Agriculture Reference
In-Depth Information
variation using EMS mutagenesis were also
unsuccessful (Wall et al., 1971).
In 1977, the
Ph1
gene was mapped to the long
arm of chromosome 5B using an X-ray mutant,
designated
Ph1b
(Sears 1977). The mutant carried
a deletion that included
Ph1
and was later shown
to encompass some 70 Mb of DNA (Gill et al.,
1993b). Using additional mutant lines, the
Ph1
region was subsequently reduced to less than
3 Mb in wheat (Gill et al., 1993b) and to an
orthologous 400-kb region in rice, spanned by the
markers
Xrgc846
and
Xpsr150A
(Roberts et al.,
1999). Synteny between wheat and rice was gen-
erally conserved in the
Ph1
region (Foote et al.,
1997), but mapping was hampered by the techni-
cal diffi culty of hybridizing genes from rice, which
diverged some 50 MYA from wheat, to the large
wheat genome. Therefore,
B. sylvaticum
, a Pooid
species 35 MY distant from wheat, was used as
an intermediate (Foote et al., 2004).
Brachypo-
dium sylvaticum
BAC clones were identifi ed with
rice probes, sequenced at low redundancy, and
used as a source for the development of new
markers. The
Brachypodium
markers were used
for mapping, and also for screening a Chinese
Spring bread wheat BAC library (Griffi ths et al.,
2006). This resulted in fi ve BAC contigs of 500 kb,
300 kb, 1.2 Mb, 1 Mb, and 200 kb. Thirty-three
wheat BACs spanning the 5 contigs were
sequenced. This region contained 34 genes.
Expression profi ling of all 34 genes failed to show
variation in lines with and without
Ph1
. Homol-
ogy with genes of known function suggested that
the
cdc2
genes, which affect chromosome conden-
sation, were the best candidates for
Ph1
. The
cdc2
genes form a gene cluster on the wheat group 5
chromosomes with at least four members on chro-
mosome 5B, one of which is chromosome-spe-
cifi c. The
cdc2
gene cluster on 5B was interrupted
by an insertion from the subtelomeric region from
chromosome arm 3AL. The authors concluded
that a tandem array of a 2.3-kb repeat unit, present
within this 3AL segment, together with the
cdc2
genes fulfi lled the functional criteria for
Ph1
activity. This was supported by the fact that the
5B-specifi c
cdc2
gene and 2.3-kb array were
present in all tetraploid wheats tested, but not in
Ae. speltoides
, the diploid species most closely
related to the presumably extant B-genome pro-
genitor. Validation of the
Ph1
candidates is in
progress.
PHYSICAL MAPPING IN
HEXAPLOID WHEAT
Hexaploid wheat is grown on over 95% of the
worldwide wheat-growing area. Breeders are
interested in utilizing its sequence to accelerate
genetic improvements for the growing demands
for high-quality food produced in an environ-
mentally sensitive, sustainable, and profi table
manner. A fi rst step in accessing the genome
sequence lies in the establishment of a physical
map. However, the construction of a physical
map of the hexaploid wheat genome is a daunting
task due to its size, prevalence of repetitive DNA,
and presence of three homoeologous genomes. To
establish a physical map with 15-fold genome
coverage, about 2.1 million BAC clones with an
average size of 120 kb would need to be fi nger-
printed, assembled into contigs, and anchored to
the genetic maps. While fi ngerprinting millions
of BAC clones is feasible using high-information-
content fi ngerprinting techniques (Luo et al.,
2003; Meyers et al., 2004), it is not clear if the
existing technology permits their specifi c assem-
bly into contigs that faithfully represent the indi-
vidual chromosomes. Moreover, anchoring the
homoeologous BAC contigs to the genetic maps
would be extremely time-consuming, would
increase costs signifi cantly, and would require the
development of a large set of genome-specifi c
markers that are not yet available in wheat. Thus,
to handle the super-sized hexaploid wheat
genome, some kind of complexity reduction
would be very useful. Until now, two main strate-
gies have been considered. The fi rst one involves
the construction of physical maps from diploid
progenitors of hexaploid wheat such as
Ae. taus-
chii
, and the second one is to construct BAC
libraries from fl ow-sorted chromosomes or chro-
mosome arms and generate physical maps for
each chromosome arm individually.
