Agriculture Reference
In-Depth Information
2002) and they include genotypic variation for
traits such as: (i) germination, biomass, earliness,
nitrogen content, and yield; (ii) amino acid com-
position; (iii) grain protein content and storage
protein genes (HMW glutenins); (iv) disease
resistances, including resistance to powdery
mildew [
Blumeria graminis
(DC) E.O. Speer f. sp.
tritici
], leaf rust (
Puccinia triticina
Eriks.), stripe
or yellow rust (
P. striiformis
Westend. f. sp.
tritici
Eriks.), stem rust (
Puccinia graminis
Pers.:Pers. f.
sp.
tritici
Eriks. & E. Henn.), and
Soilborne wheat
mosaic virus
(WSBMV); (v) high photosynthetic
yield; (vi) salt tolerance; (vii) herbicide resistance;
(viii) amylases and α-amylase inhibitors; and (ix)
micronutrients such as Zn and Fe. This is only a
preliminary list of the vast potential genetic
resources existing in wild emmer that remain to
be exploited for wheat improvement.
Quantitative trait loci (QTLs) and benefi cial
cryptic, agronomically important alleles have now
been extensively described. The current genetic
map of
T. dicoccoides
, with 549 molecular markers
and 48 signifi cant QTLs for 11 traits of agro-
nomic importance (Peng et al., 2000b), will permit
the unraveling of benefi cial alleles of candidate
genes that are otherwise hidden. These benefi cial
alleles could be introduced into cultivated wheat
by marker-assisted selection.
The Near East, in general, and Israel, in par-
ticular (Nevo 1986), are the centers of origin and
diversity of wild emmer, where it developed wide
genetic adaptations against multiple pathogens
and diverse ecological stresses. Genetic diversity
is transferable from the wild to the cultivated
gene pool, so genes of wild emmer are directly
accessible for future wheat improvement.
Consequently, exploration of
in situ
and
ex situ
collections (with optimized sampling strategies)
along with utilization programs should maximize
the contribution of wild emmer to wheat improve-
ment. Among the potential donors for improving
wheat, wild emmer occupies a very important and
unique position due to its direct ancestry of bread
wheat and its rich and largely adaptive genetic
diversity. This was fi rst suggested by Aaronsohn
(1913) and later elaborated on by many authors
(see Feldman 1977; Nevo 1983, 1995, 2001, 2006;
Xie and Nevo 2008).
There are many ongoing programs around the
world utilizing genes of wild emmer for wheat
improvement, primarily involving genes coding
for resistance to powdery mildew and the rusts,
for high protein content, and for improved baking
quality. Cultivars based on introgression of
T.
dicoccoides
genes have appeared and will continue
to appear in the near future. With
T. dicoccoides
at least three to four backcrosses with bread wheat
are a necessity in breeding programs to minimize
linkage drag (Groenewegen and van Silfhout
1988; Reader and Miller 1991). Wheat improve-
ment programs will continue utilizing
T. dicoc-
coides
and other wheat relatives (Xie and Nevo
2008). Extensive work on transferring genes for
high protein content from
T. dicoccoides
to culti-
vated wheat is currently underway in several labo-
ratories (e.g., Weizmann Institute; US Department
of Agriculture, Fargo, North Dakota; and Uni-
versity of California, Davis).
CONCLUDING REMARKS ON THE
PROCESS OF WHEAT EVOLUTION
The molecular diversity and divergence of wheat
species displays parallel ecological-genetic pat-
terning and demonstrates the following: (i) sig-
nifi cant genetic diversity and divergence exists at
single- and multilocus structures of allozymes,
random amplifi ed polymorphic DNA, simple-
sequence repeats, and single-nucleotide polymor-
phisms over very short distances of several to a
few dozen meters; (ii) genetic patterns across
coding and largely noncoding genomic regions
are correlated with, and predictable by, environ-
mental stress (climatic, edaphic, biotic) and het-
erogeneity (the niche-width variation hypothesis),
displaying signifi cant niche-specifi c and niche-
unique alleles and genotypes; and (iii) genomic
organization of wheat, including the noncoding
genome, is nonrandom, heavily structured, and at
least partly, if not largely, adaptive. The process
of wheat evolution defi es explanation by genetic
drift, neutrality, or near neutrality alone as the
primary driving forces. The main viable model to
explain wheat genomic organization seems to be
natural selection, primarily diversifying and bal-
