Agriculture Reference
In-Depth Information
mutant; for a complete review, see Sears 1977),
thus preventing multivalent chromosome forma-
tion with deleterious intergenomic exchange, was
critical for the stabilization of all polyploid wheat
species. Mutations, within the various wheat
genomes, also played a major role in allowing
wheat to increase in variability, stabilize as a
species, and become the major food crop. The
two and three genomes present within tetraploid
and hexaploid wheat, respectively, and the self-
pollinating character of all species, resulted in the
accumulation of mutations that became available
for selection. This allows for individuals within
populations to become a main driving force upon
which natural selection operates. This form of
gene formation, modifi cation, and stabilization is
one of the most powerful processes in plant
evolution.
regions (Gu et al., 2004) of many species are
mainly composed of retrotransposons, and the
vast numbers of these retrotransposons are cor-
related with genome size (Kidwell et al., 2002).
Most of the retroelements in the three wheat
genomes are not colinear, which suggests that
their present location was the result of genome
divergence after the individual A, B, and D
genome parental species were combined (Gu
et al., 2004). When analyzing the glutenin genes
of wheat, Gu et al. (2004) found that more genes
from the glutenin region of the A genome con-
tained retrotransposons than occurred in ortholo-
gous regions of either the B or D genome.
Is all or most of the noncoded DNA present
in hexaploid wheat really “junk” DNA? From
an evolutionary view, it is highly unlikely that
any genome would expend a vast percentage
of its energy production maintaining DNA that
was of little or no value. The reason behind the
presence and function of vast amounts of non-
coded DNA in the wheat genomes remains largely
unknown. To fully understand and be able to
manipulate wheat genome evolution, the function
and purpose of this noncoded DNA needs to be
investigated.
There is an abundance of data supporting the
ability of a genome to increase and/or decrease in
DNA amount over time, compared with that
observed in its original progenitor. Such genomic
changes (deletions and additions, gene conver-
sions, transposon activation and silencing, chro-
mosomal rearrangements, epigenetic events, etc.)
are known to occur widely in grass genomes
(Feldman et al., 1997; Liu et al., 1998; Ozkan
et al., 2001; Shaked et al., 2001; Kashkush et al.,
2002; Han et al., 2003; Ma et al., 2004; Ma and
Gustafson 2005, 2006) and other polyploid plant
genomes, including, for example,
Brassica napus
polyploids (for an excellent article, see Gaeta
et al., 2007). The frequency of such events is not
uniform across individual chromosomes or within
complete genomes. The selection pressures acting
on DNA deletion or insertion in either a plant or
animal genome can be different, depending on
whether or not changes are located in repeated
DNA, heterochromatin regions, or gene-rich
regions. Diaz-Castillo and Golic (2007) noted in
GENOME EVOLUTION AND
MODIFICATION
We now have a voluminous amount of informa-
tion concerning the ancestors and evolutionary
processes that created polyploid wheat. To fully
understand the genomic evolution of polyploid
wheat, it is important to ask why each of the
diploid genomes comprising polyploid wheat is so
massive relative to other grass species such as rice.
The B genome is 5.15 pg DNA (Furuta et al.,
1986); the A genome is 4.93 pg DNA (Bennett
and Smith 1976); and the D genome is 5.10 pg
DNA (Rees and Walters 1965). On the other
hand, rice contains only 0.6-1.0 pg DNA in
japonica and indica types, respectively (Bennett
and Leitch 1997; http://data.kew.org/cvalues/
introduction.html). However, the various wheat
genomes and the rice genome appear to have
similar genetic composition with a good macro-
level syntenic relationship (Gale and Devos 1998;
Sorrells et al., 2003; Tang et al., 2008). Flavell
et al. (1974) and Gu et al. (2004) established that
over 80% of the hexaploid wheat genome com-
prised noncoded highly repeated DNA sequences
and highly active and nonactive retrotransposons.
The intergenic regions (Bennetzen 2000; Feuillet
and Keller 2002; Wicker et al., 2003) and genic
