Introduction
Cotton is one of the most important commercial crops of India cultivated mainly for its fiber and other by products. Cotton, through cloth, has influenced the culture and civilizations. In the process of forming clothes and garments, it provides livelihood and employment to workers engaged in cloth making, designers, traders and the like. Cotton is one of the few crops which are accessible to development of genotypes as varieties and at the same time amenable for commercial exploitation of heterosis. Development of several hybrids during the last decade has contributed to a quantum jump in cotton productivity. Though cotton production in the country has registered marked improvement in recent years, the yield levels of hybrids appear to have reached stagnation. The important reasons attributed for this is the lack of systematic efforts made to develop hybrid oriented populations, derived lines with improved combining ability and develop new hybrids based on such genetically diverse high combiner lines.

Response to selection for any character is dependent on the existence of variability for that character. Quick gains are possible through selection during breeding. Cotton improvement programmes primarily lay emphasis on improvement of hybrids by improving the performance of hybrid parents. Emphasis is not laid on creation of variability and assessing the nature and magnitude of the variability for combining ability. Like other characters, even for combining ability, variability can be created either by inducing mutations or by crossing genotypes and generating recombinational variation. Though such studies are not available in cotton, in sorghum, attempts were made to assess the nature of induced variability for combining ability by crossing the mutant lines with male sterile tester (Shashidhar et al., 1989 and Patil et al., 1991). These studies clearly indicated that inducing mutations can be used as technique to create variability and exploit the same by practicing selection for combining ability.

The other approach to create variability is by recombination of genotypes and evaluating the recombinants in segregating generations for combining ability. In cross pollinated crops like maize, this principle is involved in breeding procedures like recurrent selection for general and specific combining ability. These procedures as such cannot be reproduced entirely in self and often cross pollinated crops like cotton because of the difficulty of intermating the selected plants. However, the segregants selected for high combining ability can be selfed to fix the high combining ability of lines in segregating generation.

In sorghum, Patil and Pandit (1991) created variability using B×R crosses and assessed F₂ segregants for combining ability with the help of male sterile testers. Similarly, Madhusudhan (1993) created variability using B R crosses and confirmed it by assessing F₃, F₄ and F₅ segregants for combining with male sterile testers. Keshall et al. (1985) selected 74 maize lines from random mating population. Out of them, 34 lines showed significant improvement in combining ability. Otte et al. (1984) also concluded in similar manner that through selection improvement in combining ability can be achieved. Atkins (1979) followed pedigree method after initial crossing of maintainer and restorer parents. The female lines thus developed revealed good combining ability for grain yield when tested with number of newly developed restorer lines.

Manivasakam and Kamalanathan (1987) revealed that mean and variability are the important factors for selection for per se performance. Mean serves as a basis for eliminating undesirable crosses and variability indicates the extent of recombinational variability available for initiating effective selection. They have emphasized the point that selection for the improvement of quantitative characters can be effective only when the segregating generations possess the potential variability. Large variability was noticed in segregating generation for yield and related traits in cotton by many workers viz., El-Gohary et al. (1985), Mahdy et al. (1987), Manivasakam and Kamalanathan (1987), Mirakhamedov et al. (1987), Mehla et al. (1988), Simongulyan and Kim (1990), Munasov et al. (1990), Tagiev (1991), Virk et al. (1991), Subramanyam et al. (1991), Akhmedov (1991), Dever and Gannaway (1992), Akumuradov and Chapau (1992); Verma et al. (2004) and Manickam and Gururajan (2004).

In cotton, there are few studies on creation and exploitation of variability for combining ability. Mallikarjun (2005) observed presence of variability for combining ability in F₄ population of two crosses. However, in many studies creation of variability for per se performance with respect to different quantitative characters is demonstrated. In this approach (Patil and Patil, 2003) in a crop like cotton diverse single cross F_1s can be identified and utilized as sources for initiating reciprocal selection for combining ability in the segregation generations of the two crosses. The genetically diverse single crosses were identified based on predicted double cross performance. The average performance of non-parental crosses was used to predict distance between the single crosses (Patil et al., 2004). Both gca and sca effects contributing to superiority of two crosses indicated that procedures such as reciprocal selection for combining ability can be utilized to enhance the heterosis level (Patil et al., 2004).

The conventional pedigree method and other breeding procedures have been successfully used for improving the per se performance with respect to yield and yield components. These approaches have not been utilized to improve combining ability as a trait in cotton. In the present study, it was hence, decided to create recombinational variability for combining ability and to assess the nature and magnitude of variability for combining ability in F₄ generation. The genotypic differences of parents used in developing populations influences magnitude and nature of recombinational variability released for different traits. There is a need of potential differences between parents to ensure larger and more useful variability released in the populations developed. Keeping this in mind, the base material of the present study was identified from the detailed initial study involving large number of crosses. In this study, two barbadense and four hirsutum lines giving best hybrid (H×B), combinations between them were selected. To create recombinational variability, the two barbadense genotypes were crossed to get F₁ and it was advanced to F₄ generation. In the present phase of this continuing study, the F₄ lines of this population (barbadense x barbadense) is utilized for assessing recombinational variability for combing ability against selected hirsutum testers. Nature and magnitude of variability for combining ability was assessed against hirsutum tester included in the heterotic box.

Keeping these aspects in view following objectives were framed for the present study.

Exploitation of heterotic groups by creation of recombinational variability in G.barbadense F₄ population for ability to combine with selected diverse G. hirsutum testers.

To estimate the per se performance of hirsutum and barbadense for developing potential inter specific hybrids.

1 Results and Discussion
This study was aimed at evaluating recombinational variability for combining ability in F₄ generation. To assess variability for combining ability, twenty eight F₄ (Gossypium barbadense L.) lines were crossed with four common diverse testers (Gossypium hirsutum L.) viz., DH 98-27 (T1), ZCH8 (T2), 178-24 (T3) and DH 18-31 (T4) for use in assessing the variability for combining ability.

1.1 Analysis of variance (RBD)
The preliminary RBD analysis was carried out for fourteen characters under study for all genotypes involved in the present investigation viz., 112 crosses (Line x Tester), 28 lines, 4 testers , two commercial checks (MRC 6918 Bt check and DCH 32 non Bt check) and eight bench mark crosses. Mean sum of squares for fourteen characters are presented in Table 1. ‘F’ test indicated highly significant variation among the genotypes for all the characters and only significant for number of sympodia per plant.

Table 1 Analysis of variance for different quantitative characters (RBD) in derived F1 crosses

 
1.2 Mean per se performance
Mean per se performance of four hirsutum females and 28 barbadense males (Table 2) and derived F1 crosses and commercial checks (Table 3) are presented below.

Table 2 Per se performance of 28 barbadense lines and 4 hirsutum testers for different quantitative characters

Table 3 Per se performance of derived F1 crosses for different quantitative characters

 
1.3 Seed cotton yield (kg ha^-1)
Seed cotton yield values ranged from 1368.15 [DB 533 x DB 534 F5 IPS 49] to 441.81 [DB 533 x DB 534 F5 IPS 33] among males/ lines, 2503.93 [DH 98-27] to 1870.91 [178-24] among females/testers and 2884.26 [DH 98-27 X (DB 533 x DB 534 F4 IPS 49)] to 1146.20 [178-24 X (DB 533 x DB 534 F4 IPS 33)] among derived F1 crosses. Per cent heterosis of F1 crosses over their respective mid parental values ranged from 108.16 [DH 98-27 X (DB 533 x DB 534 F4 IPS 49)] to -38.94 [178-24 X (DB 533 x DB 534 F4 IPS 33)]. Thirty crosses showed significant positive heterosis and only one cross showed significant negative heterosis over their mid parent. The cross DH 98-27 X (DB 533 x DB 534 F4 IPS 49) (39.97) recorded highest significant positive heterosis over commercial check MRC 6918 and the cross 178-24 X (DB 533 x DB 534 F4 IPS 33) (-44.37) exhibited lowest significant negative heterosis over MRC 6918. Two crosses showed significant heterosis in positive direction and two crosses showed significant heterosis in negative direction over MRC 6918. In case of DCH 32 non Bt check, the cross DH 98-27 X (DB 533 x DB 534 F4 IPS 49) (44.31) showed highest significant positive heterosis over this check, but the cross 178-24 X (DB 533 x DB 534 F4 IPS 33) (-42.65) recorded lowest significant negative heterosis value over DCH 32 commercial check. Two crosses exhibited significant positive heterosis over DCH 32 commercial check and only one cross showed significant negative heterosis. Majority of workers viz., Tuteja et al. (1996), Doss and Kadambavanasundaram (1997), Siruguppa and Parameswarappa (1998), Neelima (2002) and Potdukhe (2002) also reported heterosis over mid parent.

1.4 Plant height (cm)
Plant height values ranged from 100.33 [DB 533 x DB 534 F5 IPS 62] to 65.33 [DB 533 x DB 534 F5 IPS 34] among males/ lines, 100.67 [DH 18-31] to 93.50 [178-24] among females/testers and 143.83 [DH 98-27 X (DB 533 X DB 534 F4 IPS 24)] to 84.67 [DH 18-31 X (DB 533 X DB 534 F4 IPS 24)] among derived F1 crosses.

1.5 Number of monopodia per plant
Number of monopodia per plant values ranged from 2.50 [DB 533 x DB 534 F5 IPS 6] to 0.33 [DB 533 x DB 534 F5 IPS 12] among males/ lines, 1.90 [ZCH 8] to 0.50 [DH 18-31] among females/testers and 2.44 [178-24 X (DB 533 x DB 534 F4 IPS 71)] to 1.11 [ZCH 8 X (DB 533 x DB 534 F4 IPS 15) and ZCH 8 X (DB 533 x DB 534 F4 IPS 26)] among derived F1 crosses.

1.6 Number of sympodia per plant
Number of sympodia per plant values ranged from 21.17 [DB 533 x DB 534 F5 IPS 62] to 13.83 [DB 533 x DB 534 F5 IPS 32] among males/ lines, 22.83 [DH18-31] to 18.33 [178-24] among females/testers and 25.17 [178-24 X (DB 533 x DB 534 F4 IPS 14) and DH 18-31 X (DB 533 x DB 534 F4 IPS 13)] to 16.00 [DH 18-31 X (DB 533 x DB 534 F4 IPS 17)] among derived F1 crosses.

1.7 Number of bolls per plant
Number of bolls per plant values ranged from 39.33 [DB 533 x DB 534 F5 IPS 62] to 24.67 [DB 533 x DB 534 F5 IPS 34] among males/ lines, 34.67 [DH 98-27] to 24.50 [178-24] among females/testers and 66.17 [DH 98-27 X (DB 533 x DB 534 F4 IPS 36)] to 34.50 [ZCH 8 X (DB 533 x DB 534 F4 IPS 14)] among derived F1 crosses.

1.8 Mean boll weight (g)
Mean boll weight values ranged from 3.23 [DB 533 x DB 534 F5 IPS 30] to 1.67 [DB 533 x DB 534 F5 IPS 33] among males/ lines, 4.45 [178-24 and DH 18-31] to 3.95 [ZCH 8] among females/testers and 4.20 [DH 98-27 X (DB 533 x DB 534 F4 IPS 33)] to 2.35 [178-24 X (DB 533 x DB 534 F4 IPS 12)] among derived F1 crosses.

1.9 Reproductive points on sympodia
Reproductive points on sympodia values ranged from 5.25 [DB 533 x DB 534 F5 IPS 62] to 3.08 [DB 533 x DB 534 F5 IPS 1] among males/ lines, 3.89 [DH18-31] to 3.08 [178-24] among females/testers and 5.28 [DH 98-27 X (DB 533 x DB 534 F4 IPS 25)] to 1.92 [DH 18-31 X (DB 533 x DB 534 F4 IPS 48)] among derived F1 crosses.

1.10 Sympodial length at 50 per cent plant height (cm)
Sympodial length at 50 per cent plant height values ranged from 39.75 [DB 533 x DB 534 F5 IPS 62] to 24.17 [DB 533 x DB 534 F5 IPS 8] among males/ lines, 38.92 [DH 18-31] to 28.50 [DH 98-27] among females/testers and 54.92 [DH 98-27 X (DB 533 x DB 534 F4 IPS 44)] to 24.42 [DH 18-31 X (DB 533 x DB 534 F4 IPS 8)] among derived F1 crosses.

1.11 Inter branch distance (cm)
Inter branch distance values ranged from 9.33 [DB 533 x DB 534 F5 IPS 6] to 6.17 [DB 533 x DB 534 F5 IPS 16] among males/ lines, 9.17 [178-24 and DH 18-31] to 8.67 [DH 98-27 and ZCH 8] among females/testers and 9.50 [178-24 X (DB 533 x DB 534 F4 IPS 13)] to 5.33 [ZCH 8 X (DB 533 x DB 534 F4 IPS 17)] among derived F1 crosses.

1.12 Seed index (g)
Seed index values ranged from 10.94 [DB 533 x DB 534 F5 IPS 34] to 6.41 [DB 533 x DB 534 F5 IPS 62] among males/ lines, 11.67 [178-24] to 9.72 [DH 98-27] among females/testers and 17.50 [DH 98-27 X (DB 533 x DB 534 F4 IPS 32)] to 7.00 [[DH 98-27 X (DB 533 x DB 534 F4 IPS 13)] among derived F1 crosses.

1.13 Ginning outturn (%)
The values of ginning outturn ranged from 34.00 [DB 534 x DB 533 F5 IPS 22] to 24.77 [DB 533 x DB 534 F5 IPS 49] among males/ lines, 39.14 [DH 18-31] to 34.25 [178-24] among females/testers and 35.31 [178-24 X (DB 533 x DB 534 F4 IPS 44)] to 13.81 [ZCH 8 X (DB 533 x DB 534 F4 IPS 33)] among derived F1 crosses.

1.14 Lint index (g)
Lint index values ranged from 4.72 [DB 533 x DB 534 F5 IPS 25] to 2.03 [DB 533 x DB 534 F5 IPS 62] among males/ lines, 6.56 [DH 18-31] to 6.14 [DH 98-27] among females/testers and 7.40 [DH 98-27 X (DB 533 x DB 534 F4 IPS 32)] to 2.71 [ZCH 8 X (DB 533 x DB 534 F4 IPS 30)] among derived F1 crosses.

1.15 Photosynthetic rate (µmol CO₂ m^-2 s^-1)
The values of photosynthetic rate ranged from 27.15 [DB 533 x DB 534 F5 IPS 13] to 16.49 [DB 533 x DB 534 F5 IPS 8] among males/ lines, 29.62 [178-24] to 21.20 [DH18-31] among females/testers and 34.25 [ZCH 8 X (DB 533 x DB 534 F4 IPS 52)] to 3.15 [ZCH 8 X (DB 533 x DB 534 F4 IPS 33)] among derived F1 crosses.

1.16 Stomatal conductance (µmol m^-2 s^-1)
Stomatal conductance values ranged from 2.38 [DB 533 x DB 534 F5 IPS 55] to 0.55 [DB 533 x DB 534 F5 IPS 15] among males/ lines, 1.22 [178-24] to 0.78 [DH18-31] among females/testers and 1.17 [ZCH 8 X (DB 533 x DB 534 F4 IPS 52)] to 0.09 [178-24 X (DB 533 x DB 534 F4 IPS 15)] among derived F1 crosses.

1.17 Transpiration rate (mmol H₂O m^-2 s^-1)
Transpiration rate values ranged from 16.66 [DB 533 x DB 534 F5 IPS 105] to 7.92 [DB 533 x DB 534 F5 IPS 14] among males/ lines, 11.77 [178-24] to 10.09 [DH 98-27] among females/testers and 10.04 [DH 98-27 X (DB 533 x DB 534 F4 IPS 1)] to 2.07 [178-24 X (DB 533 x DB 534 F4 IPS 32)] among derived F1 crosses.

1.18 2.5% S L (mm)
The values of 2.5% SL ranged from 38.16 [DB 533 x DB 534 F5 IPS 34] to 30.54 [DB 533 x DB 534 F5 IPS 25] among males/ lines, 32.52 [178-24] to 27.85 [DH18-31] among females/testers and 36.79 [DH 98-27 X (DB 533 x DB 534 F4 IPS 16)] to 31.13 [DH 98-27 X (DB 533 x DB 534 F4 IPS 105)] among derived F1 crosses.

1.19 Fiber uniformity ratio (%)
Fiber uniformity ratio values ranged from 46.39 [DB 533 x DB 534 F5 IPS 62] to 42.64 [DB 533 x DB 534 F5 IPS 34] among males/ lines, 49.11 [DH 98-27] to 44.31 [178-24] among females/testers and 46.55 [DH 18-31 X (DB 533 x DB 534 F4 IPS 8)] to 42.60 [178-24 X (DB 533 x DB 534 F4 IPS 48)] among derived F1 crosses.

1.20 Fiber micronaire value (g/inch)
Fibre micronaire values ranged from 3.43 [DB 533 x DB 534 F5 IPS 25] to 2.42 [DB 533 x DB 534 F5 IPS 44] among males/ lines, 4.87 [DH 18-31] to 4.19 [178-24] among females/testers and 3.61 [DH 98-27 X (DB 533 x DB 534 F4 IPS 105)] to 2.49 [178-24 X (DB 533 x DB 534 F4 IPS 33)] among derived F1 crosses.

1.21 Fiber maturity ratio (%)
The values of fiber maturity ratio ranged from 0.63 [DB 533 x DB 534 F5 IPS 25] to 0.55 [DB 533 x DB 534 F5 IPS 44, DB 533 x DB 534 F5 IPS 32 and DB 533 x DB 534 F5 IPS 38] among males/ lines, 0.72 [DH18-31] to 0.68 [178-24] among females/testers and 0.67 [MRC6918] followed by the cross DH 98-27 X (DB 533 x DB 534 F4 IPS 71) (0.65) to 0.54 [DCH32] followed by the cross ZCH 8 X (DB 533 x DB 534 F4 IPS 30) (0.55) among derived F1 crosses.

1.22 Fiber strength (Tenacity) (g/tex)
Fiber strength values ranged from 30.37 [DB 533 x DB 534 F5 IPS 44] to 26.52 [DB 533 x DB 534 F5 IPS 52] among males/ lines, 22.90 [DH 98-27] to 20.01 [DH 18-31] among females/testers and 31.22 [ZCH 8 X (DB 533 x DB 534 F4 IPS 30)] to 22.91 [DH 98-27 X (DB 533 x DB 534 F4 IPS 105)] among derived F1s.

1.23 Fiber elongation (%)
Fiber elongation values ranged from 6.84 [DB 533 x DB 534 F5 IPS 105] to 6.27 [DB 533 x DB 534 F5 IPS 62] among males/ lines, 5.88 [ZCH 8] to 5.41 [DH18-31] among females/testers and 6.85 [178-24 X (DB 533 x DB 534 F4 IPS 49)] to 5.88 [DH 98-27 X (DB 533 x DB 534 F4 IPS 105)] among derived F1 crosses.

1.24 Efficiency of tester (S)
Choice of tester in assessing the combining ability of segregating lines is one of the important aspects in hybrid breeding programme. Though this aspect should assume equal importance in all the crops where hybrid improvement is practiced, the studies on choice of testers, description of ideal tester etc. are limited to maize. In maize different authors have expressed different views on efficiency of tester in assessing combining ability.

Hull (1945) visualized a model of over dominance responsible for heterosis and suggested that, tester with homozygous recessive alleles at all loci would be most effective. He used variance among derived F1 crosses as an index of measuring efficiency of a tester. Rawlings and Thompson (1962) extended this idea and suggested that, testers with low per se performance would be most effective because of accumulated recessive alleles (homozygocity) at most loci. Rawlings and Thompson (1962) and Allison and Curnow (1966) suggested even for a repeated selection for low yield within a population to develop good testers. However, this concept was counter argued as not effective by different maize workers (Hallauer and Miranda, 1981). This group of workers argued that, testers with poor performance would have lot of disadvantages in applied breeding programmes and as an alternative they suggested an unrelated elite tester may be more effective in evaluating lines. An elite unrelated tester has an advantage of identifying the lines with good combining ability and the lines identified in this manner can be directly used in breeding programmes.

Several studies have been conducted to compare the testers with broad and narrow genetic base (Horner et al., 1973; Russell et al., 1973 and Walejko and Russell, 1977). Russell and Eberhart (1975) proposed that, in receprocal recurrent selection an inbred line derived from opposite population would be ideal choice as a tester, while Comstock (1979) suggested that the opposite population would be efficient tester. Ultimately all these studies indicated that, the lines or populations which are genetically diverse can act as efficient testers. The effective way of identifying genetically diverse tester should again be based on per se performance of derived F1 crosses (test cross F1s). Patil (1995) used two parameters i.e., mean of derived F1 (test cross F1s) and variance of these group of F1 crosses in qualifying the efficiency of testers.

Applying this approach in present study, twenty eight F4 lines were selected and they were crossed to four testers to obtain 112 derived F1 crosses. Based on mean and its variability parameters comparison was made (Table 4) among the testers to assess efficiency of testers in distinguishing combining ability of F4 lines. The set of 28 lines distinguished for the ability to combine with each tester. The efficiency of tester in distinguishing of these F4 lines was determined based on mean seed cotton yield of 28 crosses and co-efficient of variability. In derived F1 crosses, tester DH 98-27 (T1) revealed high mean and also higher co-efficient of variance as compared to the other three testers for seed cotton yield. Both DH 98-27 and DH 18-31 (Plate 1) were found to be efficient testers on the basis of high mean and high co-efficient variance. Considering mean and co-efficient variance as parameters T1 was found to be more efficient in distinguishing the barbadense F4 lines for their combining ability for utilized to distinguish lines.

Table 4 Efficiency of testers against the F4 barbadense lines in terms of seed cotton yield (kg/ha)

Plate 1 Most productive hirsutum testers included in recombinational variability study

 
1.25 Sub-grouping and characterization of lines for combining ability
Based on the mean (of all the crosses), the group of crosses were classified into four classes and given the ranking 1 (>mean + SD), 2 (mean to mean + SD), 3 (mean to mean – SD) and 4 (<mean – SD) as suggested by Patil (1995). Each line was characterized with respect to its combining ability status (pattern) considering the four testers, with the help of this information, the nature and magnitude of variability for combining ability was assessed. The information obtained on the four testers DH 98-27 (A), ZCH8 (B), 178- 24 (C) and DH 18-31 (D) are presented separately.

A method of sub-grouping the F4 lines against each tester was done. Based on this elite combiner F4 lines were identified against each tester. This approach helped in sub- grouping the F4 lines against a pair of hirsutum testers and identifying lines for deriving sub populations against hirsutum testers. By following the method of determining the combining ability pattern given above each line of F4 barbadense lines was categorized and compared with the other lines.

Based on the graphical presentation given for tester DH 98-27 (T1)/A (Figure 1), it is evident that entire group of lines revealed large variability for ability to combine with this tester. The F4 lines combine better with the tester DH 98-27 as evidenced by the higher frequency of crosses falling under the category A1 [DB 533 x DB 534 F4 IPS 49, DB 534 x DB 533 F4 IPS 22, DB 533 x DB 534 F4 IPS 52, DB 533 x DB 534 F4 IPS 105, DB 533 x DB 534 F4 IPS 17, DB 533 x DB 534 F4 IPS 16 and DB 533 x DB 534 F4 IPS 48]. Whereas, DB 533 x DB 534 F4 IPS 49 alone line was belonging to this higher combiner category against this common tester DH 98-27 (T1)/A.

Figure 1 Sub-grouping of recominant population based on cross performance with hirsutum testers DH 98-27, ZCH 8, 178-24 and DH 18-31

 
The F4 barbadense lines reveal large variability in their combinations with the tester ZCH 8 (T2)/B, as evidenced by higher frequency of crosses falling under this sub-group B1 [DB 533 x DB 534 F4 IPS 49, DB 534 x DB 533 F4 IPS 22, DB 533 x DB 534 F4 IPS 52, DB 533 x DB 534 F4 IPS 105, DB 533 x DB 534 F4 IPS 17 and DB 533 x DB 534 F4 IPS 16] also belonging to this higher combining category against the tester ZCH8 (T2)/B.

With respect to tester 178-24 (T3)/C, it is observed that entire group of lines revealed better variability for ability to combine with this tester. Among the groups of lines tested, the F4 lines combine better with the 178-27 (T3)/C as evidenced by the higher frequency of crosses falling under the category C1 [DB 533 x DB 534 F4 IPS 49 and DB 534 x DB 533 F4 IPS 22]. Whereas, DB 533 x DB 534 F4 IPS 49 were belonging to this higher combiner category against this common tester 178-24 (T3)/C.

With respect to tester DH 18-31 (T4)/D, it is observed that entire group of lines revealed better variability for ability to combine with this tester. Among the groups of lines tested, the F4 lines combine better with the DH 18-31 (T4)/D as evidenced by the higher frequency of crosses falling under the category D1 [DB 533 x DB 534 F4 IPS 49, DB 534 x DB 533 F4 IPS 22, DB 533 x DB 534 F4 IPS 52 and DB 533 x DB 534 F4 IPS 17]. Whereas, DB 533 x DB 534 F4 IPS 49 were belonging to this higher combiner category against this common tester DH 18-31 (T4)/D.

Based on the graphical presentation given for tester DH 98-27 (T1)/A and ZCH8 (T2)/ B (Figure 2), it is evident that lines viz., DB 533 x DB 534 F4 IPS 49, DB 534 x DB 533 F4 IPS 22, DB 533 x DB 534 F4 IPS 52 and DB 533 x DB 534 F4 IPS 105 revealed large variability for ability to combine with these two testers. The lines DB 533 x DB 534 F4 IPS 49 and DB 534 x DB 533 F4 IPS 22 exhibited large variability for ability to combine with two hirsutum testers DH 98-27 (T1)/A and 178-24 (T3)/C, ZCH8 (T2)/B and 178-24 (T3)/C, ZCH8 (T2)/B and DH 18-31 (T4)/D and 178-24 (T3)/C and DH 18-31 (T4)/D. While, the barbadense lines DB 533 x DB 534 F4 IPS 49, DB 534 x DB 533 F4 IPS 22 and DB 533 x DB 534 F4 IPS 52 recorded large variability for ability to combine with two hirsutum testers DH98-27 (T1)/A and DH 18-31 (T4)/D. Both the lines DB 533 x DB 534 F4 IPS 49 and DB 534 x DB 533 F4 IPS 22 have combined very well with four hirsutum testers. This suggest that these two barbadense lines can recombined to initiate second phase of creating recombinational variability for combining ability. The segregating F4 lines obtained from this cross can be cross to these four hirsutum testers and especially DH 18-31 and DH 98-27 because the mean of crosses with T1 and T4 testers is very high.

Figure 2 Sub-grouping of recombinant population based on cross performance against two hirsutum testers (ZCH 8 and DH98-27, 178-24 and DH 98-27, DH 18-31 and DH 98-27, 178-24 and ZCH 8, DH 18-31 and ZCH 8 and DH 18-31 and 178-24

 
1.26 Transgressive segregation for combining ability
The per se performance and per cent superiority of derived F1 crosses over superior bench mark crosses (SBMC) and per cent inferiority of derived F1 crosses over inferior bench mark crosses (IBMC) for seed cotton yield were presented in Table 5.

Table 5 Mean and per cent improvement of derived F1 crosses involving F4 lines of cross bench mark crosses for seed cotton yield

 
1.27 With tester T1 (DH 98-27)
With tester DH 98-27 (T1) the per cent superiority of derived F1 crosses over superior straight cross produced as many as 28 positive transgressive segregants. Among these DB 533 x DB 534 F4 IPS 49 (78.36), DB 534 x DB 533 F4 IPS 22 (73.20), DB 533 x DB 534 F4 IPS 52 (63.06) and DB 533 x DB 534 F4 IPS 105 (61.19) F4 lines have produced significant transgressive segregants. Whereas, there are no negative transgressive segregants over inferior straight cross for combining ability. When the performance of derived F1 crosses was compared with superior bench mark crosses (SBMC) on the basis of per cent superiority (improvement in combining ability), it was evident that the derived F1 crosses expressed substantially high amount of superiority over superior straight cross. This indicates that the segregants showed improvement in their ability to combine with tester concerned in desirable direction. It also indicated that the segregants are genetically more diverse from tester T1. The segregants have genotypic constitution complimentary to that found in the tester T1. In other words, they accumulate more number of dominant favourable alleles which impart diversity to their genetic constitution as compared to the genetic constitution of the tester T1 (DH 98-27).

Further, the most practical measure of the potentiality of derived F1 crosses is obtained by studying the heterosis over commercial check (standard heterosis).The heterosis values of derived F1 crosses over commercial checks (MRC 6918 and DCH 32), only two of the derived F1 crosses were significantly superior over the two commercial checks. This indicates that more number of F1 crosses are inferior over commercial check.

1.28 With tester T2 (ZCH 8)
Along with tester ZCH 8 (T2), the per cent superiority of derived F1 crosses over superior straight cross had produced fifteen positive transgressive segregants. Among these, DB 533 x DB 534 F4 IPS 49 (31.62), DB 534 x DB 533 F4 IPS 22 (23.51) and DB 533 x DB 534 F4 IPS 52 (19.34) F4 lines have produced highest positive transgressive segregants. Whereas, per cent inferiority of derived F1 crosses over inferior straight cross had produced 5 negative transgressive segregants. Comparing the mean yields of derived F1 crosses and superior straight cross, it was evident that the derived F1 crosses expressed substantially marginal amount of superiority over superior bench mark crosses (SBMC). This reflected the extent of transgressive segregation for combining ability occurring in desirable direction. Despite all, along with superior straight crosses, five derived F1 crosses showed negative inferiority over inferior straight cross. This proved the transgressive segregation for combining ability occurring in undesirable direction also.

The heterosis values of derived F1 crosses over commercial checks (MRC 6918 and DCH 32), non of the derived F1 crosses were shown significant superior over the two commercial checks.

1.29 With tester T3 (178-24)
With tester 178-24 (T3), the per cent superiority of derived F1 crosses over superior straight cross produced as many as 16 positive transgressive segregants. Among these, DB 533 x DB 534 F4 IPS 49 (23.49), DB 534 x DB 533 F4 IPS 22 (17.42) and DB 533 x DB 534 F4 IPS 52 (13.43) F4 lines have produced highest transgressive segregants. Whereas, the per cent inferiority of derived F1 crosses over inferior straight cross revealed four negative transgressive segregants. Comparing the mean yields of derived F1 crosses over superior straight cross, it was evident that the derived F1s expressed marginal amount of superiority over superior straight cross indicating presence of genetic diversity between the F4 lines and the concerned tester. This indicated the extent of transgressive segregation for combining ability in desirable direction.
 
Two of the derived F1 crosses were significantly superior over Bt commercial check (MRC 6918) and one cross showed significantly superior over non Bt commercial check (DCH 32).

1.30 With tester T4 (DH 18-31)
Along with tester DH 18-31 (T4), the per cent superiority of derived F1 crosses over superior straight cross had produced 28 positive transgressive segregants. Among these, eighteen F4 lines have significant transgressive segregants and DB 533 x DB 534 F4 IPS 49 (56.01), DB 534 x DB 533 F4 IPS 22 (55.17) and DB 533 x DB 534 F4 IPS 52 (54.42) F4 lines recorded highest value of positive transgressive segregants. Whereas, there are no negative transgressive segregants for combining ability.Comparing the mean yields of derived F1 crosses and superior straight cross, it was evident that the derived F1 crosses expressed substantially high amount of expression. This reflected the extent of transgressive segregation for combining ability occurring in desirable direction. Despite all, along with superior straight crosses, non of derived F1 crosses hybrids showed negative inferiority over inferior straight cross. This proved the transgressive segregation for combining ability occurring in undesirable direction also.

The heterosis values of derived F1 crosses over commercial checks (MRC 6918 and DCH 32), non of the derived F1 crosses were shown significant superior over the two commercial checks.

In general, the derived F1 crosses expressed substantially high amount of transgressive segregation for combining ability for DH 98-27 (T1) and DH 18-31 (T2) testers. Among the population used in the combining ability study, DB 533 x DB 534 F4 IPS 49, DB 534 x DB 533 F4 IPS 22 and DB 533 x DB 534 F4 IPS 52 F4 lines combined very well with all the four testers. Being F4 lines productive with all the four testers over DB 533 and DB 534 itself indicates that when even good combiner parents can be used for creating recombinational variability. These parents reveal diversity in the identity of favourable alleles contained in them. We can expect segregation at more number of yield influencing dominant loci. As a result of this variability is released at more number of loci and thus recombinant lines having desirable blend of favourable alleles distributed between two parents are obtained such lines give rise to superior derived F1 crosses.

It also indicated that in general, the segregants are genetically more diverse from tester as compared to the two parents. It can be taken to infer that segregants used as female plants have the genotypic constitution complimentary to that found in the tester(s). In other words, they accumulated more number of such dominant favourable alleles, which impart diversity to their genetic constitution as compared to the genetic constitution of the testers used. Thus, the difference in performance seen between straight crosses and derived F1 crosses is attributed to greater genetic diversity existing between the tester(s) and segregants.

These results of present study are in confirmity with earlier studies in sorghum of Sridhar (1991) and Desai (1991), which also observed increased genetic diversity among the F2 and F3 segregants from a given tester as compared to the parents. The genetic diversity is very essential in making the derived F1 crosses or for that matter any F1 heterotic (Falconer, 1981). The value of F1 crosses depends on the magnitude of dominance and genetic diversity as revealed by the formula HF1 = dy2, where ‘d’ is dominance effect and ‘y’ represents genetic diversity between the parents, when straight crosses and derived F1 crosses are compared the difference in their performance is attributed to greater heterosis existing between the tester and segregants. This study clearly depicts that by crossing two genotypes we can obtain many segregants which accumulate such favourable combinations of alleles which make them genetically more diverse from a tester than the two original parents involved. There is a need to test the stability of combining ability of superior lines in succeeding advanced generations. In addition to this, it is required to confirm the superiority of the promising crosses for their yield potentiality by testing them in large scale plots.

2 Material and Methods
2.1 Choice of the material
To create recombinational variability for combining ability, the elite barbadense lines DB 533 and DB 534 were crossed during 2007-2008. During two seasons 2008-2009 and 2009-2010 these barbadense crosses were advanced to F2 and F3 generations, respectively. The F3 lines were evaluated for productivity and fiber quality parameters realizing the emphasis laid on developing ELS (Extra Long Stable) cotton hybrids out of 171 F3 lines, only those F3 lines with acceptable fiber strength were utilized in the study on recombinational variability of combining ability. During 2010 -2011 those twenty eight F4 lines of barbadense cross DB 533 × DB 534 depending on the higher value of fiber tenacity, were crossed with the selected four hirsutum testers viz., DH 98-27 (T1), ZCH 8 (T2), 178-24 (T3) and DH 18-31 (T4) selected based on earlier study. Each barbadense F4 line was involved in a set of crosses (112 crosses refer to as derived F1 crosses) were subjected to Line x Tester analysis.

2.2 Crossing Programme
The crossing programme was taken up during 2010. The F4 lines and four common testers were sown on staggered dates. To obtain derived F1 crosses seeds, the flower buds of the proper size from testers (used as female) were hand emasculated in the evening between 3.00 to 6.00 pm. The emasculated flowers were covered by butter paper packets for avoiding out crossing as well as ensuring their easy identification at the time of crossing. The emasculated flowers were pollinated during the next day morning between 9.30 am and 11.30 am by brushing the pollen from one of the F4 lines (used as male) on the stigmatic surface. The pedicel of each pollinated flower was tied with price label bearing date and lines number for identification of crossed bolls. In this manner derived F1 crosses seeds were obtained. Simultaneously, the barbadense population of F4 lines was selfed and material was advanced to F5 generation during the same year.

2.3 Evaluation of derived F1 crosses and F5 barbadense lines
There was a need for improving performance of inter specific hybrids. This was possible through genetic improvement of barbadense varietal lines. So that both productivity and fiber quality of barbadense were improved. An improved barbadense varietal base is essential for improving performance of inter specific hybrids. The entire experimental material was planted on a medium black soil at College of Agriculture, Dharwad under irrigated condition. All the 53 F5 (included Suvin variety as check) lines, four hirsutum testers and derived F1 crosses along with the straight crosses (Bench Mark Crosses (BMC)) and ruling commercial checks (MRC 6918 Bt check and DCH 32 non Bt check) were sown during kharif 2011 in a Randomized Block Design with two replications and a spacing of 90 cm between rows and 60 cm between the plants within a row. Recommended fertilizer doses were applied and other cultural practices were carried out at regular interval. Plant protection measures were taken at appropriate time to control pests and diseases. To facilitate Line Tester analysis, the crosses obtained were randomized and were sown in one block along with checks, bench mark crosses and parents were sown in adjoining block.

3 Conclusion
In the study on recombinational variability for combining ability the most potential crosses were identified viz., DH 98-27 X (DB 533 x DB 534 F4 IPS 49), DH 98-27 X (DB 534 x DB 533 F4 IPS 22) and DH 98-27 X (DB 533 x DB 534 F4 IPS 52) will be tested in multi location trials on larger plot size to confirm their yield potential and to know their stability of performance over different agro-climatic situations. It is possible to release them based on confirmation of their potentiality.

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