Agriculture Reference
In-Depth Information
habit in hexaploid wheat,
T. aestivum
L. (2
n
= 6
x
= 42, genome BBAADD). In this context,
Vrn
denotes the dominant allele,
vrn
denotes the
recessive allele,
VRN
denotes the locus without
allele specifi city, and nonitalicized VRN denotes
the gene product or protein. This nomenclature
applies to symbols for other genes. The
VRN-1
genes were mapped in colinear regions of the long
arm of chromosome 5A, followed by original
VRN-2
on 5B, and original
VRN-3
on 5D
(reviewed in Dubcovsky et al., 1998). These three
genes on group 5 were subsequently found to be
homoeoallelic to each other; therefore, they were
renamed as
VRN-A1
,
VRN-B1
, and
VRN-D1
,
respectively, by McIntosh et al. (1998). The
orthologous gene
VRN-H1
in
Hordeum vulgare
(originally
SH2
) (Laurie et al., 1995) and
VRN-
R1
in
Secale cereale
L. rye (originally
SP1
)
(Plaschke et al., 1993) were also mapped in colin-
ear genomic regions in these species.
The orthologous
VRN-A
m
1
gene in diploid
wheat
T. monococcum
(2
n
= 2
x
= 14, genome
A
m
A
m
) was mapped in association with growth
habit in an F
2
segregating population generated
from a cross between spring type G2528 and
winter type G1777 (Dubcovsky et al., 1998). This
segregating population of diploid wheat hosted
subsequent work to successfully clone the
VRN-
A
m
1
gene using the positional cloning approach
(Yan et al., 2003).
Orthologous genes in hexaploid wheat for
VRN-A
m
2
could be
VRN-A2
,
VRN-B2
, and
VRN-D2
, but these proposed genes have never
been genetically detected in hexaploid wheat. One
reason is that only when all three homoeologous
genes have a recessive allele could spring growth
habit caused by this group of genes be detected in
this species (Dubcovsky et al., 1998).
VRN-B3 on the short arm of chromosome 7B
The origin and nomenclature of the
VRN-B3
gene in wheat has a more complicated history.
The
VRN-H3
locus (also
SH3
or
SGH3
) was fi rst
reported on chromosome 1H in barley based on
its loose linkage (45 cM) with the
BLP
(black
lemma and pericarp) locus on this chromosome
(Yasuda 1969). The chromosomal location of
VRN-H3
was determined by mapping a popula-
tion generated from crossing spring type 'Tammi'
with winter type 'Hayakiso 2'. When the Tammi-
derived spring barley genetic stock, BGS213,
which had the expected allele combination of
recessive
vrn-H1
and dominant
Vrn-H2Vrn-H3
(Takahashi and Yasuda 1971), was crossed with
winter barley
H.
vulgare
ssp. s
pontaneum
C. Koch
(Thell) expected to have recessive
vrn-H1vrn-H3
and dominant
Vrn-H2
, segregation for growth
habit in the F
2
population was controlled by
a single locus,
VRN-H3
, as expected (Yan
et al., 2006). However, this locus was not
mapped to chromosome 1H, the expected loca-
tion of
VRN-H3
; instead, it was mapped to
the short arm of chromosome 7H. This fi nding
of
VRN-H3
on
chromosome 7H in barley lead
to application of gene symbol
VRN-B3
for
an orthologous gene in hexaploid wheat on
chromosome 7B.
Chromosome 7B of hexaploid wheat was
already reported to carry a fl owering time gene
designated as
E
in a mapping population gener-
ated from a cross between 'Chinese Spring'
(CS) and a line with the substituted 'Hope'
chromosome 7B in CS [CS(Hope7B)] (Law
1966; Law and Wolfe 1966). The
E
locus was
later found to be sensitive to vernalization
and was renamed
VRN5
(Law and Worland
1997) but was later renamed again as
VRN-B4
VRN-A
m
2 on chromosome 5A
m
in a genomic region
translocated from chromosome 4A
m
A second vernalization gene was found in the
diploid wheat
T. monococcum
and designated
VRN-A
m
2
; it was located in the distal region of
chromosome 5A
m
L within a segment that was
translocated from chromosome 4A
m
(Dubcovsky
et al., 1998). This gene resides at a chromosomal
location orthologous to the
VRN-H2
locus
(originally
SH
) on chromosome 4H in barley
(Takahashi and Yasuda 1971). The
VRN-A
m
2
gene was also mapped in an F
2
segregating popu-
lation generated from a cross between spring type
DV92 and winter type G3116, facilitating its sub-
sequent cloning via the positional cloning
approach (Yan et al., 2004b).
