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.

Heterosis is defined as the increased vigour of the F₁ generation over the mean of the parents or over the better parent (Hayes et al., 1955). Shull (1914) first coined the term heterosis. Heterosis has been observed for yield and other characters in cotton by many workers. Commercial exploitation of hybrid vigour in cotton has been successful in India with release of Hybrid 4 in 1969.

Heterosis produced by the joint effects of all the loci as the sum of their separate contributions can be represented by the formula (Falconer, 1981).

HF1 = ∑dy²

Where,
d = Magnitude of dominance
y = Allelic frequency differences at a locus in the parental populations

The genetic causes involved in the expression of heterosis are dominance and non-allelic interactions (Hayes and Foster, 1976). The magnitude of heterosis can be maximized if the parents are genetically diverse from each other. Parents should differ for maximum number of yield influencing loci so that F₁ exhibits the dominance effect at as many of the yield influencing loci as possible.

Heterosis works as a basic tool for improvement of crops in form of F₁ and F₂ populations, and economic heterosis (over standard cultivar). It also contributes to choose genotypes with desired genetic variance, vigor and maternal effects. Therefore, it is essential to have detailed information about desirable parental combiners in any breeding program, which can reflect a high degree heterotic response. In intra- and inter-specific heterosis, yield increase over better parent or greater than best commercial cultivar (useful heterosis) has been documented (Baloch et al., 1993; Galanopoulou and Roupakias, 1999; Wei et al., 2002; Yuan et al., 2001 & 2002; Khan et al., 2007; Khan, 2011). Both positive and negative heterotic values have been detected, demonstrating potential of hybrid combinations for traits improvement in breeding programs (Hassan et al., 1999; Khan et al., 2009). F1 hybrids with high heterosis were also associated with higher inbreeding depression; therefore, moderate type of heterosis has some stability in segregating populations (Tang et al., 1993; Soomro, 2000; Soomro and Kalhoro, 2000). Therefore, heterotic studies can provide basis for exploitation of valuable hybrid combinations in future breeding program.

The main objective of this study to study the heterosis of cotton inter specific crosses in respect of seed cotton yield and its attributing characters.

1 Results and Discussion
Heterosis is the superiority of F₁ over the mean of the parents or over the better parent or over the standard check (Hays et al., 1956), with respect to agriculturally useful traits. The primary objective of heterosis breeding is to achieve a quantum jump in yield and quality of crop plants.

Cotton improvement programmes primarily lay emphasis on development of hybrids, which have contributed in improving the productivity of cotton. Hybridization is the most potent technique for breaking yield barriers. Selection of parents on the basis of phenotypic performance alone is not a sound procedure, since phenotypically superior lines may yield poor combinations. It is therefore essential that parents should be chosen on the basis of their combining ability. Combining ability analysis is the most widely used biometrical tool for identifying prospective parents and for formulating breeding procedures most likely to succeed.

Results of heterosis values over mid parent and commercial checks for various characters were studied to assess the variability for combining ability were given in Table 1 and 2.

Table 1 Heterosis over mid parent and commercial checks for plant height, no of monopodia, no of sympodia, no of bolls and mean boll weight traits in derived F1 crosses

Table 2 Heterosis over mid parent and commercial checks for reproductive points on sympodia, length of sympodia at 50% height, inter branch distance, seed index and ginning outturn in derived F1 crosses

 
1.1 Seed cotton yield (kg·ha^-1)
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. 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. 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.

1.2 Plant height (cm)
Plant height is one of the important character of growth and development of the cotton canopy. There was increase in number of sympodia per plant and number of bolls per plant with increase in plant height. Per cent heterosis of F₁ crosses over their respective mid parental values ranged from 68.57 [ZCH 8 X (DB 533 x DB 534 F₄ IPS 14)] to -6.96 [DH 98-27 X (DB 533 x DB 534 F₄ IPS 14)]. Majority of the crosses exhibited significant heterosis over mid parent in positive direction. Significant positive revealed by 51 crosses. Significant and positive heterosis for plant height was earlier reported by Singh and Narayanan (1993), Bhatade and Rajeshwar (1994), Sambamurthy et al. (1995) and Rajput et al. (1997).

The cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 17) (33.80) showed highest significant positive heterosis over commercial check MRC 6918, the cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 14) (-21.24) exhibited lowest heterosis over MRC 6918 and only three crosses showed significant heterosis in positive direction. In case of DCH 32 non Bt check, the cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 17) (23.46) showed highest positive heterosis over this check, but the cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 14) (-27.33) recorded lowest significant negative heterosis value over DCH 32 commercial check. Only one cross had significant negative heterosis over DCH 32 commercial check.

1.3 Number of monopodia per plant
The monopodial branches utilize more energy and delay the reproductive parts. More vegetative branches are undesirable and any genetic parameter in negative direction is desirable. Per cent heterosis of F₁ crosses over their respective mid parental values ranged from 446.82 [178-24 X (DB 533 x DB 534 F₄ IPS 38)] to -41.58 [DH 98-27 X (DB 533 x DB 534 F₄ IPS 62)]. Majority of the crosses exhibited significant heterosis over mid parent in positive direction. Seventy two crosses showed significant positive heterosis and three crosses revealed significant negative heterosis over their mid parent. In many crosses, heterosis over mid parent has been observed as significant positive heterosis. Similar reports were made by Reddy (2001), Neelima (2002) and Potdukhe (2002).

The cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 23) (22.00) recorded highest positive heterosis over Bt check MRC 6918, the crosses DH 98-27 X (DB 533 x DB 534 F₄ IPS 62) (-44.50) and DH 98-27 X (DB 533 x DB 534 F₄ IPS 34) (-44.50) exhibited lowest negative significant heterosis over MRC 6918 and twenty eight crosses showed significant negative heterosis. In case of DCH 32 check, the cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 23) (46.55) showed highest significant positive heterosis over this check, but the crosses DH 98-27 X (DB 533 x DB 534 F₄ IPS 62) (-33.33) and DH 98-27 X DB 533 x DB 534 F₄ IPS 34 (-33.33) recorded lowest significant negative heterosis value over DCH 32 commercial check. Six crosses had significant negative heterosis and eighteen crosses exhibited significant positive heterosis over DCH 32 commercial check.

1.4 Number of sympodia per plant
Per cent heterosis of F1 crosses over their respective mid parental values ranged from 52.83 [DH 18-31 X (DB 533 x DB 534 F₄ IPS 15)] to -17.95 [DH 98-27 X (DB 533 x DB 534 F₄ IPS 71)]. Thirteen crosses showed significant positive heterosis over their mid parent. Significant heterosis over mid parent for this character was observed by Reddy (2001), Neelima (2002) and Potdukhe (2002).

The crosses DH 98-27 X (DB 533 x DB 534 F₄ IPS 30) (29.08) and DH 18-31 X (DB 533 x DB 534 F₄ IPS 71) (29.08) recorded highest positive heterosis over commercial check MRC 6918 and the cross 178-24 X (DB 533 x DB 534 F₄ IPS 15) (-17.95) exhibited lowest negative heterosis over MRC 6918. There is no significant heterosis respect to this trait over MRC 6918 Bt commercial check between crosses. In case of DCH 32 non Bt check, the crosses DH 98-27 X (DB 533 x DB 534 F₄ IPS 30) (17.07) and DH 18-31 X (DB 533 x DB 534 F₄ IPS 71) (17.07) showed highest positive heterosis over this check, but the cross 178-24 X (DB 533 x DB 534 F₄ IPS 15) (-25.58) recorded lowest negative heterosis value over DCH 32 commercial check. There is no significant of heterosis respect to this trait over DCH 32 commercial check between crosses.

1.5 Number of bolls per plant
Per cent heterosis of F1s over their respective mid parent values ranged from 135.61 [ZCH 8 X (DB 533 x DB 534 F₄ IPS 30)] to -5.27 [DH 18-31 X (DB 533 x DB 534 F₄ IPS 33)]. Sixty nine crosses showed significant positive heterosis over their mid parent. Siruguppa and Parameswarappa (1998), Reddy (2001), Neelima (2002) and Potdukhe (2002) also revealed the heterosis over mid parent for this trait.

The cross ZCH 8 X (DB 533 x DB 534 F₄ IPS 30) (40.78) recorded highest significant positive heterosis over commercial check MRC 6918 and the cross DH 18-31 X (DB 533 x DB 534 F₄ IPS 33) (-26.60) exhibited lowest negative heterosis over MRC 6918. Only one cross showed significant heterosis over MRC 6918 in positive direction. In case of DCH 32 check, the cross ZCH 8 X (DB 533 x DB 534 F₄ IPS 30) (59.45) showed highest significant positive heterosis over this check, but the cross DH 18-31 X (DB 533 x DB 534 F₄ IPS 33) (-16.86) recorded lowest negative heterosis value over DCH 32 commercial check. Two crosses exhibited significant positive heterosis respect to this trait over DCH 32 commercial check between crosses.

1.6 Mean boll weight (g)
In cotton, boll weight and boll number in inter specific and intraspecific hybrids are reported as major components of heterosis in yield (Pavasia et al., 1999b). Per cent heterosis of F₁ crosses over their respective mid parent values ranged from 32.28 [ZCH 8 X (DB 533 x DB 534 F₄ IPS 49)] to -34.94 [DH 98-27 X (DB 533 x DB 534 F₄ IPS 38)]. Five crosses showed significant negative heterosis and one cross recorded significant positive heterosis over their mid parent.

The cross ZCH 8 X (DB 533 x DB 534 F₄ IPS 49) (15.07) recorded highest positive heterosis over commercial check MRC 6918 and the cross 178-24 X (DB 533 x DB 534 F₄ IPS 33) (-35.62) exhibited lowest significant negative heterosis over MRC 6918. Eight crosses showed significant heterosis over MRC 6918 in negative direction. In case of DCH 32 check, the cross ZCH 8 X (DB 533 x DB 534 F₄ IPS 49) (27.27) showed highest positive heterosis over this check, but the cross 178-24 X (DB 533 x DB 534 F₄ IPS 33) (-28.79) recorded lowest negative heterosis value over DCH 32 commercial check. The crosses exhibited non significant heterosis respect to this trait over DCH 32 commercial non Bt check.

1.7 Reproductive points on sympodia
Per cent heterosis of F₁ crosses over their respective mid parent values ranged from 59.75 [DH 18-31 X (DB 533 x DB 534 F₄ IPS 6)] to -57.53 [DH 98-27 X (DB 533 x DB 534 F₄ IPS 49)]. Twenty three crosses showed significant negative heterosis and twelve recorded significant positive heterosis over their mid parent.

The cross 178-24 X (DB 533 x DB 534 F₄ IPS 12) (81.30) recorded highest significant positive heterosis over Bt check MRC 6918 and the cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 13) (-34.31) exhibited lowest significant negative heterosis over MRC 6918. Thirty six crosses showed significant heterosis over MRC6918 in positive direction, while three crosses recorded significant heterosis in negative direction. In case of DCH 32 check, the cross 178-24 X (DB 533 x DB 534 F₄ IPS 12) (51.00) showed highest significant positive heterosis over this check, but the cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 13) (-45.29) recorded lowest significant negative heterosis value over DCH 32 commercial check. Thirteen crosses exhibited significant positive heterosis and nine recorded significant negative heterosis respect to this trait over DCH 32 commercial check. Significant positive heterosis over mid parent and commercial check was reported by Mallikarjun (2005), Somashekhar (2006) and Deepakbabu (2007).

1.8 Sympodial length at 50 per cent plant height (cm)
Per cent heterosis of F₁ crosses over their respective mid parent values ranged from 84.71 [ZCH 8 X (DB 533 x DB 534 F₄ IPS 23)] to -32.74 [DH 98-27 X (DB 533 x DB 534 F₄ IPS 30)]. Eleven crosses showed significant positive heterosis and only two crosses recorded significant negative heterosis over their mid parent. Significant positive heterosis over mid parent and commercial check was reported by Mallikarjun (2005), Somashekar (2006).

The cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 38) (75.25) recorded highest significant positive heterosis over commercial check MRC 6918 and the cross DH 18-31 X (DB 533 x DB 534 F₄ IPS 16) (-22.08) exhibited lowest negative heterosis over MRC 6918. Five crosses showed significant heterosis over MRC 6918 in positive direction. In case of DCH 32 check, the cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 38) (36.43) showed highest significant positive heterosis over this check, but the cross DH 18-31 X (DB 533 x DB 534 F₄ IPS 16) (-39.34) recorded lowest significant negative heterosis value over DCH 32 commercial check. Eleven crosses exhibited significant negative heterosis and only two recorded significant positive heterosis respect to this trait over DCH 32 commercial check.

1.9 Seed index (g)
Per cent heterosis of F1s over their respective mid parent values ranged from 80.45 [DH 98-27 X (DB 533 x DB 534 F₄ IPS 32)] to -29.96 [DH 98-27 X (DB 533 x DB 534 F₄ IPS 13)]. Twenty five crosses showed significant positive heterosis and seven crosses recorded significant negative heterosis over their mid parent. Heterosis over mid parent was reported by Reddy (2001), Neelima (2002) and Potdukhe (2002), Karande et al. (2004) and Maisuria et al. (2006).

The cross DH 98-27 X (DB 533 x DB 534 F4 IPS 32) (83.87) recorded highest significant positive heterosis over Bt check MRC 6918 and the cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 13) (-26.43) exhibited significant negative heterosis over MRC 6918. Fourteen crosses showed significant heterosis in positive direction and two crosses recorded significant negative heterosis over MRC 6918. In case of DCH 32 check, the cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 32) (84.16) showed highest significant positive heterosis over this check, but the cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 13) (-26.32) recorded lowest significant negative heterosis value over DCH 32 commercial check. Fourteen crosses exhibited significant positive heterosis and only two recorded significant negative heterosis respect to seed index over DCH 32 commercial check.

1.10 Ginning outturn (%)
Per cent heterosis of F₁ crosses over their respective mid parent values ranged from 11.31 [DH 18-31 X (DB 533 x DB 534 F₄ IPS 105)] to -58.48 [ZCH 8 X (DB 533 x DB 534 F₄ IPS 33)]. Twenty five crosses showed significant negative heterosis over their mid parent. Mid parental heterosis for GOT was reported earlier by Reddy (2001), Neelima (2002), Potdukhe (2002) and Sivaprasad (2003).The cross DH 18-31 X (DB 533 x DB 534 F₄ IPS 105) (25.65) recorded highest significant positive heterosis over commercial check MRC 6918 and the cross ZCH 8 X (DB 533 x DB 534 F₄ IPS 33) (-51.63) exhibited lowest significant negative heterosis over MRC 6918. One cross showed significant heterosis in negative direction and two crosses recorded significant positive heterosis over MRC 6918. In case of DCH 32 check, the cross DH 18-31 X (DB 533 x DB 534 F₄ IPS 105) (6.74) showed highest positive heterosis over this check, but the cross ZCH 8 X (DB 533 x DB 534 F₄ IPS 33) (-58.91) recorded lowest significant negative heterosis value over DCH 32 non Bt check. Sixteen crosses exhibited significant negative heterosis over DCH 32 commercial check.

1.11 Lint index (g)
Per cent heterosis of F₁ crosses over their respective mid parent values ranged from 53.71 [DH 98-27 X (DB 533 x DB 534 F₄ IPS 32)] to -44.65 [ZCH 8 X (DB 533 x DB 534 F₄ IPS 30)]. Twenty four crosses showed significant negative heterosis and four crosses showed significant positive heterosis over their mid parent. Heterosis over mid parent was reported by Doss and Kadambavanasundaram (1997), Reddy (2001), Neelima (2002) and Potdukhe (2002). The cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 32) (82.61) recorded highest significant positive heterosis over commercial check MRC 6918 and the cross ZCH 8 X (DB 533 x DB 534 F₄ IPS 30) (-33.29) exhibited lowest negative heterosis over MRC 6918. Twenty one crosses showed significant heterosis in positive 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 32) (45.34) showed highest significant positive heterosis over this check, but the cross ZCH 8 X (DB 533 x DB 534 F₄ IPS 30) (-46.91) recorded lowest significant negative heterosis value over DCH 32 commercial check. Seventeen crosses exhibited significant negative heterosis over DCH 32 commercial check and only one cross showed significant positive heterosis.

2 Materials and Method
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.

Heterosis of F₁ over mid parent (MP) and commercial check (MRC 6918 and DCH 32) were calculated by methods of Turner (1953) and Hayes et al. (1955) as given below. 

Per cent heterosis in F₁ over mid parent (MP) = (F₁ - MP)/MP × 100

Where, Mid parent (MP) = (P₁ + P₂)/2

Per cent heterosis in F₁ over commercial check (CC) = (F₁ - CC)/CC × 100

Where, MP = Mid parent, CC = Commercial check

Mean sum of squares due to error from RBD analysis was considered to compute standard error (S.E.) of estimated heterosis as follows.

S.E. for heterosis over mid parent
S.E. (Hmp) = [(3/2 × EMS)/r]^0.5

S.E. for heterosis over commercial check
S.E. (Hcc) = (2 EMS/r)^0.5

Where, EMS = Error mean sum of squares

The critical difference values in each case were worked out by multiplying their corresponding S.E. values with table t  value at error degree of freedom at 5 and 1 per cent levels of significance.

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