Agriculture Reference
In-Depth Information
developing countries. Pastas, couscous, bulgur,
and local breads are processed from durum
wheat.
Tetraploid wheat was domesticated some
10,000-12,000 years ago, either in the Middle
East, eastern Turkey, or western Iran, where the
habitats of
Aegilops speltoides
, the probable donor
of the B genome, and
T. urartu
, the donor of the
A genome, overlap (Feldman 2001). This amphi-
ploid, called wild emmer or
T. turgidum
ssp.
dicoc-
coides
, had a brittle rachis, was diffi cult to thresh,
and was therefore unsuitable for cultivation.
Mutation later gave rise to
T. turgidum
ssp.
dicoc-
cum
, which was free-threshing with a nonbrittle
rachis and was more suitable for cultivation. Some
8,000 years ago, these cultivated tetraploids spread
from the Fertile Crescent region into western
Iran, where they likely hybridized with
Aegilops
tauschii
, the donor of the D genome (Kihara 1944;
McFadden and Sears 1946) (syn.
Ae. squarrosa
and
T. tauschii
, see Chapter 1), giving rise to
hexaploid wheat. Feldman (2001) believed that
this hybridization involved the cultivated tetra-
ploid,
T. dicoccum
, rather than wild emmer. As
cultivation spread from the Fertile Crescent, so
did the geographic range of wheat. Evidence sug-
gests that wheat cultivation began in 6500 BC in
the Fertile Crescent, spreading as far as modern-
day France and Egypt by 4000 BC, and east to
India and eastern China by 3000 and 1500 BC,
respectively (Feldman 2001).
In the process of evolution and domestication,
both tetraploid and hexaploid wheat passed
through signifi cant genetic bottlenecks, greatly
reducing genetic variability in their cultivated
forms. Mutation gave rise to free-threshing, non-
brittle domesticated forms of einkorn and emmer
wheat which were selected and grown by farmers,
thereby greatly truncating existing variation. The
formation of hexaploid wheat was subject to even
greater truncation. Evidence suggests that the
spontaneous hybridization of
T. dicoccum
and
Ae.
tauschii
likely involved a few independent events,
creating a signifi cant founder effect (Appels and
Lagudah 1990).
Farmers have continually improved the adap-
tation and productivity of wheat over the centu-
ries through continuous selection of better plants
as seed stocks for the following year. Large-scale
wheat breeding based on planned hybridizations
only began in the early 20th century, following
the rediscovery of Mendel's laws of heredity
(Sneep 1966). The success of modern wheat
breeding has been built upon exploitation of
useful genetic variation in the wheat gene pool.
Much of the improvement in wheat productivity
can be attributed fi rst to improved disease resis-
tance and second to the maintenance of this
resistance over time. As pathogens mutate and
overcome host resistance, so have plant breeders
sought new sources of resistance from an ever-
dwindling reservoir of still-effective genes. Sig-
nifi cant but reduced genetic progress has been
made in the improvement of wheat for tolerance
to abiotic stresses, largely because genetic varia-
tion is less plentiful, the inheritance of these traits
is complex, and heterogeneity in the environment
limits response to selection.
In their search for new genetic variation, wheat
breeders have used adapted cultivars, landraces,
and translocated chromosome segments from
wild relatives to extend the pool of useful genes.
Nevertheless, variability in existing cultivars and
landraces is still subject to the founder effect and
alien translocations are diffi cult to produce, are
often unstable, and can bring with them unfavor-
able gene linkages. The
de novo
synthesis of new
primary synthetics is an effective way to over-
come the founder effect in wheat. Hundreds of
accessions of
Ae. tauschii
are held in gene banks
around the world, with still more
in situ
in native
habitats, and extensive collections of cultivated
and wild tetraploids are readily available. Vari-
ability among these wild diploid and tetraploid
progenitors of hexaploid wheat has likely changed
through natural selection over thousands of years.
Many of these species have been collected in
some of the harshest environments on earth, and
it is likely that useful genes for adaptation to
these conditions have accumulated with time.
This chapter explores the genetic variability
available in primary synthetic hexaploid wheat
and the application of this variation in wheat
breeding.
