Agriculture Reference
In-Depth Information
where P
1
×
identifying parental lines which will produce produc-
tive progeny in a wide range of hybrid cross. Generally,
it is not possible to cross all possible parental lines in
pair-wise combinations, as the number of crosses to be
made and evaluated increases exponentially with the
increased number of parents. It is therefore more usual
to cross each parent under evaluation to a common
test parent or tester
. The tester used is common to
a set of evaluations and therefore, general combining
ability is determined by comparing the performance of
each progeny, assuming that the only difference between
the different progenies can be attributed to the differ-
ent inbred parents. Testers are usually highly developed
inbred lines, which have proved successful in hybrid
combinations in the past. A far better prediction of gen-
eral combining ability would be achieved if more than
one tester were used. This, however, is not common
practice and breeders have tended to test more inbreds
than to increase the number of test parents used.
Evaluation of specific combining ability (or actual
individual hybrid combination performance) is carried
out when the number of parents is reduced to a reason-
able level. The number of possible cross combinations
differs with the number of parents to be tested. The
number of combinations is calculated from:
[
P
3
is the performance of the F
1
progeny
from the cross between P
1
and P
3
. Note again that the
two single crosses used in the double cross do not appear
in the prediction equation.
The assumptions underlying this will not be dis-
cussed here, but we simply note that this is what is
carried out quite often in practical breeding.
Backcrossing in hybrid cultivar
development
Backcrossing has featured quite highly in hybrid breed-
ing schemes. Backcrossing is used in hybrid develop-
ment for two purposes:
To introduce a single gene into an already desirable
inbred parent
•
•
To produce near isogenic lines, which can reduce
the cost of seed production, by
convergence
, called
convergent improvement
This is used to make slight
improvements to specific hybrid cross combinations
and involves recurrent backcrossing between a sin-
gle cross hybrid (F
1
=
P
1
×
P
2
)
and both of its
two parents (P
1
and P
2
)
. The result will be to pro-
duce from P
1
,P
1
∗
(where P
1
∗
, is a near isogenic line
(
−
)
]
/
n
n
1
2
for pair-wise crosses
of P
1
)
. Similarly near isogenic lines are developed
for P
2
. These can be used in a modified single cross
([P
1
×
[
(
−
)(
−
)
]
/
n
n
1
n
2
2
for three-way crosses
P
1
∗
]×
P
2
), or a double modified single cross
[
(
−
)(
−
)(
−
)
]
/
8 for double crosses
where
n
is the number of parents to be evaluated. For
example if 20 parents are to be tested then there would
be: 20 crosses to a single tester; 190 pair-wise cross,
3420 three-way cross and 14 535 double cross combi-
nations possible. It is therefore common to predict the
performance of three-way and double crosses from sin-
gle cross performance rather than actually test them.
The three-way cross [(P
1
×
n
n
1
n
2
n
3
(
[
P
1
×
P
1
∗
]×[
P
2
×
P
2
∗
]
)
. The aim of this is to increase
the efficiency, and reduce cost, of seed production.
Hybrid seed production and
cultivar release
The inbred lines used as parents are increased in exactly
the same way as pure-line cultivars and hence no further
description is needed here.
The first, and highest priority of hybrid seed produc-
tion is to complete the task as cheaply as possible with
the maximum proportion of hybrid offspring. There
are four basic means that have been used to produce
commercial amounts of hybrid seed:
P
2
)
×
P
3
] performance is
predicted from the equation:
1
/
[
(
P
1
×
P
3
)
+
(
P
2
×
P
3
)
]
2
where P
1
×
P
3
is the performance of the F
1
progeny
from the cross between P
1
and P
3
. It is noted that the
actual single cross in the hybrid predicted (P
1
×
P
2
)
is
not used in the prediction.
To predict the performance of a double cross [(P
1
×
P
2
)
×
(
Mechanical production
In hermaphroditic plants,
female are emasculated and pollination is achieved
either
P
3
×
P
4
)
] the following equation is used:
/
[
(
P
1
×
P
3
)
+
(
P
1
×
P
4
)
+
(
P
2
×
P
3
)
+
(
P
2
×
P
4
)
]
1
4
by
hand
or
naturally.
In
diclinous
plants
