Introduction
Cotton, being the king of fibers in preparing human apparel has played a key role in civilization of mankind. Cotton is providing livelihood directly and indirectly to over 60 million people and accounting for about 16 per cent of India’s export earnings. India was the first country to introduce commercial cultivation of intra hirsutum hybrids in 1960s and then interspecific hybrids in 1970s. The success of interspecific hybrids has led to overcome the acute shortage of ELS (Extra long stable) cotton which the country was experiencing during 1970s and the prominent hybrids from University of Agricultural Sciences, Dharwad namely Varalakshmi and DCH 32 were instrumental in saving very critical foreign exchange running to hundreds of crores of rupees.

Later in 90s, due to increase of severity of pests and the inherent susceptibility to them which was contributed by barbadense parents, the performance of interspecific hybrids declined. This popularity of interspecific hybrids declined and the intra hirsutum hybrids have occupied major area under hybrids.

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 recombinational variability realized in a segregating generation like F₃/F₄ can be evaluated by crossing these lines with opposite testers representing opposite heterotic group.

If more lines are found to be giving superior crosses with a tester then it is possible to initiate multiple crosses among such lines selected for combining ability and this can lead to creation broad gene pool of recombination variability for combining ability as the population developed in this manner based on number of components improved in ability to combine with the tester. This heterotic gene pool can be exploited for developing superior hybrid combinations with the tester concerned. Several criteria have been suggested to choose promising heterotic groups: (i) high mean performance and large genetic variance in the hybrid population in the target region(s), (ii) high per se performance and good adaptation of the parent populations, and (iii) a higher ratio of the variance due to general (σ2 GCA) versus specific combining ability (σ2 SCA) (Melchinger and Gumber, 1998; Reif et al., 2005a).

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.

Fan et al. (2001) used a diallel design to study combining abilities among 10 maize lines (five lines from the International Maize and Wheat Improvement Center [CIMMYT] and five major commercial lines from China). According to SCA_PY method, they classified CML171, CML161, CML166 into one heterotic group; Chang 631/o2, Zhongxi 096/o2 into another heterotic group; and Qi 205 into a third heterotic group.

Huang and Li (2001) evaluated 45 maize inbred lines from China, U.S. Corn Belt, and tropical regions by using 44 restriction fragment length polymorphism (RFLP) markers equally distributed across the 10 maize chromosomes. The 45 inbred lines were grouped into six heterotic groups: Mo17 was assigned to group II; Tangsipinttou (TSPT) and Huangzao 4 (HZ4) were assigned to group IV and Dan 340 was assigned to group VI.

Menkir et al. (2004) used two testers representing the two heterotic patterns to test 38 tropical maize inbred lines. The two testers successfully classified 23 of the 38 tested inbred lines into two heterotic groups based on the SCA_PY method.

Fischer et al. (2010) identified two heterotic groups in triticale by employing principle coordinate analysis and an enumeration algorithm for maximizing F₁ performance, mid parent heterosis, and σ2 GCA/ σ2 SCA ratio for grain yield. Twenty one inbred lines and their 210 diallel crosses were evaluated for grain yield at five agroecologically diverse locations in Germany.

Realizing the need for developing potential interspecific (G. hirsutum x G. barbadense) hybrids, a detailed study was initiated at UAS Dharwad to identify hirsutum and barbadense genotypes capable of giving potential interspecific hybrids. Based on this study done during earlier years in 2009-2010 two barbadense DB 533 and DB 534 and four hirsutum DH 98-27 (T₁), ZCH 8 (T₂), 178-24 (T₃) and DH 18-31 (T₄) lines giving best hybrids (H x B) combinations between them were selected. After identifying this heterotic box, these barbadense lines DB 533 and DB 534 were crossed in 2007-2008 and the material was advanced during succeeding years. To confirm the potentiality of heterotic group, the detailed evaluation of interspecific hybrids involving these lines along with other crosses was taken up during 2010-2011. 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. 3) Comparison of hybrids based on heterotic box with checks and other interspecific hybrids.

1 Results
Brbadense lines were crossed to hirsutum testers in Line x Tester fashion to determine whether the heterotic groups (2 barbadense and 4 hirsutum lines) formed is potential. This Line x Tester study is denoted as YHB trial fast to distinguish it from other studies. After determining potential of the heterotic box, the 8 interspecific crosses were compared with commercial checks and many other potential interspecific crosses developed by the centre. This study distinguished as best HB trial and the results are presented below.

1.1 Line x Tester for confirmation of interspecific heterotic groups
1.1.1 Analysis of variance for combining ability
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 inter specific crosses 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

 
1.1.2 Combining ability effects
The estimates of general combining ability effects of females and males are presented in Table 3 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

 
1.1.3 Seed cotton yield (kg/ha)
The estimates of gca effects of tester parents 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, of which two lines exhibited positive significant gca effects recorded by the line DB 533 (242.92) and DB 534 (75.54).

1.1.4 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).

1.1.5 Number of sympodia per plant
The estimates of general combining ability effects indicated not significant differences among all hirsutum testers and four barbadense lines.

1.1.6 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.

1.1.7 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).

1.1.8 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).

1.1.9 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.

1.1.10 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.

1.2 Comparison of hybrids based on heterotic box with checks and other crosses
The mean performance of 49 best HB interspecific hybrids and three commercial checks (Bt check MRC 6918 and non Bt checks RAHB 87 and DCH 32) are briefly presented in Table 4.

Table 4 Per se performance of inter specific hybrids (Best HB) involved in heterotic box for different quantitative characters

 
1.2.1 Seed cotton yield (kg ha^-1)
Among 52 hybrids, seed cotton yield values ranged from 2277.81 [ZCH 8 x DB 533] to 1232.69 [DH 11-20 x DB 534]. Fourteen hybrids had high performance with respect to seed cotton yield as compared to RAHB 87 (2087.13), two hybrids exhibited higher seed cotton yield than MRC 6918 (2070.00) and twenty three hybrids had higher values of seed cotton yield than DCH 32 (1767.23).

1.2.2 Plant height (cm)
Plant height values ranged from 301.00 [DH 102-23 x DB 534] to 178.00 [DH 18-31 x DB 532] and the results showed that ten hybrids recorded higher plant height than DCH 32 (256.67), thirty hybrids recorded high performance with respect to plant height as compared to RAHB 87 (210.67) and seven hybrids had higher plant height than MRC 6918 (199.00).

1.2.3 Number of monopodia per plant
Among 52 hybrids evaluated, number of monopodia per plant values ranged from 4.00 [DH 43-44 x DB 532] to 0.67 [DH 18-31 x DB 533] and twenty six hybrids recorded higher number of monopodia per plant than RAHB 87 (2.00), eight hybrids recorded high performance with respect to number of monopodia per plant as compared to DCH 32 (1.33) and two hybrids DH 57-95 x DB 533 and DH 10-2 x DB 531 exhibited equal performance of number of monopodia per plant to the Bt check MRC 6918 (1.00).

1.2.4 Number of sympodia per plant
Among all the hybrids evaluated, number of sympodia per plant values ranged from 39.33 [DH 40-29 x DB 532] to 23.00 [DH 18-31 x DB 532] and the results showed that three hybrids recorded high performance with respect to number of sympodia per plant as compared to DCH 32 (34.67), twelve hybrids recorded higher number of sympodia per plant than RAHB 87 (31.33) and seventeen hybrids had higher number of sympodia per plant than MRC 6918 (28.67).

1.2.5 Number of bolls per plant
Number of bolls per plant values ranged from 47.33 [DH 22-76 x DB 534] to 21.00 [DH 102-23 x DB 534], seventeen hybrids exhibited high performance with respect to number of bolls per plant as compared to RAHB 87 (33.00), one hybrid DH 18-31 x DB 534 recorded equal value of performance to the Bt check MRC 6918 (32.67) and ten hybrids had higher number of bolls per plant than DCH 32 (30.67).

1.2.6 Mean boll weight (g)
Mean boll weight values of 52 best HB hybrids ranged from 5.20 [DH 39-84 x DB 531, DH 11-8 x DB 532, DH 53-2 x DB 534, DH 84-7 x DB 533 and DH 8-7 x DB 532] to 3.50 [DH 11-8 x DB 531, 177-24 x DB 531, DH 11-8 x DB 534 and DH 91-1 x DB 531] and the results showed that seven hybrids recorded higher mean boll weight than DCH 32 (5.00), eight hybrids showed high performance with respect to mean boll weight as compared to RAHB 87 (4.60) and two hybrids had higher mean boll weight than MRC 6918 (4.40).

1.3 Reproductive points on sympodia
Among all hybrids, Reproductive points on sympodia values ranged from 7.33 [DH 25-22 x DB 533 and DH 25-3 x DB 533] to 4.33 [DH 21-29 x DB 532 and RAHB 87]. Thirty four hybrids had higher reproductive points on sympodia than MRC 6918 (5.17) and DCH 32 (5.17) and twelve hybrids showed higher reproductive points on sympodia than RAHB 87 (4.33).

1.3.1 Sympodial length at 50 per cent plant height (cm)
Sympodial length at 50 per cent plant height values ranged from 89.50 [DH 102-23 x DB 534] to 50.00 [RAHB 87] and the results showed that thirty three hybrids recorded high performance with respect to sympodial length at 50 per cent plant height as compared to DCH 32 (62.33), four hybrids recorded higher sympodial length at 50 per cent plant height than MRC 6918 (60.67) and twelve hybrids had high performance with respect to sympodial length at 50 per cent plant height as compared to RAHB 87 (50.00).

1.3.2 Inter branch distance (cm)
Among 52 hybrids, inter branch distance values ranged from 37.00 [DH 102-23 x DB 534] to 20.67 [DH 11-8 x DB 533] and the results showed that fifteen hybrids recorded high performance with respect to inter branch distance as compared to DCH 32 (30.33), eight hybrids recorded higher inter branch distance than MRC 6918 (29.33) and six hybrids showed higher values of inter branch distance than RAHB 87 (27.67).

1.3.3 Seed index (g)
Seed index values of 52 YHB hybrids ranged from 15.00 [DH 28-36 x DB 531, DH 91-1 x DB 531 and DCH 32] to 11.00 [DH 22-76 x DB 534, DH 57-95 x DB 533, DH 47-67 x DB 532, ZCH 8 x DB 533, DH 11-8 x DB 531, 179-20 x DB 532, DH 44-14 x DB 534, DH 25-3 x DB 533 and RAHB 87]. Two hybrids had equal performance of seed index to the check DCH 32 (15.00), twenty one hybrids exhibited high performance with respect to seed index than MRC 6918 (12.00) and eight hybrids showed equal values of seed index to the check RAHB 87 (11.00).

1.3.4 Ginning outturn (%)
Ginning outturn values of 52 best HB hybrids ranged from 39.74 [RAHB 87] to 27.76 [DH 39-84 x DB 531]. Twenty one hybrids exhibited high performance with respect to ginning outturn as compared to DCH 32 (34.38) and fourteen hybrids had higher values of ginning outturn than MRC 6918 (32.44).

1.3.5 Lint index (g)
Lint index values of all best HB hybrids ranged from 5.16 [DH 28-36 x DB 531 and DCH 32] to 3.50 [177-24 x DB 531] and the results exhibited that eighteen hybrids exhibited higher lint index than RAHB 87 (4.37) and twenty one hybrids had higher values of lint index than MRC 6918 (3.89).

1.3.6 2.5% span length (mm)
2.5% span length values of all hybrids ranged from 40.40 [177-24 x DB 531] to 32.40 [DH 8-7 x DB 531]. Six hybrids showed high performance with respect to 2.5% span length as compared to DCH 32 (39.60), twenty nine hybrids exhibited higher 2.5% span length than RAHB 87 (35.40) and four hybrids had higher values of 2.5% span length than MRC 6918 ( 34.90).

1.3.7 Fiber uniformity ratio (%)
Among 52 hybrids, fiber uniformity ratio values ranged from 49.00 [DH 11-8 x DB 533] to 43.00 [DH 28-36 x DB 531, DH 53-2 x DB 534, DH 25-22 x DB 533, DH 18-19 x DB 533 and ZCH 8 x DB 534]. Five hybrids showed higher fiber uniformity ratio than two checks MRC 6918 (46.00) and RAHB 87 (46.00) and fifteen hybrids exhibited equal performance of fiber uniformity ratio to the check DCH 32 (45.00).

1.3.8 Fiber micronaire value (g/inch)
Fiber micronaire values ranged from 3.60 [DH 8-7 x DB 531] to 2.50 [DH 25-22 x DB 533]. The results showed that one hybrid DH 8-7 x DB 531 showed highest value of micronaire than RAHB 87 (3.50) and DCH 32 (3.50) and fourty three hybrids exhibited higher fiber micronaire values than MRC 6918 (2.60).

1.3.9 Fiber strength (Tenacity) (g/tex)
The tenacity values ranged from 30.60 [DH 11-8 x DB 531] to 22.70 [DCH 32]. One hybrid DH 11-8 x DB 531 showed higher tenacity value than MRC 6918 (27.40), thirty hybrids exhibited high performance with respect to tenacity value as compared to RAHB 87 (24.90) and sixteen hybrids had higher values of tenacity than DCH 32 (22.70).

1.3.10 Fiber elongation (%)
Fiber elongation values ranged from 6.70 [DH 22-76 x DB 534] to 5.60 [177-24 x DB 531], the results recorded that one hybrid DH 22-76 x DB 534 exhibited higher fiber elongation value than RAHB 87 (6.40), ten hybrids had high performance with respect to fiber elongation than DCH 32 (6.10) and twenty hybrids showed higher values of fiber elongation than MRC 6918 (5.80).

2 Discussion and Conclusion
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. A study of set of hybrids involving the line as a common parent gives an idea about the combining ability pattern of the concerned line. The higher or lower performance of the hybrids is itself taken as reflection of genetic distance existing between the parents. It has been possible to identify heterotic combinations (potential crosses) based on their per cent superiority over the commonly used check. When these crosses show up to be consistently potential, they are considered while forming heterotic groups involving parents of such crosses.

The exercise of identifying diverse groups is a continuous process because the new breeding lines developed and stabilized and those lines obtained from other sources are included in developing crosses and these lines could be added in different heterotic groups after studying their combining ability behavior (pattern) by crossing with representative genotypes of different groups. Thus the grouping of genotypes is continuously revised and refined. In the recent years, the concept of developing heterotic groups 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, Patil et al., 2011).

2.1 Line x Tester for confirmation of interspecific heterotic groups
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. 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. The Line x Tester study was planned to assess the relative potential of the barbadense and hirsutum in developing interspecific hybrid combinations. The objective of this part of the study was to determine the relative of 4 barbadense and 23 hirsutum lines. Barbadense were used as lines and hirsutum were used as testers.

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 Comparison of hybrids based on heterotic box with checks and other crosses
The efforts on creating recombinational variability have begun with two barbadense and four hirsutum lines. However, to compare the potentiality of these hybrids with other potential hybrids developed over years at Dharwad centre and best commercial hybrids (Bt and non Bt). A separate evaluation of these hybrids was taken up.

Study on this set of crosses was denoted as best HB trial. The data generated on 49 best HB crosses of this study is utilized to confirm the performance of eight crosses of the heterotic box. The potential of 8 bench mark crosses of heterotic box [ZCH 8 x DB 533, DH 98-27 x DB 534, DH 18-31 x DB 533, ZCH 8 x DB 534, 178-24 x DB 533, DH 98-27 x DB 533, DH 18-31 x DB 534 and 178-24 x DB 534] recorded higher per se performance for seed cotton yield ranking 1, 2, 4, 6, 8, 10, 11 and 14 out of 49 crosses. These crosses also recorded superior value for yield attributing characters like number of bolls and mean boll weight. The mean performance of these crosses associated with this heterotic box was found to be 2174 kg/ha as compared to the overall mean of best HB interspecific hybrids which was 1921kg/ha.

The crosses namely ZCH 8 x DB 533, DH 98-27 x DB 534, DH 18-31 x DB 533, ZCH 8 x DB 534 and 178-24 x DB 533 recorded highest yield combining good fibre micronaire and fibre length. These crosses are examples for blending of both quality and yield.

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