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Legume Genomics and Genetics (online), 2010, Vol. 1, No.2, 7-10
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9
single recessive locus. After several backcrosses with
the wild type in which 3:1 ratios of the wild type and
mutant were observed consistently in the segregating
populations,
kew3
behaved as a stable mutant. How-
ever, in the backcross with the wild type, the phenol-
type of
kew2
could disappear in the following generations.
Since the phenotype of
kew3
mimicked the one of
kew1
,
allelic testing was conducted. It was found that
kew1
and
kew3
were allelic, since all the F
1
progenies displayed
the mutant phenotype as the one of
kew1
and
kew3
.
However, in the allelic test of
kew2
, the F
1
progenies
of the cross between
kew1
and
kew2
were segregated
as either the wild type or
kew1
. These suggest that in
the
kew2
genetic background, the
kew1
locus must be
mutated but should be in a heterozygous state, sug-
gesting that other genetic locus/loci were contributing
to the mutant phenotype in the original
kew2
mutant.
2 Discussion
Floral zygomorphy (flower with bilateral symmetry), as
a specialized forms of flower symmetry, is an important
evolutionary trait in flowering plants with multiple
origins.
The molecular analysis on the mechanism in
the control of zygomorphic development was first
conducted in a model plant,
Antirrhinum majus
, that
was commenced from the screening of floral mutants
from mutagenesis experiments (Carpenter and Coen,
1990). Combination of precise genetic analyses and
transposon-tagging approach, a few key regulatory
genes were identified and cloned. In the past ten years,
we have explored the model papilionoid legume,
Lotus
japonicus
, which has the advantages for molecular
genetic studies due to its relative smaller size, simple
genome structure, feasible for genetic transformation and
other characteristics. In this study, we reported our effort
to conduct large scale mutagenesis in
L. japonicus
.
Ionizing radiation, such as γ and X–ray, are the efficient
physical mutagens, which produce deletions leading to
chromosome break and chromosome aberrations. On
the other hand, most chemical mutagens normally cause
base pair substitutions, especially from GC to AT in the
case of EMS. With the difference in the mechanism, the
different mutagenesis, such as, physical and chemical
mutagens have different hotspots, produced different
spectrum (Van der Veen. 1966; Feldmann, 1991). In our
mutagenesis in
L. japonicus
, a few
kew
mutants were
identified in the γ-ray mutagenized M2, but EMS did
not produce the
kew
mutation yet in our experiment.
This could either be a by-chance case or alternatively
indicates that different type of muta- tions might be
obtained through different mutagenesis.
In
Antirrhinum
majus
, two TCP genes,
CYCLOIDEA
(
CYC
) and
DICHOTOMA
(
DICH
), and two MYB
genes,
RADIALIS
(
RAD
) and
DIVARICATA
(
DIV
) play
key role in the development of zygomorphic flower,
and their interplay determine the petal differentiation
(Carpenter and Coen 1990; Luo et al., 1996; 1999;
Almeida et al., 1997;
Galego and Almeida, 2002). It
has been shown that both
CYC
and
DICH
control the
dorsal and lateral petal identity during zygomorphic
development. In
cyc dich
double mutant, all petals
resemble the ventral petals of wild type.
RAD
and
DIV
interplay with
CYC
and
DICH
, and two pairs of key
regulators control the asymmetry of the whole flower, as
well as the one of lateral and dorsal petals in
Antirrhinum
majus
(Corley et al., 2005).
L. japonicus
belongs to Papilionoideae, whose floral
asymmetry is thought to have an independent origin,
different from the one in
Antirrhinum majus
. In our
recent study on the floral zygomorphy in pea, another
well known model plant in papilionoid legumes, we
identified two kinds of key regulatory genes in the
control of zygomorphic flower development, and pro-
posed that they determine the two kinds of asymmetries
independently: the dorsoventral (DV) and organ internal
(IN) asymmetries in the floral plane and organ plane
respectively (Wang et al., 2008).
K
and
KEW
play the
important role in the control of lateral petal development
and thus are the key regulatory genes in the DV pathway.
As a comparison, no single locus which controls the
lateral petal development has been identified in
Antirr-
hinum majus
.
In this study, we characterized two
kew
mutants. Gene-
tic analysis indicated that they were allelic to
kew1
, a
mutant locus being identified as the homolog one of
k
based on the comparative genomic data in the previous
study (Feng et al., 2006).
K
has been cloned and en-
codes a TCP factor PsCYC3 (Wang et al., 2008). The
most closely homolog of PsCYC3 in
L. japonicus
is
LjCYC3. However, in the genomic sequence of
LjCYC3
,
no mutation has been identified yet. With more alleles
of
kew1
were isolated, the molecular base for the
kew