Agriculture Reference
In-Depth Information
of the recurrent parent phenotype. Also impor-
tant is to use a recurrent parent that will retain its
value during the backcrossing procedure, as the
backcross-derived line is equivalent to the recur-
rent parent plus the added trait. Examples of
successful backcross-developed lines include
'Prowers' (Quick et al., 2001a) and 'Prairie Red'
(Quick et al., 2001b).
The backcross method has been used effec-
tively as a short-term breeding strategy to
incorporate dominant genes for the control of
devastating pathogens, such as that causing stem
rust, in otherwise highly productive and adapted
cultivars (Campbell et al., 1967; Campbell 1970;
Green and Campbell 1979). The emergent stem
rust race Ug99 (race TTKSK) in East Africa has
virulence to gene
Sr31
derived from the 1RS·1BL
translocation, and another variant of Ug99
(TTKST) subsequently identifi ed has virulence
to genes
Sr24
and
Sr31
. Both genes are deployed
in numerous cultivars on a global scale. The
Global Rust Initiative (http://www.globalrust.
org/index.cfm?m=1) is a concerted effort to
develop a global response to the emergence of
these devastating virulent races of stem rust.
Backcross breeding will be one of the strategies
used to incorporate resistance genes into adapted
cultivars in various countries. This illustrates a
good example of opportunities to use DNA
molecular markers linked to known genes that
confer some resistance—for example,
Sr2
(Hayden
et al., 2004) and
Sr26
(Mago et al., 2005)—without
actually having to introduce the pathogen into
regions where it does not exist, thus putting local
wheat production at great risk should the intro-
duced pathogen escape from the testing site. Final
verifi cation may be obtained in the fi eld in coun-
tries where the pathogen already exists, after the
backcrossing has been completed. Of course,
many other genes and traits have been incorpo-
rated using backcross and molecular marker
breeding (Dubcovsky 2005).
In addition to adding a gene to a recurrent
parent, backcrossing is used to make alloplasmic
lines (Kofoid and Maan 1982). Alloplasmic lines
are used to study the effects of different cyto-
plasms on various traits. However, the most
common use of alloplasmic lines was in develop-
ing cytoplasmic male sterility (CMS) for hybrid
wheat production (reviewed by Edwards 2001).
The three components of CMS hybrid wheat are
a restorer line (R-line), the maintainer line (B-
line), and the male-sterile line (A-line). Once a
good B-line × R-line combination has been iden-
tifi ed, the B-line needs to be converted into an
A-line via backcrossing. Usually at least six back-
crosses are needed for this conversion, but it
should be understood that many of these back-
crosses can occur while the A-line seed is being
increased to commercial seed quantities. Every
time an A-line is crossed by the same B-line to
increase seed, it is a backcross. Hence a desirable
B-line is converted by making two or three back-
crosses by hand in the greenhouse or fi eld to a
previously existing A-line, followed by a variable
number of crosses (i.e., backcrosses) that are
needed to obtain commercial volumes of A-line
seed for hybrid wheat production.
MAJOR ISSUES ALL WHEAT
BREEDERS FACE
Early- vs late-generation selection
Refl ections on desirable gene quotient (Table
13.1) and minimum population size (Table 13.2)
reinforce the theory that the earliest inbreeding
generations have the highest frequency of geno-
types with the desirable alleles, albeit pre-
dominately in the heterozygous condition. The
challenge then is to identify those genotypes in
the earliest possible generation and to select them
for further inbreeding.
In the 20-gene example a minimum of 315 F
2
plants would have to be retained for the chance
occurrence of one containing all the desirable
alleles (Table 13.2). From Table 13.3 it can be
determined that two classes of genotypes occur
with the largest frequency (18% each): one class
homozygous for the desirable allele at 7 loci and
heterozygous at the remaining 13 loci, and another
class homozygous and heterozygous at 6 and 14
loci, respectively. The challenge is to identify
these unique F
2
genotypes and concentrate sub-
sequent selection among F
2
-derived lines while
