Agriculture Reference
In-Depth Information
PRIMARY SYNTHETIC
HEXAPLOID WHEAT
New genetic variability for tolerance to
biotic stress
Rust diseases
The primary synthetics have proven to be a valu-
able source of genetic variability for disease resis-
tance. Rust is the most important disease of wheat
worldwide and is controlled by both race-specifi c
and race-nonspecifi c genes. Much of the variabil-
ity in race-specifi c or major gene resistances has
eroded with time as pathogen mutation has over-
come the effectiveness of these genes, particularly
when deployed singularly under disease pressure.
Major gene resistance for stem rust (caused by
Puccinia graminis
f. sp.
tritici
) and for leaf rust
(caused by
P. triticina
Ericks.) has been reported
in synthetic wheat (Kerber and Dyck 1969, 1978;
Villareal et al., 1992; Innes and Kerber 1994).
Aguilar-Rincón et al. (2000) also reported resis-
tance to leaf rust among a set of fi ve primary
synthetics and determined that the genes confer-
ring resistance were different, indicating they
could be pyramided in cultivars to improve resis-
tance. However, it is not clear whether or not
these genes differ from those previously reported
in the wheat gene pool. Assefa and Fehrmann
(2004) found that 8% of 169
Ae. tauschii
acces-
sions exposed to stem rust were resistant. When
primary synthetics were developed from these
resistant sources, however, expression varied
from resistant to susceptible due to poorly under-
stood suppression. Among the materials they
tested, several primary synthetics were found to
carry different resistance genes.
Interestingly, a number of authors also reported
both suppression and overexpression of stripe rust
(caused by
P. striiformis
Westend. f. sp.
tritici
)
resistance in primary synthetics. Kema et al. (1995)
reported that stripe rust resistance expressed in
both
Ae. tauschii
and
T. dicoccum
was not expressed
in the resultant primary synthetic. Similarly,
Ma et al. (1995a) examined 74 primary synthetics
and concluded that stripe rust resistance present
in the tetraploid and diploid progenitors was
not expressed or was partially expressed in some
combinations. They found both major gene
and adult-plant (or race-nonspecifi c) resistance
The fi rst published amphiploid was produced in
the late 19th century from a cross between wheat
and rye (
Secale cereale
L.) (Wilson 1876). With
the advent of colchicine in the 1930s it became
possible to develop hybrids between wheat and
Aegilops
spp. These successful hybridizations
demonstrated that species could be artifi cially
formed via allopolyploidy (Feldman 2001). The
fi rst artifi cial hybridization between tetraploid
wheat and
Ae. tauschii
, and hence the fi rst primary
synthetic, was reported by McFadden and Sears
(1946). Large-scale development of synthetic
wheat began at the International Maize and Wheat
Improvement Center (CIMMYT) in the mid-
1980s (van Ginkel and Ogbonnaya 2007). Many
of these primary synthetics have been screened
for various traits and crossed in breeding pro-
grams around the world.
Figure 16.1 describes the three types of crosses
generally made to produce new primary synthetic
hexaploid wheat. This genetic resource can have
wide genetic diversity and can vary considerably
in agronomic type. Synthetics formed from
modern durum wheat tend to carry less diversity
than synthetics formed from either of the tetra-
ploid emmer types, but they have better agro-
nomic type and are therefore more easily used in
applied wheat breeding.
×
Triticum turgidum
ssp.
Aegliops
dicoccoides
(wild Emmer)
tauschii
dicoccum
(cultivated Emmer)
(Goatgrass)
durum
(Pasta wheat)
AABB
DD
AABBDD
Triticum aestivum
L.
Primary synthetic hexaploid bread wheat
Fig. 16.1
Typical crosses that give rise to primary synthetic
hexaploid wheat.
