Agriculture Reference
In-Depth Information
was subsequently released as 'Lillian' (DePauw et
al., 2005). Uauy et al. (2005) reported that the
marker
Xucw71
was closely linked to
Gpc-B1
(0.3
cM). DePauw et al. (2007) provided evidence
using the nucleolus organizer region (NOR)
marker and
Xucw71_5utr
cleavage amplifi cation
polymorphic sequence (CAPS) marker that the
chromosomal region with
Gpc-B1
in Lillian was
smaller than in the parent 90B07-AU2B and
grandparent Glupro.
Another novel use of molecular markers has
been to develop a triple hemizygous stock to fi x
multiple Fusarium head blight (FHB, caused by
Fusarium graminearum
Schwabe) QTLs on 3BS,
5AS, and 6BS (Thomas et al., 2006). The rapid
fi xation was achieved by hemizygosity of the
chromosome arms that carry the QTLs (3BS,
5AS, and 6BS) in the F
1
, followed by reversion to
euploidy upon inbreeding. Elite hemizygous
parents were developed by combining nonrecip-
rocal translocations on 3BL, 5AL, and 6BL. Even
though rapid fi xation of the FHB QTLs and an
increase in the number of lines with all three
QTLs are expected, the disadvantage remains
that any undesirable genes on the same chromo-
some arm will be retained, as no recombination
within the chromosome arm occurs.
will completely turn over three to fi ve times. This
concept is important to understanding that
program continuity is critical, and that when a
new technology is said to not affect wheat breed-
ing for 25 years, from a breeder's perspective that
is not a long time. Also, plant breeding really does
not have a start or stop point but is a continuum
where new crosses are made every year, popula-
tions or lines are selected and advanced every
year, and if the program is productive, cultivars
are released every few years and more frequently
than every 7 or 12 years.
A typical breeding program can be described
by the fl ow diagram in Fig. 13.2. Every breeding
program starts with an objective, such as improv-
ing grain yield. Once the objective is decided, the
most common way to introduce new variation
into a breeding program is to introduce new
germplasm. However, if the objective is not
achieved by introduced germplasm, the next step
is to make a cross or sexual hybrid between parents
that have all or most of the complementary traits.
Every program must decide which parents to use
based upon the objective and the appropriate type
of cross: single (A/B), three-way (A/B//C),
double or four-way cross (A/B//C/D), and
others.
Once there was considerable debate on whether
elite-by-elite crosses (containing the best alleles)
THE PRACTICE OF WHEAT BREEDING
In the preceding sections, we have described
much of the theory and research laying the foun-
dation of present-day wheat breeding. In this
section, we will attempt to illustrate how the
theory and research is actually used to create new
cultivars. At the international level, CIMMYT
recently reviewed their strategy for cultivar devel-
opment (Ortiz et al., 2007) and their review is
an excellent outline of practical plant breeding
methods.
Ideotype
Germplasm
Ideotype Identified?
Hybridize + Evaluate + Select
YES
Evaluation for Release
NO
Extension of the theory
It should be remembered that normally it takes at
least 7 years to develop a spring wheat cultivar
and 12 years to develop a winter wheat cultivar.
For a 35-year career a wheat breeder's germplasm
Meritorious Type
Commercial Production
Fig. 13.2.
Flow chart of steps to develop and commercialize
a wheat cultivar.
