Principles of Plant Genetics and Breeding

Single cross
A × B
AB
Proportion A = 50%
B = 50%
(a)
Backcross
1st BC
2nd BC
3rd BC
A × B
F 1 × B
BC 1F 1 × B
BC 2F 1 × B
BC nF 1 × B
of the existing desirable traits. Hence, one (or several)
parent(s) serves as a donor of specific genes and is usually
involved in the cross only once. Subsequent crosses
entail crossing the desirable parent (recurrent parent)
repeatedly to the F 1 , in order to retrieve all the desirable
traits. A commonly used convergent cross is the backcross
(Figure 10.1b) (see Chapter 16).
Issue of reproductive isolation barriers
Hybridization is often conducted routinely without
any problems when individuals from the same species
are involved, provided there are no fertility-regulating
mechanisms operating. Even when such mechanisms
exist, hybridization can be successfully conducted by
providing appropriate pollen sources. Sometimes, plant
breeders are compelled to introduce desired genes from
distant relatives or other species. Crossing plants from
two different species or sometimes even plants from different
genuses is more challenging and has limited
success. Often, the breeder needs to use additional techniques
(e.g., embryo rescue) to intervene at some point
SEXUAL HYBRIDIZATION AND WIDE CROSSES IN PLANT BREEDING 171
Three-way cross
A × B
AB × C
ABC
Proportion A = 25%
B = 25%
C = 50%
Proportion of B
50%
75%
87.5%
(b)
Double cross Diallel cross
A × B A × B
A B C D
AB × CD
ABCD
Proportion A = 25%
B = 25%
C = 25%
D = 25%
Convergent cross
(1) A × B C × D E × F G × H
Proportion of A
50%
Figure 10.1 The basic types of crosses used by plant breeders. Some crosses are divergent (a) while others are
convergent (b).
A
B
C
D
E
AA
AB
AC
AD
AE
(2) A × B A × C A × D A × E
BA
BB
BC
BD
BE
25%
13%
CA
CB
CC
CD
CE
Reciprocal
50% A
50%
50%
DA
DB
DC
DD
DE
in the process in order to obtain a mature hybrid plant.
This kind of crossing involving parents from different
species is called a wide cross and is described further
below.
Reproductive isolation barriers may be classified into
three categories as suggested by researchers such as G.
L. Stebbins, T. Dobzhansky, and D. Zohary (Table 10.2).
E
EA
EB
EC
ED
EE
Selfs
Table 10.2 A summary of the reproductive isolation
barriers in plants as first described by G. L. Stebbins.
External barriers
Spatial isolation mechanisms: associated with geographic
distances between two species
Prefertilization reproductive barriers: prevents union of
gametes. Includes ecological isolation (e.g., spring and
winter varieties), mechanical isolation (differences in floral
structures), and gametic incompatibility
Internal barriers
Postfertilization reproductive barriers: leads to
abnormalities following fertilization (hybrid inviability or
weakness and sterility of plants)