Agriculture Reference
In-Depth Information
both parents. They concluded that the tetraploid
and diploid parents of the synthetic materials
contributed favorable alleles for quality. Primary
synthetics with higher grain protein content,
longer kernels, greater kernel weight, and
improved SDS-sedimentation compared to Seri
82, an adapted cultivar, have also been observed
(Lage et al., 2006).
Grain quality is not only defi ned in terms of
product processing, but also encompasses nutri-
tional quality. Zinc, Fe and vitamin A defi cien-
cies, particularly in women and children, occur in
many developing countries where wheat is the
primary source of calories (Ortiz-Monasterio et
al., 2007). No variation for vitamin A or beta-
carotene content has been discovered in the wheat
gene pool. However, signifi cant variation in Zn
and Fe grain concentration has been identifi ed in
a range of materials, ranging from adapted culti-
vars to landraces and the tetraploid and diploid
progenitors of wheat (Ortiz-Monasterio et al.,
2007). Calderini and Ortiz-Monasterio (2003)
tested a range of primary synthetics and con-
cluded that their higher grain micronutrient con-
centration was a result of higher nutrient uptake
effi ciency.
the tetraploid used to make the synthetic is less
adapted, such as
T. dicoccum
or
T. dicoccoides
,
then even larger populations will be required.
Characteristics of the primary synthetic which
must be removed are tenacious glumes and rachis,
speltoide head shape, generally poor agronomic
type, and depending on market requirements, the
soft red-grained seed and generally poor process-
ing quality.
An additional limitation is the relatively high
frequency of F
1
hybrid necrosis when crossing
primary synthetics with adapted wheat. Hybrid
necrosis is controlled by genes at two loci named
Ne1
and
Ne2
(Pukhalskiy et al., 2000). In the
heterozygous state for both loci (
Ne1/ne1
and
Ne2/ne2
), necrosis occurs and plants die in early
development. Bread wheat and durum wheat
differ in the frequency of these alleles; hence
crosses between primary synthetic wheat carrying
the durum complement of necrosis genes and
bread wheat will often produce hybrid necrosis.
If a specifi c cross combination is sought by the
breeder and experience indicates that hybrid
necrosis is a problem, it will be necessary to
conduct a bridging cross with a compatible parent
before top-crossing the resultant F
1
with the
desired cultivar.
The breeder should be prepared to conduct
two cycles of breeding before materials eligible for
commercialization are identifi ed. Experience at
CIMMYT indicated that when derivatives
expressing the desired trait from the fi rst round
of backcrossing with primary synthetics were
crossed with adapted wheat, the resultant progeny
were signifi cantly better adapted (Lage and
Trethowan 2008). Alleles controlling grain quality
are, to a large extent, randomly distributed in
primary synthetic wheat. Favorable linkage blocks
built up over time and conserved in many breed-
ing programs are disrupted when adapted culti-
vars are crossed with primary synthetics. Many
breeders, particularly those from developed coun-
tries where specifi c market classes are targeted
because of associated price premiums, tend to be
reluctant to use primary synthetics. In these
instances a concerted parent building approach is
required before this new variation can be used
extensively.
STRATEGIES FOR USING PRIMARY
SYNTHETICS IN APPLIED
WHEAT BREEDING
Primary synthetic hexaploids are a valuable source
of variability for a host of traits conferring resis-
tance to biotic and abiotic stresses. However,
while they can be directly crossed with cultivated
wheat and do not require embryo rescue, breeders
must be aware of potential problems when intro-
ducing this variation into their cultivar develop-
ment program.
Much of the new variation in primary synthet-
ics is not useful variation. When crossing with
primary synthetic wheat, it is advisable to make
at least one backcross and to grow signifi cantly
larger BC
1
F
1
and BC
1
F
2
generations, thereby
allowing greater selection pressure for desired
agronomic type in the segregating generations. If
