Agriculture Reference
In-Depth Information
sequencing of selected BAC clones, mapping of
low-copy sequences identifi ed within the BACs,
and rescreening of the BAC libraries led to a
contig in
T. durum
that spanned the
Pm3b
locus.
The physical-to-genetic distance for that region
was shown to be 900 kb cM
−1
. The contig con-
tained three resistance-gene-like (RGL) sequences,
designated
TdRGL-1
,
TdRGL-2
, and
TdRGL-3
,
which were members of a large gene family on the
wheat group 1 chromosomes (Yahiaoui et al.,
2004). Using primers to conserved regions, fi ve
RGL genes were amplifi ed from chromosome 1A
from a near-isogenic line containing
Pm3b
. Four
of these were mapped, but none cosegregated
with
Pm3b
. No specifi c markers could be devel-
oped for the fi fth RGL gene. Differences in gene
content in the
Pm3
region between
T. monococ-
cum
,
T. durum
, and
Pm3
-resistant and
Pm3
-sus-
ceptible
T. aestivum
lines, identifi ed through a
combination of sequencing and hybridization
experiments, led to the isolation of a DNA frag-
ment that cosegregated with
Pm3b
and was 100%
identical to the RGL gene that had not been
mapped previously. The identity of this gene was
confi rmed both by analysis of a γ-irradiation
mutant that had become susceptible to powdery
mildew due to a single base-pair deletion in the
Pm3
gene, and a transient single-cell assay
showing reduced fungal growth in cells bom-
barded with the
Pm3
candidate gene.
VRN-1, VRN-2,
and
VRN-3
Vernalization is the requirement for a period of
cold to accelerate fl owering initiation. It is a
mechanism to protect the cold-sensitive meri-
stems from damage during the winter. In the
diploid
T. monococcum
, vernalization response is
regulated mainly by two genes,
VRN-1
and
VRN-2
:
VRN-2
is located distally on chromo-
some arm 5AL in the region translocated from
4AL (Dubcovsky et al., 1998);
VRN-1
is located
more proximally on 5AL. In hexaploid wheat,
the main vernalization gene is
VRN-1
with
homoeoloci on chromosome arms 5AL, 5BL, and
5DL. Spring habit is dominant at the
VRN-1
loci, but recessive at the
VRN-2
loci. Because
winter habit is ancestral, the
VRN-2
genes can
confer spring habit in hexaploid wheat only if
homozygous recessive at the
VRN-A2
,
VRN-B2
,
and
VRN-D2
loci. This explains why phenotypic
variation at the
VRN-2
locus has not been
observed in hexaploid wheat. A third vernaliza-
tion response locus,
VRN-4
, was fi rst reported in
the 1960s and located on wheat chromosome arm
7BS (Law 1966). This gene was identifi ed in a
Chinese Spring ('Hope 7B') substitution line.
Because the presence of
VRN-4
in the cultivar
Hope could not be confi rmed in a subsequent
study by Goncharov and Gaidalenok (1994), it
was suggested that
VRN-4
might, in fact, be
VRN-1
that had been translocated from the
group 5 chromosomes to 7BS during the devel-
opment of the Chinese Spring (Hope 7B) substi-
tution line. Isolation of the
VRN-4
gene showed
that this gene is indeed different from
VRN-1
.
Based on its orthology to the barley
VRN-H3
gene, the wheat
VRN-4
gene was renamed
VRN-
3
(McIntosh et al., 2007).
Low-density genetic mapping had shown the
VRN-1
gene to be fl anked distally by WG644 and
proximally by CDO708. These markers were
used to identify the orthologous region in rice,
which was subsequently used as a source of
markers for the
VRN-1
region. Two markers
were used to screen an F
2
population of 3,095
T.
monoccocum
F
2
plants for recombination events
surrounding
VRN-1
(Yan et al., 2003). All rice
Genes involved in adaptation
Five genes involved in adaptation have been iso-
lated using a map-based cloning approach in
wheat. This includes the three vernalization
response genes
VRN-1
,
VRN-2
, and
VRN-3
(as
elaborated in Chapter 3), the gene that confers the
spelt phenotype to wheat,
Q
, and the gene that
controls homologous pairing in polyploid wheat,
Ph1.
In contrast to the isolation of the disease
resistance genes, which were rarely conserved
between wheat and rice, the map-based cloning
strategies employed for the isolation of
VRN-1
,
VRN-2
,
VRN-3
,
Q
, and
Ph1
relied heavily on the
use of the rice genomic sequence as a source of
markers.
