Agriculture Reference
In-Depth Information
tion over the past 10-13 million years, which is
the estimated age of the Triticeae tribe (Wolfe
et al., 1989; Gaut 2002).
which diverged from wheat some 35 MYA
(Bossolini et al., 2007) (Fig. 15.1). Two
Brachy-
podium
species have received research attention
as potential model systems for wheat.
Brachypo-
dium sylvaticum
is a tetraploid species with a basic
chromosome number of 7 and a 1C DNA content
of 460 Mb (Bennett and Smith 1991). A BAC
library of
B. sylvaticum
has been constructed and
used in the comparative analysis of wheat-
Brachypodium
relationships at the sequence level
as discussed further in the section, “Colinearity
at the DNA sequence level.”
Brachypodium dis-
tachyon
is a diploid (2
n
= 2
x
= 10) with a 320-Mb
genome, and this species has been selected for
whole-genome shotgun (WGS) sequencing.
Currently, a 4X shotgun sequence is avail-
able (http://www.brachybase.org/) and an 8X
sequence is expected to be released in spring
2009. A comparative map of the
B. distachyon
genome is under development and will provide
information on the relationship at the map level
between the fi ve
B. distachyon
chromosomes and
the seven wheat homoeology groups. Preliminary
analysis of the 4X
B. distachyon
sequence with
bin-mapped wheat ESTs has indicated that four
of the fi ve
B. distachyon
chromosomes may be
colinear with a single wheat homoeology group
each. The fi fth and largest
B. distachyon
chromo-
some appears to show homoeology with three
wheat chromosome groups (M. Bevan, pers.
comm.).
Pooideae subfamily
The Pooideae subfamily includes the supertribes
Triticodeae and Poodea. The former contains the
Triticeae tribe, and the latter the Aveneae and
Poeae tribes. The two supertribes diverged
approximately 25 MYA (Gaut 2002) (Fig. 15.1).
Oat (
Avena sativa
) is the major cereal belonging
to the Aveneae tribe, and the Poeae tribe includes
the forage crops ryegrass (
Lolium perenne
) and
fescue (
Festuca pratensis
). Comparative relation-
ships have been established between wheat and
oat (Van Deynze et al., 1995a), and between
wheat, ryegrass, and fescue (Jones et al., 2002;
Alm et al., 2003). The
Lolium
and fescue genomes,
each of which has a basic chromosome number of
seven, are highly colinear with the wheat genome.
The major difference between the two Poeae
species and wheat is the presence of a segment
with homology to the long arm of wheat chromo-
some 5 in the distal region of the short arm of a
chromosome that is otherwise completely syn-
tenic with wheat chromosome 4. This 4S/5L
rearrangement is different from the 4L/5L trans-
location that is present in some Triticeae genomes
(Table 15.1).
The wheat-oat relationship is more complex.
Oat, as wheat, has a basic chromosome number of
seven, and at least fi ve of the seven oat chromo-
somes have undergone rearrangements relative to
wheat (Van Deynze et al., 1995a). Most of these
rearrangements are specifi c to oat or, possibly, the
Aveneae tribe. The 4S/5L rearrangement that
differentiates the Poeae and Triticeae species is
present in oat, indicating that it took place either
in the lineage leading to the supertribe Poodeae
or in the Triticodeae lineage.
In recent years, understanding the relationship
between the genomes of wheat and
Brachypodium
distachyon
has become a priority. All major crops
within the Pooideae subfamily have large genomes
with genome sizes between 2,000 and 5,500 Mb.
The closest small-genome wheat relatives are
species belonging to the genus
Brachypodium
,
Poaceae family
Pairwise comparisons at the map level between
genetic maps of wheat and those of non-Pooideae
grasses include an analysis of the wheat homoeol-
ogous group 7 chromosomes with maize (
Zea
mays
) chromosome 9 (Devos et al., 1994) and
whole-genome studies of wheat-rice (Ahn et al.,
1993; Kurata et al., 1994) and wheat-maize rela-
tionships (Van Deynze et al., 1995b). Despite
50-70 million years of evolutionary divergence
between grass species (Wolfe et al., 1989) (Fig.
15.1), variation in chromosome numbers (
n
= 12
for rice,
n
= 10 for maize, and
n
= 7 for wheat),
and large differences in genome size (1C = 400 Mb
for rice, 1C = 2,500 Mb for maize, and 1C =
