Introduction
Cotton is a major fiber crop and contributes a large share in the economy of India. Cotton is the main source of foreign exchange earnings. There is need to enhance cotton yield and fiber quality to become a key producer in the global cotton market. All cotton breeders aim to develop cotton cultivars with good fiber quality and yield. Yield of cotton can be improved by improving characteristics that make genotype of cotton plant, such as; developmental characters (optimum plant height, number of sympodial and monopodial branches and number of nodes or internodal length), economic characters (number of bolls per plant, boll size or weight, yield of seed cotton) and quality traits (lint percentage or ginning out turn, lint index, seed index, staple length, fiber strength and fiber fineness). Plant breeder should combine these desirable components of yield and quality.

Combining ability method is important in the breeding programme as it provide information’s about the heritability of crossing parents involved in the production of hybrid cotton seeds. It provides a specific guide line to the plant breeder about the establishment of a unique breeding experiment for the evolution of spectacular cotton varieties.

The combining ability describes the breeding value of parental lines to produce hybrids, general and specific combining ability as defined by (Sprague and Tatum, 1942) who stated that gca effects were due to an additive type of gene action, but sca effects were due to genes which exhibit non additive (dominant and epistatic) type of gene action. Combining ability analysis helps in the evaluation of inbreds in terms of their genetic value and in the selection of suitable parents for hybridization. The superior specific cross combinations were also identified by this technique.

General combining ability is the average performance of a genotype in cross combinations involving a set of other genotypes. It is specific for the set of lines and testing environment, whereas specific combining ability is the average performance of a specific cross combination expressed as deviation from the population mean (Sprague and Tatum, 1942). The gca effect reflects the breeding value of the parental genotypes and assists in identifying genotypes to be used for developing superior populations. Specific combining ability effects represent the non-reliable component of the genotypic value arising due to contribution from dominance deviation and interaction deviation. Hence, sca effect is the main cause for superiority of a cross. It is inferred that superiority of a cross can not be fixed through selection.

Basal et al. (2009) studied eight barbadense cotton cultivars and fifteen F₁ hybrids obtained by crossing five lines and three testers in the line x tester mating system. The predominance of non-additive gene action was estimated for all the traits except for seeds per boll, which were controlled by additive type of gene action due to high GCA variance.

El- Mansy et al. (2010) hybridized nine diverse G.barbadense cotton genotypes i.e.; Pima S6 as American Egyptian cotton, Karshenky 2 as Russian genotypes, Suven as Indian genotype and six Egyptian genotypes i.e. Monoufi, dandara, Giza 86, Giza87, Giza 89 x Pima S6 and Giza 92, following half diallel crossing system in order to investigate the genetic mechanism controlling variation. Analysis of combining ability revealed significant gca and sca for most studied characters indicating importance role of additive and non-additive effects in the inheritance these characters.

Mohammad and Nadali (2010) determined combining ability and heterosis in crosses of G.hirsutum and G.barbadense for agro morphological traits and yield. High variation was observed for characteristics among parents and the F₁ combinations. Barbadense 5539 and Termeze14 (G.barbadense) had negative gca for plant height, bolls per plant and sympodia branches per plant. Barbadense genotypes also showed negative gca for monopodia per plant and boll weight. The GCA: SCA ratios for the studied traits were higher than one indicating the presence of additive genetic effects for most of the characteristics studied.

The term heterotic group refers to “a group of related or unrelated genotypes from the same or different populations, which display similar combining ability and heterotic response when crossed with genotypes from other genetically distinct germplasm groups” (Melchinger and Gumber, 1998).

In the recent years the concept of developing heterotic populations is put to test in self pollinated crops like cotton, segregating populations based on diverse pairs of genotypes can be the ideal base material required for implementing procedures like reciprocal selection for improving combining ability (Patil and Paltil, 2003 and Patil et al., 2011). In hybrid research study on cotton, a large number of crosses involving varietal lines are used for assessing combining ability status. On constantly observing the most potential crosses attempts are made to infer about the causes of high heterosis.

Utilization of heterosis depends on genetic diversity existing between the parents, magnitude of dominance at the yield influencing loci and the genetic distance between the chosen parental genotypes. It is possible to maximize heterosis by enhancing genetic distance between two chosen parental populations. Many population improvement schemes are followed in cross pollinated crops to increase genetic diversity, to create heterotic groups and exploit them. These schemes can be extended to self pollinated crops by introducing slight modifications in the procedures to suit the crossing system of self pollinated crops. In present study heterotic box was developed by involving barbadense and hirsutum varieties and it was exploited by creating recombinational variability for combining ability.

If two lines A and B are found to give potential crosses with testers T₁ and T₂, it is possible to increase the genetic distance between these opposite pairs A, B vs, T₁ and T₂ by following population improvement scheme for improving combining ability defined in cross pollinated crops by introducing suitable changes to match the crossing system seen in self pollinated crops. The ongoing study at ARS, Dharwad on evaluation of interspecific hybrids led to formation of a heterotic box of two barbadense lines DB 533 and DB 534 and hirsutum testers DH 98-27, ZCH 8, 178-24 and DH 18-31.

The main objectives of this study are 1: development of hirsutum vs barbadense heterotic groups, 2: To determine combining ability effects (gca and sca), variances (GCA and SCA), combining ability patterns of these barbadense and hirsutum lines.

1 Results
1.1 Analysis of variance for combining ability
The Analysis of variance for combining ability done with respect to hybrids are summarized in Table 1 for eight characters. Among the lines (males), the mean sum of squares (MSS) were not significant for all characters except mean boll weight and seed cotton yield which showed highly significant. Testers (females) exhibited significant difference for four characters seed index, ginning outturn, lint index and seed cotton yield which showed highly significant. Whereas, line x tester interactions were highly significant for all characters except plant height and number of sympodia per plant which recorded not significant differences.

Table 1 Analysis of variance for combining ability of Line x Tester interspecific crosses (YHB) for different quantitative characters

 
The estimates of variance due to general combining ability (GCA), variance due to specific combining ability (SCA), the magnitude of SCA variance were greater than GCA variance for all 8 characters and the variance ratio was less than half in these traits (Table 2).

Table 2 Variance due to general and specific combining ability for different quantitative characters

1.2 Combining ability effects
The estimates of general combining ability effects of females and males are presented in Table 3 and their specific combining ability effects are presented in Table 4 for all the characters. 

Table 3 Estimates of general combining ability effects of parents for different quantitative characters included in Line x Tester interspecific crosses (YHB)

Table 4 Estimates of specific combining ability effects of Line x Tester interspecific crosses (YHB) for different quantitative characters

 
1.3 Seed cotton yield (kg/ha)
The estimates of gca effects of tester parents (Figure 1) in the population based crosses were found to be significant differences for sixteen testers, of which ten testers recorded positive significant gca effects and the tester DH 98-27 showed maximum value of gca effect for seed cotton yield trait (333.42). Three barbadense lines recorded significant gca effects (Figure 1), of which two lines exhibited positive significant gca effects recorded by the line DB 533 (242.92) and DB 534 (75.54). Among 92 crosses, seventeen crosses differed significantly for sca effects and the hybrid DH 11-8 × DB 534 (565.35) recorded highest positive sca effect.

Figure 1 Estimates of general combining ability effects of parents for seed cotton yield included in Line x Tester interspecific crosses (YHB)

1.4 Plant height (cm)
The estimates of general combining ability effects among hirsutum testers recorded significant negative value in the tester DH 75- 23 (-21.36) and the highest positive gca effect was exhibited by the tester DH 45-23 (9.60). All four barbadense lines exhibited not significant gca effects and the line DB 531 showed highest positive gca effect (2.88). Four crosses differed significantly for sca effects, of these two had positive sca effects showed by the crosses DH 18-31 x DB 532 (52.60) and DH 31-2 × DB 531 (43.87), whereas the crosses DH 23-4 × DB 533 (-44.04) and DH 18-31 × DB 531 (-67.46) recorded significant negative sca effects for plant height. 

1.5 Number of sympodia per plant
The estimates of general combining ability effects indicated not significant differences among all hirsutum testers and four barbadense lines. Three crosses expressed significant sca effects, of these one hybrid exhibited positive sca effect showed by the cross DH 31-2 × DB 532 (5.06) and two hybrids recorded negative sca effects DH 23-4 × DB 533 (-5.64) and DH 31-2 × DB 534 (-5.97).

1.6 Number of bolls per plant
Out of the 23 testers, 15 testers showed significant gca effects for number of bolls per plant. ZCH 8 (2.02), DH 37-4 (1.62), DH 8-7 (1.85), DH 29-1 (1.33), DH 35-17 (5.29), DH 13-7 (1.49) and DH 23-21 (5.18) exhibited significant positive sca effects in the desirable direction, whereas testers DH 18-31 (-3.25), 178-24 (-3.34), DH 24-4 (-2.38), DH 11-8 (-1.03), DH 23-4 (-3.19), DH 75- 23 (-2.63), DH 91-1 (-2.38) and DH 45-23 (-1.23) showed significant negative gca effects in desirable direction. Out of 4 lines, three lines exhibited significant gca effects in desirable direction, the lines DB 534 and DB 532 exhibited positive significant gca effects 1.06 and 0.52 respectively. While the line DB 533 (-1.51) recorded significant negative gca effect. Out of 92 crosses, fifty three crosses recorded significant sca effects in desirable direction. Twenty five crosses showed positive significant sca effects and the cross DH 23-1 x DB 532 (8.61) exhibited higher value of positive significant sca effect. Whereas, twenty eight crosses recorded negative significant sca effects.

1.7 Mean boll weight (g)
The estimates of gca effects of tester parents in the population based crosses were found to be significant in twelve testers, out of which six were positive significant and other were negative significant differences, shown the highest values by the testers DH 49-1 (0.34) and DH 18-31 (0.25). Among the lines, three showed significant differences of gca effects and the line DB 534 had significant positive of gca effect (0.16). Ninteen crosses expressed significant sca effects, out of which nine recorded positive significant sca effects and the cross DH 31-2 × DB 533 (0.77) had higher value of positive significant sca effect.

1.8 Seed index (g)
Nine tester parents exhibited significant gca effects, four testers had significant gca in positive direction and highest was recorded in DH 13-7 (1.18). Among the lines, DB 534 showed positive significant gca effect (0.27). Thirteen crosses revealed significant sca effects, of these six crosses showed positive sca effects and the highest was displayed by the cross DH 23-21 × DB 532 (2.61).

1.9 Ginning outturn (%)
Six hirsutum tester parents displayed significant gca effects, of which three testers exhibited significant positive gca effects and highest value was recorded by DH 98-27 (2.87). Among the barbadense lines, DB 532 (-0.61) exhibited significant negative gca effect. Eighteen crosses expressed significant sca effects, of these eight crosses showed positive sca effects and the hybrid 178-24 × DB 533 recorded highest positive sca effect (6.70).

1.10 Lint index (g)
Seven hirsutum testers showed significant gca effects, of which four testers had positive gca effects and the tester DH 13-7 (0.84) recorded highest value of gca effect. There are no significant differences among all barbadense lines and DB 534 (0.14) exhibited highest value of gca effect. Out of 92 crosses studied, seven crosses recorded significant sca effects for this character, of which five showed significant in positive direction and the cross 178-24 × DB 533 recorded highest significant positive sca effect (1.91).

1.11 Pooled score for gca effects
Based on simple pooled gca score method (Table 5), the hirsutum testers DH 13-7 and DH 98-27 (Decreasing order) are recognized as the most potential parents. Among the barbadense lines, the lines DB 534 and DB 533 based on simple pooled gca score method showed the most general combiner parents. Based on per cent gca method, the hirsutum testers DH 13-7, DH 98-27 and DH 45-23 (Decreasing order) were the most potential combiners (Table 6). Among the barbadense lines, the lines DB 5343 and DB 533 based on per cent gca score method showed the most potential parents. Similarly, based on weighted gca method the most potential combiners were found to be the hirsutum testers DH 13-7, DH 98-27 and DH 18-31. Among the barbadense lines, the lines DB 534 and DB 533 based on weighted gca method is the most potential parents (Table 7).

Table 5 Pooled scores of hirsutum testers and barbadense lines based on Simple pooled gca score

Table 6 Pooled scores of hirsutum testers and barbadense lines based on per cent pooled gca score

Table 7 Pooled scores of hirsutum testers and barbadense lines based on weighted per cent pooled gca score

 
2 Discussion and Conclusion
2.1 Combining ability for confirmation of heterotic groups
In hybrid research study on cotton, large number of crosses involving varietal lines are used for assessing combining ability status. On constantly observing the most potential crosses attempts are made to infer about the causes of high heterosis. What are the combinations that give potential crosses? What would be the probable cause for high potentiality revealed by the F₁? What is the genetic base or is there any physiological mechanism linked to high productivity of F₁ etc., are the questions which are examined and on the basis of the information available, heterotic groups are developed (Patil et al., 2011). The most potential crosses observed in present study have been examined and based on this the combining ability behavior (Pattern) of the line involved is determined. With the help of this information diverse groups are formed which are capable of giving potential hybrids between them.

The research programme on development of hybrids at UAS Dharwad has focused attention on developing heterotic groups, meant for evolving intra hirsutum hybrids and interspecific hybrids. Efforts are made to develop different heterotic groups like Stay green x Compact, Robust x Compact, Robust x Higher RGR and Stay green x higher RGR i.e. (Patil and Patil, 2003). These studies have also shown ways of exploiting heterotic groups by following novel approaches of creating recombinational variability for combining ability and exploiting the same through reciprocal selection for combining ability (Mallikarjun, 2005 and Somashekar, 2006).

The ongoing study at ARS, Dharwad on evaluation of interspecific hybrids led to formation of a heterotic box of two barbadense lines DB 533 and DB 534 and hirsutum lines DH 98-27, ZCH 8, 178-24 and DH 18-31. To create recombinational variability DB 533 and DB 534 crossed was made and material was advanced to F4 generation with an objective of evaluating recombinational variability for combining ability in F₄ generation. The Line x Tester study was planned to assess the relative potential of these barbadense and hirsutum lines and confirmed appropriateness of the choice of this heterotic box of two barbadense and four hirsutum lines. The objective of this study was to determine the relative ranking of the selected barbadense and hirsutum lines when compared with the new lines developed during this period.

Among four barbadense lines positive gca effects for seed cotton yield was recorded by DB 533 and DB 534 confiriming the potential of these barbadense lines in developing productive interspecific hybrids. Among 23 hirsutum testers, four testers DH 98-27, ZCH 8, 178-24 and DH 18-31exhibited good positive values of gca effects for seed cotton yield and were located among top ten testers ranks 2, 8, 1 and 7 respectively when compared to other hirsutum testers.

2.2 Pooled score for gca effect
The overall combining ability status of barbadense lines and hirsutum testes was determined by working out pooled gca score. Three methods namely simple gca score, per cent gca and weighted gca method were used in arriving at the best general combiner lines. The overall ranking from these approaches differed and the weighted gca approach helped in preciese identification of potential combiners.

2.3 Simple pooled gca score method
In this approach, significant gca effect in desirable direction is given positive weightage (+1) and negative weighatage (-1) is given for gca effect in undesirable direction (Arunachalam and Bandopadhyay, 1979). These values are added over different yield influencing characters to arrive at pooled score of gca effects. The inherent disadvantage with this system is that all the parents with significant gca effects in desirable direction get the same score (positive). Hence, it is not possible to quantify the magnitude of difference existing among the genotypes of this group which get a positive score. Therefore, it is necessary develop a system of working out pooled scores of gca by utilizing the actual gca values and ensuring quantification of every possible difference existing in gca effects between only two parents.

2.4 Per cent gca method
When the actual gca values are added across characters to arrive at pooled score, problem arises because of difference in unit of measurement of each character. Absolute values of gca effects may be big (plant height) or small (boll weight) depending on the character and if used, the importance of the character may not be projected correctly. If the raw values of gca effects are added across the characters, the character with higher per se effect influence the pooled scores most as against the character with low per se gca values. To overcome this disadvantage, the raw gca values have to be converted into per cent gca values.

Thus, by working out per cent gca values, the minute differences in gca values are also focussed and the possible problem arising out of the differences in unit of measurement, high and lower per se gca values associated with the type of character concerned are overcome.

2.5 Weighted per cent gca method
In this method further improvement is brought about in arriving at the pooled gca scores of the parents. In per cent gca method, the per cent gca values are straight away added across the characters which means each character including yield and yield components are all given equal weightage. The experience of the breeders would suggest sometimes that, in arriving the pooled score, it is desirable to attach differential weightages to each of the characters studied depending upon its economic importance, contribution to yield etc. These weightages can be multiplied with per cent gca values of corresponding characters and then added to arrive at the pooled gca score for each parent. In the present study weightages for different yield related characters were worked out by consulting the senior breeder related to cotton viz., Dr. S. S. Patil, Senior Scientist, Agricultural Research Station, Dharwad and Dr.B.C.Patil, principal scientist (Plant physiology), ARS, Dharwad Farm.

Based on weighted gca method the most potential combiners were found to be the hirsutum testers DH 13-7, DH 98-27 and DH 18-31. Among the barbadense lines, the lines DB 534 and DB 533 based on weighted gca method is the most potential parents.

3 Materials and Methods
The main objective of this Line x Tester study was to determine the combining ability status of the barbadense and hirsutum lines included in the heterotic box. These lines were compared with other barbadense and hirsutum lines. For the pattern of combining ability and the potentiality of the interspecific hybrids developed based on them.

In this experiment, 92 (YHB trial) interspecific crosses (G.hirsutum x G.barbadense) along with three checks (MRC 6918 Bt check, RAHB 87 and DCH 32 non Bt checks) were subjected to Line x Tester analysis (23 hirsutum testers and 4 barbadense lines). This experiment was laid out in Randomized Block Design (RBD) with two replications. Each entry was sown in 2 row plots spaced at 90 cm x 60 cm with recommended dose of fertilizer and seeds were sown on 28-6-2010, 2-3 seeds were dibbled per spot in each row and thinning was attended to retain one healthy plant per hill at 25 days after sowing. All the recommended package of practices were followed to raise healthy crop.

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