Genetic Distance and Heterosis Through Evaluation Index in the Silkworm, Bombyx mori L.

Problem statement: Synthesis of new-gene combinations by genetic manipulation is one of the powerful tools in exploiting the commercial qualities of plants and animals. Hybrid performance is evaluated from extensive yield trials that are costly and time consuming. Approach: Four silkworm races belonging to two different voltine groups (two multivoltine races namely Pure Mysore and Nistari and two bivoltine races namely C108 and NB4D2) and the twelve regular and reciprocal hybrids derived from them were reared under standard laboratory condition analyzing six quantitative traits namely cocoon weight, shell weight, shell ratio, filament length, denier and renditta. The data of the pure races was analysed for the estimation of evaluation index to study the genetic divergence between the races, where as evaluation index, heterosis and overdominance effects were studied in twelve hybrid combinations. Results: Varied heterotic effects were observed for different traits for hybrid combination. Cocoon weight and shell weight has maximum heterosis over the mid parent in Pure Mysore × Nistari (27 and 42% respectively), whereas C108 × Nistari have shown maximum shell ratio (30%) among the hybrids. The maximum filament length for heterosis was observed in the Nistari × Pure Mysore. Heterosis for cocoon weight, shell weight, shell ratio, filament length, denier and renditta based on evaluation index confirmed the above results. Conclusion: The investigation indicates that optimum level of genetic divergence between parents is necessary to obtain heterosis in F1 generation.


INTRODUCTION
suitable F hybrid for better silk production. They Synthesis of new-gene combinations by genetic of the quantitative characters. Ramesh and Subramanya manipulation is one of the powerful tools in exploiting [10] have found that the hybrid combinations manifested the commercial qualities of plants and animals. The positive heterosis value for the cocoon traits studied. heterosis is observed when the animals of different Kumaresan et al. [11] observed that heterotic effect for genetic backgrounds are mated and this mating system is different traits for different hybrid combination in their called the line crossing or crossbreeding. Perusal of experiment. literature related to heterosis manifestation in Bombyx Silkworm breeding programs are based on the mori has been demonstrated by many breeders [1][2][3][4][5][6]. development and selection of outstanding hybrids from Heterosis, expressed as the improvement in a character inbred lines [12]. Developing and selecting inbred lines for shown by a hybrid over their mid-or better parental value, performance is quite easy, although time consuming. is a vital measure of the genetic progress made in plant, Hybrid performance is evaluated from extensive yield animal and silkworm selection. Tayade [7] studied trials that are costly and time consuming. In any hybrid heterosis effect on economic traits of new hybrids of program, a large number of crosses are made, while only silkworm, Bombyx mori L. and reported that heterosis was a few good hybrids are obtained. This process is a result of overdominance and proposed that linked extremely labour intensive, time consuming and tedious favorable dominant gene also contributed to heterosis. and hence often, alternative methods are explored for Ravindra Singh et al. [8] demonstrated that the degree of analyzing the potential parental material. It is an already heterosis varied considerably for different characters with established fact that the amount of yield heterosis the maximum heterosis for cocoon shell weight, cocoon obtained by hybrids depends largely on the genetic weight and thickness of the silk filament. Das et al. [9] divergence of the populations from which the parental carried out a series of out-breed crosses to select a lines have been extracted [13]. The level of genetic 1 observed that heterotic effects were significant for most diversity between the two parents being used for crossing e = [(A-B) × 10 / C] + 50 has been proposed as a possible predictor of F 1 performance [14]. D statistic by Mahalanobis [15] has 2 been extensively used in predicting the hybrid performance on the basis of morphological traits. However, these methods require extensive field tests and crossing and hence, a need is now felt to determine genetic distance at the molecular level and thus to predict hybrid performance [16].
Small populations are particularly vulnerable to inbreeding and loss of genetic diversity through genetic drift [17]. Levels of genetic diversity within and divergence among populations are expected to vary with degree of fragmentation. As geographic distance between fragments or time since isolation increases, genetic diversity within small populations is expected to decrease and populations will diverge [18].
Prediction of hybrid performance has been of primary interest to essentially all hybrid breeding programs and has attracted an enormous amount of effort. The present studies were undertaken to prediction and estimate the level of heterosis and overdominance among F hybrids of two bivoltine (C and NB D ) and 1 108 4 2 two multivoltine (Pure Mysore and Nistari) races of silkworm, Bombyx mori L. This information's would be useful to investigate the performance and relationship of F hybrids and parents and to select suitable parents and 1 population for designing an effective silkworm breeding programme.

MATERIALS AND METHODS
The silkworm races in the present study includes, two bivoltine races NB D and C and two multivoltine 4 2 108 races Nistari and Pure Mysore races which were drawn from the germplasm bank of the Department of Studies in Sericulture Science, University of Mysore, Mysore, India. The F hybrids of regular and reciprocals along with 1 parents were reared in replicates of three each following the rearing methodology described by Narasimhanna and Krishnaswamy The percent increase or decrease of F hybrids over 1 mid parent as well as better parent was calculated to estimate possible heterotic effects for above mentioned parameters [21]. The't' test was manifested to determine whether F 1 hybrid means were statistically different from mid parent and better parent means as follows [22].
The 't' value for overdominance was calculated following the formula. To estimate significant differences among mean value and evaluation index of parents and F hybrids, 1 the data were subjected by analysis of variance using SAS software (version 9). Evaluation index was used to determine genetic relationships among pure races. The cluster analysis was carried out using the UPGMA The dendrogram produced on unweighted pair method (Unweighted Pair Group Method Algorithm) group method with arithmetic means (UPGMA) developed by Sokal and Michener [23]. and cluster analysis indicated that the two bivoltines

RESULTS
the two multivoltines Pure Mysore and Nistari The mean value of four parental races and 12 F The distance between C and NB D was 0.33 1 hybrids for six cocoon traits namely cocoon weight, and distance between Pure Mysore and Nistari was shell weight, shell ratio, filament length, denier and 0.44. The distance between two clusters calculated renditta are presented in Table 1 and relevant statistical revealed a value of 1.196 (Table 4). These results analysis through ANOVA is presented in Table 2. It is indicated that being inbreed population and evident from the data presented in Table 1 and 2 that all differing voltinism, the higher genetic distance was the six traits measured revealed high significant difference between multivoltine and bivoltine in the above (P<0.01) except the trait denier.
C and NB D formed into one cluster, where as,  Means having the same letters do not differ significantly Means having the same letters do not differ significantly at 0.05 level of probability crosses. on evaluation index showed clear separation of races with specific voltinism (Figure 1).

Filament Length: Positive and significant heterosis
Cocoon Weight: Positive and significant heterosis over and significant overdominance was observed from 1 cross mid parent was observed from 8 crosses and positive and out of 12 crosses. Also, Farooq et al.
[32] observed significant heterosis over better parent was observed heterosis for filament length in some crosses in their from 2 crosses out of 12 crosses ( results are in conformity with the report of Malik [33] Shell Weight: The eleven crosses exhibited positive overdominance about the denier trait. and significant estimates for heterotic effects and positive and significant heterosis over better parent Denier: No positive heterosis and overdominance was observed from 4 crosses. Krishnaswami et al. [26] heterosis were observed. and Satenahalli [28] also reported high heterosis for The estimate of heterosis of F 's over mid and the shell weight trait. The superiority of F hybrids better parent (s) for mean value of the six traits and 1 and their parents with regard to economically important evaluation index value are presented in Table 5 and 6, traits such as hatchability, larval duration, cocoon respectively. Estimation of heterosis and overdominance weight and shell weight have also been studied by for evaluation index was similar with estimation of Katsumata [29].
heterosis and overdominance based on mean value.   Ht= Relative Heterosis, OD = Overdominance. Tabulated 't' value at 5% and 1% for d.f. is 2.042 and 2.750, respectively 30 * = Significant, **= highly significant  Nistari and Nistari × C showed positive and 108 significant evaluation index average and over than 50. Nistari × Pure Mysore and Pure Mysore × Nistari exhibited positive and significant evaluation index average but under 50 (Table 8). Figure 2 revealed positive correlation between relative heterosis and overdominance.

CONCLUSION
It is concluded from present studies that cross C × Pure Mysore, NB D × Nistari, C × Nistari, C × 108 4 2 108 108 NB D and Nistari × C could be further evaluated 4 2 108 for selecting high cocoon characters genotypes due to its highest heterotic value for important evaluation index. As well as, evaluation index is a valuable method for determining genetic variability among silkworm races. This investigation exhibited no significant correlation between all cocoon characters based on mean value and genetic distance useing evaluation index ( Table 8). The optimum level of genetic distance is necessary to obtain heterosis. Arunachalam et al. [34] reported that there is an optimum level of genetic divergence between parents to obtain heterosis in F generation and they also 1 reported that it may not be logical to advocate the use of extremely divergent parents to obtain heterotic combinations. As well as, heterosis estimation for mean evaluation index is more important than heterosis estimation for each trait individually and crossing between bivoltine female × multivoltine male is useful than reciprocal crossing.