Principles of Plant Genetics and Breeding

(a)
(b)
Seed parent
(male-sterile)
S
rfrf
S ×
Rfrf
Hybrid
(male-fertile)
hence isogenic (genetically different at only a specific
locus).
To maintain the A-line, the breeder crosses sterile
A-line (S, rfrf ) with the B-line (N, rfrf ) to obtain
all male-sterile offspring. The B-lines (maintainer
lines) are maintained by selfing, sibbing, or by openpollination
in isolated crossing blocks. Another inbred
parent, the fertility-restorer parent, is required in a
CMS breeding program. This parent (called the Rline)
has the genotype RfRf and is the pollen parent
in the hybrid. It is produced by first selecting a desirable
line to be converted into an R-line. It is crossed
to the restorer gene donor parent using backcrossing
with the R-line as the recurrent parent. The process
can be simplified if the donor has sterile cytoplasm (S,
rfrf ) and is used as the female parent in the cross.
S
BREEDING HYBRID CULTIVARS 345
RfRf
N
Pollen parent
(male-fertile)
Parent A Parent B Parent C Parent D
rfrf
Seed parent
×
rfrf
S
Single-cross AB
(male-sterile)
rfrf
Pollen parent
Rfrf
S
N
Figure 18.1 Breeding by CMS: (a) single cross and (b) double cross. N, normal cytoplasm; S, sterile cytoplasm.
×
Seed parent
rfrf
Double-cross ABCD
(50% fertile and 50% sterile)
S
S
rfrf
S
×
Rfrf
Single-cross CD
(male-fertile)
RfRf
Pollen parent
This strategy will eliminate the need for a testcross or
selfing after each backcross to distinguish between
fertile and sterile individuals [(S, Rfrf × N, rfrf ), S,
Rfrf (fertile), or S, rfrf (sterile), which are phenotypically
distinguishable]. Once produced the R-line is
maintained like the B-line. It should be mentioned
that inbred lines of self-pollinated species may also be
developed to incorporate a male-sterility system for
hybrid production.
3 Genetically modified inbreds. With the advent of
genetic engineering, special lines with specific transgenes
are developed for use in producing transgenic
hybrids. The details of the techniques are discussed
in Chapter 14. Inbred lines are transformed with
these special genes to develop transgenic breeding
stocks.
N