Evaluation and Identification of Superior Polyvoltine Crossbreeds of Mulberry Silkworm, Bombyx mori L.
In the present study, the existing polyvoltine germplasm resource material of the Silkworm Breeding and Research and Development Institute (APSSRDI), screened for the desired qualitative and quantitative traits. After fixation of the desired traits, 5 inbred parental breeds (APM4, APM6, APM8, APM10, APM12) utilized as Lines for the preparation of 25 crosses in LinexTester method with the five bivoltine breeds (SDD1, SDD2, SDD3, APS12, APS105) as Testers. The hybrid testing was conducted and assessed for three different seasons for their performance on eight important economical genetic traits. The data obtained on the traits such as fecundity, yield per 10,000 larvae by number, single cocoon weight, shell weight, shell ratio, filament length and reliability was analyzed with the assistance of statistical tools. Based on two popular evaluation methods such as multiple traits Evaluation Index (EI) and Sub-ordinate Function (SF) methods, the 5 new hybrid combinations (APM12xAPDR105, APM6xAPS12, APM4xPDR105, APM10xSDD1 and APM10xSDD3) shown above 50 EI values with SF values varied from 5.663 to 7.596 were identified as superior over the control hybrid and recommended for large scale in laboratory trail. Further, based on the lab and field performance promising crossbreed will be identified and adjudicated for the commercial exploitation at farmers level.
The silkworm breeds plays an imperative role to make sericulture as a successful occupation through sustainable cocoon yield with superior quality silk. Before the introduction of hybrids, pure lines were used for the production of silk. Since, the pure lines were giving lower yield and poor quality of silk, hybrids were brought into field for commercial purpose. For the ancient times, silkworm hybrids are playing significant role in tropical sericulture industry (Chandrashekharaiah and Babu, 2003; Choudhary and Ravindra Singh, 2007). Advantage of hybrid vigor has been exploited in American corn and Japanese scientists in silkworm, since almost a century ago for commercial utilization. Since, the hybrids heralded better performance than parents and exploitation of hybrid vigor was initiated as early as 1906 in silkworm breeding (Toyama, 1906). In tropical country like India, polyvoltine hybrids (polyvoltine x bivoltine) play important role in the production of silk.
Commercial utilization of hybrid vigor in the domesticated silkworm, heralded a new era in silkworm which substantially contributed to the increase of silk yield. Through conventional breeding, many promising silkworm hybrids have been developed with improved quantitative and qualitative traits.
Hybrid vigor is an important tool in increasing cocoon production, evaluation, maintenance of inbred lines and identification of promising hybrids for commercial exploitation (Nagaraju et al., 1996). Based on the importance of hybrids requirement in the field, they were extensively developed and evaluated by the silkworm breeders. Some of them have survived for long time and few of them have stirred out with in few years of introduction in the field. The main challenge for the breeder is to prioritize the order of important characters for improvement in the resultant population. Besides, it is more important for the breeder to determine important factors responsible for the animal to survive and reproduce to its full potential (Nagaraju et al., 1989). The main objective of silkworm breeding is to synthesize new genotypes with more plasticity to different climates and to select sustainable silkworm hybrids for commercial exploitation. Scientists are making constant efforts in the synthesis of superior varieties to meet the demand of sericulture farmers and silk reelers.
In the changing scenario of the globe, for the developing countries, there is a great need to develop potential silkworm hybrids of higher quality and quantity to sustain the sericulture industry since, the polyvoltine or crossbreed silkworm are well adopted for tropical climate. India being a tropical country, several factors influences the success of cocoon crops of which biotic and abiotic factors play a vital role (Basavaraja et al., 2003). The present attempt has been made to identify superior polyvoltine hybrid (polyvoltinexbivoltine) with improved quantitative and qualitative genetic traits through conventional breeding approach.
MATERIALS AND METHODS
Silkworm Breeds and Hybrid Combinations
Five promising polyvoltine breeds viz., APM12, APM4, APM6, APM8, APM10 and
five bivoltine breeds viz., SDD1, SDD2, SDD3, APS12, APDR105 were drawn from
the germplasm bank maintained in Silkworm Breeding and Molecular Genetics Laboratory,
Andhra Pradesh State Sericulture Research and Development Institute (APSSRDI).
By utilizing female component of the polyvoltine breeds as Lines and bivoltine
breeds as Testers, 25 hybrid combinations (APM12xSDD1, APM12xSDD2, APM12xSDD3,
APM12xAPS12, APM12xAPDR105, APM4xSDD1, APM4xSDD2, APM4x SDD3, APM4xAPS12, APM4xAPDR105,
APM6xSDD1, APM6xSDD2, APM6xSDD3, APM6xAPS12, APM6xAPDR105, APM8x SDD1, APM8xSDD2,
APM8xSDD3, APM8xAPS12, APM8xAPDR105, APM10xSDD1, APM10xSDD2, APM10xSDD3, APM10xAPS12
and APM10xAPDR105) were prepared by adopting LinexTester method (Griffing, 1956).
The popular crossbreed, APM1xAPS8 was used as the control hybrid to compare
new hybrid combinations.
The layings of all the new hybrid combinations were incubated in a thoroughly
disinfected rearing house by maintaining 80-85% of relative humidity and 25°C
temperature. Uniformly developed layings were selected, packed and covered with
a black cloth to ensure uniform hatching. Newly hatched larvae were brushed
on finely chopped fresh mulberry leaves. These new hybrid combinations were
reread in three replications. The young silkworm larvae were reared at 26-28°C
with relative humidity of 85-90% and 28°C temperature. After 3rd moult,
300 larvae were retained from each bed for further rearing and assessment. The
late age silkworms were reared at 24-26°C with relative humidity of 70-75%.
The larvae were fed with mulberry leaves brought from well-irrigated garden
to the silkworm rearing house (Krishnaswami, 1978) during the year, 2007-2008
in Andhra Pradesh State Sericulture Research and Development Institute (APSSRDI),
Hindupur, Andhra Pradesh, India. The disease free rearing atmosphere was maintained
in the rearing house by using certain disinfectants like lime and bleaching
powder. After the larvae attained maturity, they were mounted on plastic collapsiblemountages
for spinning. Appropriate care was taken while mounting the worms for spinning
as per the standard rearing protocol. The data pertaining to commercial important
genetic traits viz., fecundity, cocoon yield/10,000 larvae by number, pupation
rate, single cocoon weight, single shell weight, cocoon shell ratio were measured
and analyzed. Further, important post cocoon parameters like single cocoon filament
length and relabeled were estimated. The brief description of the genetic traits
which are evaluated is given below.
The trait represents the number of eggs laid by a single moth and is important
as it is directly associated with fitness of the individual. Fitness of individuals
and of the population is often expressed in terms of reproductive success.
Cocoon Yield per 10,000 Larvae by Number|
It is the total weight of live cocoons expressed in kilogram for unit number
of larvae retained after 3rd moult and it is important as directly associated
with the economics of cocoon crops.
Pupation Rate (%)
It is the number of cocoons with live pupae recovered out of the number
of larvae retained after 3rd moult expressed in percentage.
Cocoon Weight (g)
The average weight of 25 male and 25 female cocoons taken randomly on 6/7th
day after onset of spinning and measured in gram.
Cocoon Shell Weight (g)
The average weight of 25 male and 25 female cocoon shells of same cocoons
taken randomly and measured in gram.
Cocoon Shell Ratio (%)
The trait depicts the total content of shell available in the cocoons. It
is the average ratio of 25 male and female cocoon shells to the total cocoon
weight and assessed in percentage.
Silk Filament Length (m)
It represents the length of unwound silk filament from the cocoon which
is represented in meters.
It is the efficiency of reeling/unwinding of the silk from the total number
of cocoon and expressed in percentage.
|No. of reeling cocoon
||Testing cocoons No. of unreeled cocoons No. of carry over
|No. of feeding end
||Measurement of feeding end+No. of carry over cocoons No. of carry over
Multiple Evaluation Index Method
Evaluation index values were obtained for the each genetic trait with the
assistance of following formulae adopted by Mano et al. (1993).
Evaluation index (E I) = A-B/Cx10+50
||Mean of particular trait
||Overall mean of particular trait
Subordinate Function Method
For each genetic trait, subordinate function values are obtained with the
assistance of the following formula (Gower, 1971).
Xu = (Xi-X min)/(X max-X
|| Measurement of character of tested breed
|| Minimum value of the character among all the tested hybrids
||Maximum value of the character among all the tested hybrids
The evaluation index values were calculated for each of the trait and the mean value was taken into consideration. Values obtained on subordinate function method for each trait were added per each combination and ranked based on the cumulative values. The average cumulative value on the number of traits was analyzed and ranked the hybrid which possessing with highest value was ranked first and accordingly each rank was allotted to each hybrid combination.
RESULTS AND DISCUSSION
Fecundity was ranged between maximum of 529 (APM12xAPDR105) and minimum of
482 (APM4xAPDR105) control with 470 and Standard Deviation (SD) of 15 was observed.
Maximum yield/10,000 larvae by number was in APM12xAPDR105 (9511) where as minimum
in APM6xSDD1 (8733) was recorded. Higher pupation rate was recorded in APM10xSDD1
(94.89) lower was observed in APM6xSDD1 (86.44). The highest single cocoon weight
was observed in APM12xAPDR105 (1.952 g) and lowest in APM12xSDD2 (1.599 g) with
the standard deviation of 0.112 g among the hybrid combinations. With regarding
to cocoon shell ratio, maximum in APM6xAPS12 (19.95%) and minimum was found
in APM8xAPDR105 (17.29) with an average of 19.15%. Maximum filament length in
APM12xAPDR105 (936 m) where as minimum in APM6xSDD3 (709 m) with an average
of 786 m has been revealed. Highest relabeled percentage in APM12xAPDR105 (90.10)
and lowest was recorded in APM6xAPDR105 (76.90%) with an average of 86.23%.
Among all the hybrid combinations, for the trait single shell weight more CV
(7.94%), which was followed by filament length (7.52%) and single cocoon weight
(6.49%) (Table 1). It is established that most of the genetic
traits in silkworm are under polygenic control, under the influence of environmental
factors and nutrition as in other system (Miyagawa and Sato, 1954; Ueda and
Lizuka, 1962; Arai and Ito, 1963, 1967; Yokoyama, 1979).
||Silkworm performance of the new hybrid combinations on economically
important genetic traits
|C: Control; FL: Filament Length
Hence, all the hybrid combinations were reared in same environmental conditions,
fed with the same variety of leaves and important quantitative traits were measured
for the analysis of hybrid performance. When both the parental strains and hybrids
are raised in unfavorable environmental conditions, performance of hybrids will
be much superior to both the parental strains those used for the preparation
of the hybrid (Nagaraju et al., 1996). Majority of silkworm breeders
are especially interested in evolving breeds with productive merit by employing
either conventional or mutation breeding (Kovalov, 1970). Crossbreeds provide
great variability on which selection can be operated to isolate promising breed
with new gene complexes. The primary objective of the breeder is to isolate
robust silkworm hybrid with reasonably consistent levels of productivity, viability
with improved post cocoon parameters.
Therefore, the objective of silkworm breeding is not only to synthesize new genotypes or hybrid combinations but also to identify sustainable silkworm hybrids for commercial exploitation by farmers. Selection of suitable parents and information on nature and magnitude of gene action of traits of economic importance determine the success of any crop (Chouhan et al., 2000). Critical assessment of variability present in the breeding materials is one of the pre-requisites for paving the way of combining most of the desirable traits present in different genotypes into a single hybrid combination. However, the per se performance of parental breeds is not always be the good reflection of the combining ability and its analysis therefore helps the breeders to understand the nature of gene action to identify prospective parents/hybrids (Narayanaswamy et al., 2002). In this context, LinexTester studies assume significance so as to determine the nature of gene action involved in the expression of different economic traits besides identification of specific desirable combination (Choudhary and Ravindra Singh, 2007). Bhargava et al. (1995) recorded predominant role of non-additive gene action for the traits cocoon weight, cocoon shell weight, cocoon shell ratio, filament length and relabeled. Similar predominant role of non-additive gene action in the inheritance of cocoon yield by number, cocoon weight, cocoon shell weight, cocoon shell ratio and filament length was observed by Sudhakar Rao et al. (2001).
In the present study an attempt was being made to identify the superior crossbreed through conventional breeding approach by assessment on multiple traits of the developed silkworm hybrids as an important task in predicting the potential hybrid combinations. Different statistical methods are applied for the analysis of hybrid performance in both plants and animals (Hayman, 1954, 1960; Henderson, 1953, 1963, 1984; Gower, 1971; Arunachalam and Bandhyopadhyay, 1984). The comprehensive merit of the hybrid over a range of traits depends on relative superiority of many individual traits. Selection needs to be based on multiple trait analysis comprising of viable, quantitative and qualitative traits. In silkworm, good numbers of hybrids are evaluated and promising ones are selected based on the economic traits (Singh and Subba Rao, 1993). The present data was analyzed with equal weight to all the important economic traits using both multiple evaluation index (Mano et al., 1993) and subordinate function methods (Gower, 1971). These methods were successfully employed by many silkworm breeders for evaluation of the silkworm hybrids (Krishnaswami et al., 1964; Singh and Subba Rao, 1993; Sudhakar Rao et al., 2001; Ramesh Babu et al., 2002; Rao et al., 2006; Ramesha et al., 2008).
The multiple evaluation index values obtained for each of the trait on fecundity,
yield/10,000 larvae by number, pupation rate, cocoon weight, shell weight, shell
ratio, filament length and relabeled with an average value presented in Table
2. Average evaluation index
values ranged to the maximum of 63.63 (APM12xAPDR105) followed by 57.66 (APM6xAPS12),
56.79 (APM12xAPDR105) with minimum 41.76 (APM8xSDD3) among the hybrid combinations
where as control hybrid recorded 37.96 (APM1xAPS8).
||Multiple evaluation index values on economically important
genetic traits of the new hybrid combinations
|C: Control; FL: Filament length; EI: Evaluation Index
Based on the evaluation index values, the new hybrid combinations were ranked
accordingly and APM12xAPDR105 (63.63) assigned first rank followed by APM6xAPS12
(57.66), APM4xAPDR105 (56.79), APM10xSDD1 (54.61) and APM10xSDD3 (54.50) where
as the control hybrid placed in the last position. Further, the cumulative values
obtained for individual trait by applying subordinate function method (Table
3) were ranged to the maximum of 7.596 (APM12xAPDR105) and minimum of 2.652
(APM8xSDD3) where as control hybrid obtained the value of 1.963 (APM1xAPS8).
Based on cumulative values, the hybrid combinations were ranked accordingly
and APM12xAPDR105 (7.596) stood first followed by APM6xAPS12 (6.254), APM4xAPDR105
(6.152), APM10x SDD1 (5.788), APM10xSDD3 (5.663) and the control hybrid occupied
last rank as the method followed by Ramesh Babu et al. (2002) and Rao
et al. (2006).
With both the statistical methods of average evaluation index and cumulative subordinate function values were arranged in descending order and accordingly given ranks to each hybrid (Table 4). Top ranked five new hybrid combinations viz., APM12xAPDR105, APM6xAPS12, APM4xAPDR105, APM10xSDD1 and APM10xSDD3) which stood high in both evaluation index and subordinate function methods were identified as potential hybrid combinations. These short listed hybrid combinations were also ranked higher than the control hybrid (APM1xAPS8).
||Subordinate function values on economically important genetic
traits of the new hybrid combinations
|C: Control, FL: Failament length,SF: Cumulative Subordinate
||Ranking of new hybird combination in ascending order based
on EI and subordinate values
|C: Control, EI: Evaluation index, SF: Subordinate function
Among all the hybrids tested in the laboratory, APM12xAPDR105 was well performed and adjudicated for the commercial exploitation. The findings of this study prove the superiority of this new hybrid combination with improved productivity traits than the existing hybrids which are being commercially exploited in the field after large scale laboratory and field trails.
Authors are thankful to Dr. Chandrashekharaiah, former Director, APSSRDI, Kirikera-515 211, Hindupur, A.P., India for his encouragement and support for the present investigation.
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