoltine testers, the highest fecundity was recorded in APS8
(511) and lowest in APS45 (499). For the trait cocoon yield per 10,000 larvae
by weight, ranged between 16.954 kg (APS45) to 17.854 kg (APS8) was revealed.
The average pupation rate was recorded 87.17%. The highest single cocoon weight
was recorded in APS8 (1.885 g) followed by APS12 (1.879 g) and APS45 (1.865).
With regard to shell weight, the average was recorded 0.363 g with the maximum
of 0.368 g (APS12) and minimum of 0.358 g (APS45). Maximum filament length in
APS12 (904 mts.) and minimum in APS45 (886 mts.) was recorded. The highest neatness
in APS8 (89%) and lowest in APS45 (87%) was disclosed (Table 1).
Performance of the Hybrid Combinations
The silkworm rearing performance on economical traits among new silkworm
hybrid combinations, number of eggs per brood ranged between 451 (APMG4xAPS12)
to 521(APMG3xAPS45) with an average of 489 (Table 2). With
regard to cocoon weight per 10,000 larvae by weight varied between 15.130 kg
(APMG2xAPS12) to 19.483 kg (APMW2xAPS45). The pupation rate differs from 90.00
(APMW3xAPS12) to 95.84% (APMW6xAPS12) with an average of 93.89%. The highest
single cocoon weight was observed in APMW1xAPS8 (2.004 g) and lowest in APMG4xAPS8
(1.645 g). Average shell ratio was observed 18.95% with the highest of 20.37%
in APMG4xAPS12 and lowest of 17.42% in APMG4xAPS45. The maximum filament length
was observed in APMG3xAPS12 (978 mts.) and minimum in APMG2xAPS8 (745 mts.).
Manifestation of Hybrid Vigor
Six hybrids viz., APMG1xAPS8, APMG1xAPS45, APMG3 xAPS12, APMW1xAPS8, APMW2xAPS8
and APMW4xAPS45 were established as good heterotic combinations with significant
hybrid vigor over mid parents for all the economic characters studied. High
heterotic effect for fecundity (5.57%) was shown by APMG3xAPS45 followed by
APMW2xAPS8 (5.36).
| Table 1: | Mean rearing performance on the genetic traits for the lines and testers |
| Table 2: | Mean rearing performance on the genetic traits of new silkworm hybrid combinations |
The significant hybrid vigor was found to exhibit in APMW2xAPS45 (26.59%) for the cocoon yield per 10,000 larvae followed by APMW4xAPS45 (25.94%), APMG4xAPS45 (22.81%). Five hybrid combinations were found to exhibit negative heterosis for yield. For the trait pupation, all the hybrid combinations showed positive heterosis with maximum of 8.36% in APMW6xAPS12, followed by APMW6xAPS45 (8.16%). Maximum heterosis of 22.05% was observed for the single cocoon weight in APMW1xAPS8 followed by APMW2xAPS45 (22.08%). The highest of 31.86% for shell weight was observed in APMW1xAPS8 followed by APMW2xAPS45 (24.71%) and in all the hybrids positive heterosis was observed. Maximum of 19.60% heterosis was observed in APMW1xAPS45 for the filament length. With regard to reliability maximum of 19.50% was observed in APMG2xAPS8 and for the neat ness maximum of 9.64% (APMG3xAPS8) heterosis was observed over mid parent (Table 3).
The hybrid combination, APMW2xAPS45 was exhibited significant hybrid vigor over better parent for seven out of nine characters studied (Table 4). Some of the hybrids viz., APMG1xAPS8, APMG1xAPS12, APMG3xAPS12, APMW1xAPS8 and APMW1xAPS8 exhibited positive heterosis for 9 out of 9 traits over better parent heterosis. Maximum significant hybrid vigor over better parent was found to be exhibit in APMG3xAPS45 (4.41%) for the fecundity, yield per 10,000 larvae by weight in APMW2xAPS45 (14.92), pupation rate in APMW6xAPS12 (7.81), single cocoon weight in APMW2xAPS45 (7.08%), shell weight in APMW1xAPS8 (7.46%), shell ratio in APMG4xAPS8 (5.41%), filament length in APMG3xAPS12 (8.19%), reliability in APMG2xAPS8 (13.10%) and neatness in APMG3xAPS8 (2.25%).
| Table 3: | Manifestation of hybrid vigor in the new hybrid combinations over the mid parent |
| Table 4: | Manifestation of hybrid vigor in the new hybrid combinations over the better parent |
| Table 5: | Evaluation index values on the genetic traits for the new hybrid combinations |
Multiple Evaluation Index Values
With an objective for identification of the superior hybrid combinations
based on Evaluation index values were calculated for the each genetic trait
and presented in the Table 5. Among the hybrids evaluated,
13 combinations were scored more than 50 evaluation index value (Table
6, Fig. 1). The top ranked hybrid combinations based on
the average evaluation index values viz., APMW1xAPS8 (59.58) and APMG1xAPS8
(58.68) were identified for further study (Fig. 2, 3).
Heterosis, the function of various gene frequencies, over dominance observed to be highly variable and basically it depends on the characters as well as parental strains utilized in the hybridization programs (Falconer, 1988). Hybrid vigor is very important in silkworms breeding (Toyama, 1906; Harada, 1961) and it has been successfully utilized at commercial level all over the world. Majority of the genetic traits under the control of polygenic nature and influenced by environment in goats as revealed by Singh et al. (2009) and silkworm is not exceptional (Gokulamma and Reddy, 2005). In the present study, the hybrid vigor was observed over Mid Parent Heterosis (MPH) and Better Parent Hetrerosis (BPH) in many crosses involving polyvoltine x bivoltine breeds might be due to the complementary gene action of the parents. Highest heterosis was observed for cocoon yield by weight (26.59%) and shell weight (31.86%) revealed the magnitude of genetic diversity of the parental material and the predominance of the complimentary type of gene action in the parents is in conformity with the observations of Sengupta et al. (1971). Genetically, hybrid vigor is manifested high in single cross hybrids as compared to three way and double cross hybrids and results obtained was corroborate with the earlier studies of Watanabe (1961) and Yokoyama (1963).
| Table 6: | Average EI values of the hybrid combinations |
| Fig. 1: | Evaluation of hybrid combinations |
The present study clearly showed heterosis for many yield contributing genetic characters but no single hybrid combination found to be positive heterosis for all the economical traits are in agreement with the observations of Datta et al. (2001).
| Fig. 2: | Silkworm larvae and cocoons of APMW1xAPS8 hybrid combination |
| Fig. 3: | Silkworm larvae and cocoons of APMG1xAPS8 hybrid combination |
The high degree of heterosis in specific crosses for some characters in this study may be due to additive gene effects (Udupa and Gowda, 1988; Rao et al., 2004, 2006). Expression of hybrid vigor was very high in some economic characters like cocoon yield, cocoon weight and shell weight. In the present observation, thirteen hybrid combinations manifested heterosis over mid parent for fecundity. Majority of the hybrid combinations were manifested positive heterosis over the mid and better parent for cocoon yield, cocoon weight and shell weight (Table 3 and 4). Further, it is corroborate with the earlier studies for the evaluation and identification of prospective polyvoltinexbivoltine hybrids/cross breeds (Singh et al., 1998, 2000; Rao et al., 2004; Umadevi et al., 2005; Ramesha et al., 2008).
In the present study of silkworm hybrid evaluation, we targeted certain quantitative as well as qualitative traits that contribute to the better performance of the breed/hybrids. Based on the expression of heterosis over mid parent and better parent for different important economical characters, they could be utilized for improvement of specific characters in specific hybridization programs.
CONCLUSION
Among the newly developed thirty silkworm hybrid combinations evaluated in the present study, two hybrids viz., APMW1xAPS8 and APMG1xAPS8 were adjudicated as superior heterotic hybrid combinations based on genetic manifestation of hybrid vigor studies and average multiple evaluation index values. These hybrid combinations are recommended for large scale laboratory trials and further for commercial exploitation at the farmer’s level.
ACKNOWLEDGMENTS
The authors wish to thank the Former Director and colleagues of the Moriculture Division as well as other staff of APSSRDI, Kirikera for their encouraging support in the present investigation and preparation of the manuscript.
ANNEX
Systematic classification of domesticated silkworm, Bombyx mori L.