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Cotton Genomics and Genetics
46
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 F
1
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).
HF
1
=
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 F
1
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 F
1
and F
2
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). F
1
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 F
1
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.
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 F
4
IPS 49)] to -38.94 [178-24 X
(DB 533 x DB 534 F
4
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
Cotton Genomics and Genetics