Agriculture Reference
In-Depth Information
regions around the world and in germplasm col-
lections. Over the past century of modern breed-
ing, attempts to produce cultivars that meet the
advanced agriculture demands of an ever-increas-
ing population has resulted in the landraces being
almost wholly displaced by genetically uniform
cultivars. The result of modern agriculture has
been a marked narrowing of the genetic base in
probably all advanced agricultures (Harlan 1975,
1976, 1992). While wheat yields have kept up
with population demands in advanced agricul-
tures (e.g., Avery 1985), genetic homogeneity
has also dramatically increased due to modern
agricultural practices (Frankel and Bennett
1970; Frankel and Hawkes 1975; Harlan 1975,
1976, 1992; Frankel and Soulé 1981; Nevo et al.,
1982; Nevo 1983, 1986, 1989, 1995, 1998, 2001;
Plucknett et al., 1983, 1987; Lupton 1987; Nevo
and Beiles 1989). Consequently, the genetic base
of many cultivated crops, including wheat, has
been narrowed and placed under serious risk
(Frankel and Soulé 1981; Plucknett et al., 1987).
A global network of gene banks has been estab-
lished to provide plant breeders with the genetic
resources for maintaining germplasm collections
and for developing more resistant and tolerant
crops that will improve production (Lupton
1987; Plucknett et al., 1987; Brown et al., 1989,
1990).
Dynamic
in situ
conservation of landraces and
wild relatives, the best hope for improving culti-
vated plants (Feldman and Sears 1981), is being
actively discussed as an optimal conservation
strategy (Hawkes 1991; Heyn and Waldman 1992;
Valdes et al., 1997; Maxted et al., 1997; Nevo
1998). Just as important as the conservation of
diverse germplasm is the achievement of a more
effi cient and comprehensive utilization of con-
served wild gene pools. It is essential to be able to
effi ciently predict, screen, and evaluate promising
genetic diversity and resources, thereby optimiz-
ing crop improvement (Nevo 1983, 1989, 1992,
1995, 2001; Peng et al., 2000a,b,c). The analysis
of genetic diversity across the geographic range,
at both the macro- and microscale, will unravel
patterns and forces driving wheat genome evolu-
tion and lay open the full potential of its genetic
resources for utilization.
ORIGIN, DOMESTICATION, AND
EVOLUTION OF WHEAT
Modern wheat cultivars belong primarily to two
polyploid species: hexaploid bread wheat [
T. aes-
tivum
(2
n
42 chromosomes)] and tetraploid
hard or durum-type wheat [
T. turgidum
L. (Thell.)
(2
n
=
6
x
=
28)] used for macaroni and low-rising
bread. The cultivated diploid species
T. monococ-
cum
L. einkorn wheat (2
n
=
4
x
=
14) is a relic and
is only found in some mountainous Mediterra-
nean regions. Wheat is predominantly self-
pollinated; hence, genetic diversity is represented
in the wild by numerous clones, in vast national
and international germplasm collections, and in
current cultivation by some 25,000 different cul-
tivars. Wild and primitive wheat forms have
hulled grains and brittle ears that disarticulate at
maturity into individual spikelets, with each
spikelet having a wedge-shaped rachis internode
at its base, and an arrowlike device that inserts the
seed into the ground (Zohary 1969). By contrast,
all cultivated wheat forms have nonbrittle ears
that stay intact after maturation, thus depending
on humans for reaping, threshing, and sowing.
The nonbrittleness and nakedness of cultivated
wheat is controlled by the Q locus (Luo et al.,
2000), located on chromosome 5 of genome A,
which may have arisen from the
q
gene of the
hulled varieties by a series of mutations (Feldman
et al., 1995).
=
2x
=
Polyploidy, a form of plant evolution
The evolution of the genus
Triticum
serves as
one of the best models of polyploidy, one of the
most common forms of plant evolution (Elder
and Turner 1995; Soltis and Soltis 1999). The
gradual shift to a steady-production-based agri-
culture has been the main driving force behind
the domestication of wheat. The evolution of
domestication can be considered as the evolution
of new crop species by natural and artifi cial
selection, and the evolution of human civilization
as we know it. Unfortunately, this has resulted in
a massive population explosion and greatly
increased world hunger in many regions of
the world.
