Agriculture Reference
In-Depth Information
grees in CIMMYT's rainfed breeding program
contain at least one synthetic progenitor
(Trethowan et al., 2003). Derived synthetics were
distributed worldwide in CIMMYT's interna-
tional yield trial network, and collaborators in
many countries have returned performance data.
One such trial is the Semi-Arid Wheat Yield
Trial (SAWYT), which targets rainfed environ-
ments. Lage and Trethowan (2008) analyzed the
yield performance of those materials from the
deployment of the fi rst synthetic derivatives in
yield trials in 1996, when they comprised 8% of
the total entries, to 2006 when 46% of the entries
were derived from synthetic wheat. Between the
years 1996 and 2006, the average contribution of
primary synthetics to pedigrees in the SAWYT
decreased from 75% to 19% based on coeffi cients
of parentage. The extent of this reduction was
supported by subsequent microsatellite (SSR)
analyses of recent derivatives, which indicated
that only a small portion of the primary synthetic
genome remained (Zhang et al., 2005). The
average yield rank of the synthetic-derived mate-
rials in the 50-entry SAWYT improved from
30th to 25th position over the period. Synthetic
derivatives were identifi ed that were superior to
the local check cultivars across a wide range of
global environments and yield levels. Those
derivatives possessing a smaller portion of the
primary synthetic genome tended to perform
better, indicating that at least one backcross is
required to improve agronomic type suffi ciently
to exploit underlying variation for yield and
adaptation.
CIMMYT screens the derived materials in the
Sonoran Desert in northwestern Mexico using
limited irrigation to simulate specifi c stress
regimes. There was interest in Australia, where
wheat is frequently grown in moisture-stressed
conditions, in testing the value of the materials
bred and selected in Mexico. When the synthetic
derivatives developed in Mexico were tested
under Australian conditions in multienviron-
ment trials, a yield advantage between 8% and
30% compared with the best local check cultivar
was observed (Ogbonnaya et al., 2007a). In a
separate analysis of a different set of 156 deriva-
tives, 56% of the materials were found to be
higher yielding than the best local check cultivar
across multiple locations (Dreccer et al., 2007).
Both authors noted strong genotype × environ-
ment interactions and concluded that northern
Australian wheat-growing environments were
similar to those in Mexico, while southern
environments ranked materials differently. The
most likely driver of this north-south division
was phenology; later-maturing materials tended
to perform better in cooler southern areas of Aus-
tralia (Mathews et al., 2007). Azizinya et al.
(2005) evaluated synthetic derivatives developed
in Mexico in Iran and found lines that outper-
formed the local check cultivars, producing
heavier, longer spikes with better spikelet
fertility.
The preceding results show that the perfor-
mance of synthetic derivatives developed under
managed stress in one country can be transferred
to dry environments in other locations around the
world. In a different comparison, Gororo et al.
(2002) produced synthetic hexaploid derivatives
from primary synthetics developed in Australia
and tested those materials in both Australia and
Mexico. The synthetic derivatives produced in
Australia were up to 32% higher yielding than
their recurrent parent and exceeded the check in
38 of 42 comparisons in multienvironment trials
in both countries.
The evaluation of derivatives in multienviron-
ment trials clearly indicates that useful variation
for adaptation, particularly to drought stress, can
be obtained from synthetic wheat. Reynolds et al.
(2007) and Reynolds and Trethowan (2007)
evaluated synthetic backcross derivatives under
drought stress and found that the 15%-to-33%
yield advantage of the derivatives compared with
their recurrent parents could be attributed to a
greater investment in root biomass deeper in the
soil profi le. Interestingly, this investment in deep
roots at the expense of surface roots led to a 46%
lower root-shoot ratio under drought stress. Syn-
thetic derivatives were also able to better maintain
greater seed weight under drought and high-
temperature stress (Trethowan et al., 2005). Larger
seed size may also have favorable implications
