To understand the molecular mechanisms forming the seed size of rice,
we are carrying out the identification of causal genes in small-round
seed mutants. Previously, two genes, D1 and D11, which control
the seed size, were isolated (Ashikari et al. 1999, Fujisawa et al. 1999,
Tanabe et al. 2005). D1 and D11 genes are located in the
position 15.4 Mb of chromosome 5 and 23.4 Mb of chromosome 4, respectively.
In this paper, we report the mapping position of the causal gene in a
new small-round seed mutant of rice, TCM1173.
TCM1173 was obtained from Taichung 65 (T65) library mutagenized with N-methyl-N-nitrosourea
treatment and renamed srs3 in this paper. Gross morphology of srs3
was shown in Fig. 1. The plant height of srs3 was similar to that
of the recurrent parent, but the seed size of srs3 was smaller
than that of its parent.
To identify the SRS3 gene, srs3 (japonica) was crossed
with Kasalath (indica). The F1 plants were self-pollinated
to obtain F2 seeds. We screened 90 F2 plants bearing small-round
seeds. Genomic DNA from leaf tissues was extracted by the CTAB method.
The genetic linkage between the SRS3 locus and molecular markers was determined
by PCR using the sequence tagged site (STS) markers reported by the Rice
Genome Program and microsatellite markers (McCouch et al. 2002a, b). In
addition to these markers, the eight primers for the four STS markers
on rice chromosome 5, 5-2500F (5'-GGATCACGACGAGGTAGAAC-3') and 5-2500R
(5'- CGCTTGGCCTTCTCCGCATC-3'), 5-3000F (5'-TCGGATGGTACCCTACGAAC-3') and
5-3000R (5'-TCAATGGCAGTTTCCAAGTTTC-3'), 5-3247F (5'-TCCCACAAGCGCTACCTCAG-3')
and 5-3247R (5'-CGAAAATTCCCCGGACGCAG-3'), and 5-3599F (5'-GCAGTGGTCTCCAAACTGATG-3')
and 5-3599R (5'-CTTAACAGCTGTCAAACTAGAG-3') were designated. The SRS3
gene was mapped between 5-3000 and 5-3247 markers on the chromosome 5
and co-segregated with the C52717 marker (Fig. 2). Because the SRS3
gene is mapped between the 5-3000 and 5-3247 markers, which are located
in the position 3 Mb and 3.25 Mb on chromosome 5, respectively, srs3
may not be allelic to d1, in which mutated gene is mapped on the
location 15.4 Mb of chromosome 5. We are constructing a more precise linkage
map of the SRS3 locus using more than 1000 F2 plants bearing small-round
seeds and STS markers between 5-3000 and 5-3247 markers.
Acknowledgements
We thank Dr. H. Satoh, Institute of Genetic Resources, Kyushu University,
for the gift of TCM1173 mutant.
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