Basic HTML Version
Cotton Genomics and Genetics
34
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 T
1
and T
2,
it is
possible to increase the genetic distance between these
opposite pairs A, B
vs
, T
1
and T
2
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 F
3
/F
4
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
addi
tive 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
Cotton Genomics and Genetics