Agriculture Reference
In-Depth Information
from crosses between spring and winter lines or
between parental lines with different sensitivities
to photoperiod. The only example for fi ne
mapping of
EPS
genes is
EPS-A
m
1
on chromo-
some 1A
m
in
T. monococcum
, which has been
delimited within a 0.9-cM interval and encom-
passed within a colinear region between wheat
and rice (Valárik et al., 2006).
habit (Takahashi and Yasuda 1971; Dubcovsky
et al., 1998). Epistatic interaction between
VRN-
A
m
1
and
VRN-A
m
2
was detected in a
T. monococ-
cum
population in which only these two genes
segregated for growth habit (Tranquilli and
Dubcovsky 2000).
Based on current understanding of the three
vernalization genes in diploid wheat and barley,
only the allele combination of recessive
vrn-1
and
vrn-3
and dominant
Vrn-2
(
vrn-1Vrn-2vrn-3
)
confers winter growth habit in diploid wheat
(Pugsley 1971; Takahashi and Yasuda 1971;
Tranquilli and Dubcovsky 2000; von Zitzewitz
et al., 2005; Yan et al., 2006; Sz
″
cs et al., 2007).
Due to the presence of three homoeoalleles for a
given gene, the analysis of gene action (domi-
nance or recessiveness) and multigenic epistatic
interactions will be much more complicated
in hexaploid wheat. The recombinant
vrn-A1vrn-
B1vrn-D1Vrn-A2vrn-B2vrn-D2vrn-A3vrn-
B3vrn-D3
is but one example of 2
9
gametic
possibilities for nine genes determining growth
habit in hexaploid wheat. Precise separation of
phenotypes becomes impractical with even more
genes likely involved in hexaploid wheat. Never-
theless, cloning and characterization of a gene will
greatly facilitate analysis of allelic variation and
genotypic identifi cation.
Quantitative trait loci affecting
fl owering time
Previous studies with aneuploid and substitution
lines of CS indicated the presence of genes affect-
ing fl owering time on almost every chromosome
of hexaploid wheat (Law et al., 1998). Laurie
et al. (1995) predicted 25 loci controlled the dura-
tion of the life cycle based on comparative studies
of
VRN
,
PPD
, and
EPS
genes between diploid
barley and hexaploid wheat. However, approxi-
mately 80 genes have been reported to affect fl ow-
ering time in Arabidopsis (Levy et al., 2002;
Tasma and Shoemaker 2003). Due to the pres-
ence of three homoeologous genomes, it would
not be surprising if more than 200 orthologous
genes were found to affect fl owering time in hexa-
ploid wheat.
When two parental lines with diverse genetic
backgrounds are used to generate a population,
fl owering time may be mapped by quantitative
trait loci (QTLs), and one QTL may appear in
one population but not in another. Many QTLs
for fl owering time have been reported, and the
presence of certain QTLs may be affected by ver-
nalization or photoperiod treatment, or both of
them, or neither of them. In addition, interactions
among genes or QTLs will cause greater com-
plexity of gene effects on fl owering time in hexa-
ploid wheat.
POSITIONAL CLONING OF FLOWERING
TIME GENES IN WHEAT
VRN-A
m
1
, an orthologue of
AP1
,
promotes fl owering
Using 6,190 gametes from the G2528 × G1777 F
2
population of diploid wheat
T. monococcum
,
VRN-
A
m
1
was delimited in a 0.03-cM interval contain-
ing two MADS-box genes,
AP1
(
APETALA 1
)
(Mandel et al., 1992) and
AGLG1
(
AGAMOUS
LIKE 2 in GRASSES
), which are orthologues of
two Arabidopsis meristem identity genes (Yan
et al., 2003). The Arabidopsis
AP1
gene is respon-
sible for the apical transition from vegetative to
reproductive phase (Mandel et al., 1992), whereas
AGLG1
belongs to
AGL2
, which is involved in
Epistatic interactions
Dominance or recessiveness of genes and their
epistatic interactions can be genetically deter-
mined in a segregating population. Alleles
Vrn-1
and
Vrn-3
are dominant for spring growth habit,
whereas
Vrn-2
is dominant for winter growth
