Agriculture Reference
In-Depth Information
From a practical perspective, a large number
of simply inherited dominant or recessive genes
conferring different types of resistances are still
available in wheat germplasm and wild relatives of
wheat. A solid knowledge of the mechanisms of
polyploidization will help scientists in manipulat-
ing gene pools to improve cultivated wheat. Sci-
entists and historians have long been searching for
an explanation of the evolution and domestication
of the various forms of cultivated wheat (diploid,
tetraploid, and hexaploid;
T. monococcum
,
T. tur-
gidum
,
and T. aestivum
, respectively). The origin
of polyploid wheat is complex because its evolu-
tion, since the various grass species diverged, has
involved a long-established massive intervention
of human and environmental selection pressures.
The evolution within the entire Triticeae tribe
included early widespread intra- and intergenome
hybridization followed by introgression, gene
fl ow, gene fi xation, and rapid diversifi cation within
and among the ancestral diploid and polyploid
species (Kellogg et al., 1996). Unequal rates of
evolution, parallel evolution, DNA sequence dele-
tion and/or amplifi cation, and silencing during
the evolution of present-day wheat species has
been postulated to explain the complexity in phy-
logenetic relationships (McIntyre 1988; Appels
et al., 1989; Feldman 2001).
Evolutionary studies involving various plant
taxa have demonstrated that not only wheat, but
also many polyploids, evolved from different pro-
genitor populations. Independently formed poly-
ploids most likely came in contact to hybridize
with each other, thus resulting in ever-expanding
primary and secondary germplasm pools (reviewed
by Soltis and Soltis 1999). The formation of many
allopolyploids was also accompanied by consider-
able genomic changes and structural reorganiza-
tion within all or some of the parental genomes,
including rapid nonrandom coded and noncoded
sequence elimination, genic silencing, interge-
nomic colonization by repeats and transposable
elements, intergenomic homogenization of diver-
gent DNA sequences, DNA methylation changes,
and other genomic modifi cations (Ozkan et al.,
2001; Liu and Wendel 2002; Ma and Gustafson
2005). These genomic changes have been well
demonstrated in the polyploids of the Triticeae
tribe (Feldman et al., 1997; Kashkush et al., 2002;
Han et al., 2003; Ma et al., 2004; Ma and
Gustafson 2005, 2006). Such genomic changes,
coupled with the likely repeated occurrence of
polyploid formation, also contribute to the con-
fl icting determinations of phylogenetic relation-
ships and origins of many species, including
wheat.
The origin, evolution, and domestication of
cereals were among the major events shaping the
development and expansion of human culture and
will continue to shape the world in which we live.
The domestication of cereals, which occurred
approximately 10,000 years ago, was critical in
laying the groundwork for the Neolithic revolu-
tion that transformed humanity to more central-
ized, sedentary farming societies (for a complete
discussion see Kimber and Feldman 1987; also
see especially Feldman 2001). There is no ques-
tion that the various grass species (approximately
10,000 species), growing in every habitat in the
world, and our understanding of the evolution
of grasses are critical to developing the potential
for grasses to feed the world's ever-increasing
population.
Polyploidy has been defi ned as the presence of
more than one genome per cell and is probably
the most common mode of speciation in plants
(Stebbins 1950; Wendel 2000). Polyploids are
classifi ed into autopolyploids, which are formed
from intraspecifi c chromosome doubling, and
allopolyploids, which are the result of the inter-
specifi c or intergeneric hybridization of two
or more genomes from differentiated species
(Stebbins 1947). Polyploidy is one of the most
important evolutionary events leading to a massive
increase of genetic diversity, thus allowing species
to adapt to varying environments. The most
important and best-characterized group of allo-
polyploids, from an agricultural point of view, is
the wheat genus (Kimber and Sears 1987; Feldman
2001). The evolutionary development of the
various cultivated wheat species comprises several
converging and diverging polyploid events involv-
ing several
Triticum
and
Aegilops
species from the
Triticeae tribe.
It has been estimated that the Triticeae tribe
began diverging from its progenitor approxi-
