To develop hybrid rice breeding on a commercial basis, it is essential
to have an efficient seed-production system. One realistic way to increase
the amount of seed production per unit field is to genetically improve
the floral traits in a female parent, which is expected to make them more
acceptable to pollens dispersions from male parents. An indica variety,
IR24, expresses relatively higher frequency of stigma-exserted glumes
compared with that of the typical Japanese cultivars. We here report the
initial trial of the identification of the quantitative trait loci (QTL)
controlling stigma exsertion that may be a useful indicator of seed production
Seventy-one lines of the recombinant inbred derived from the cross between
Asominori and IR24 (Tsunematsu et al., 1997) were cultivated in
the field in natural season. At 10-14 days after first panicle emergence,
three panicles from each of the ten plants in each line were taken and
served to trait evaluation. The frequency of stigma exsertion was defined
as the ratio of the number of glumes which closed palea and lemma but
still retained stigma(s) outside to the total number of the flowered glumes.
In practical evaluation at the paddy, the frequency was scored according
the ten grade system by careful observation. The representative scores
of the lines for QTL analysis were calculated by the average of the grades
in each line.
The frequency distribution of the score of stigma exsertion (Fig. 1) indicates
that both parents have several chromosomal regions increasing the frequency
of stigma exsertion. The result of the QTL analysis by qGene 2.26 (Nelson
1997) is summarized in Table 1. Two QTL increasing the frequency at Asominori
allele and seven QTL increasing at IR24 allele were estimated with a threshold
F value > 5.0. The total QTL explain about 60% of the total phenotypic
variance, which suggests that stigma exsertion is the trait which could
be genetically improved. The largest three QTL increasing the score at
IR24 allele are located in the vicinities of R1468B, R1002 and C1468 on
chromosome 3 and contribute to stigma exsertion equally. On the other
hand, one prominent QTL increasing the score at the Asominori allele in
the vicinity of XNpb331 on chromosome 4 is estimated, even though Asominori
itself does not
express high stigma exsertion.
We have consequently developed backcrossed lines which possess some of
the combinations of the candidate regions under the Japonica genetic background.
A detailed analysis is underway to confirm the accuracy of the gene actions
of the QTLs detected here.
The authors thank Dr. A. Yoshimura, Kyushu University, for providing the
Tsunematsu, H., A. Yoshimura, Y. Harushima, Y. Nagamura, N. Kurata, M.
Yano, T. Sasaki and N. Iwata, 1997. RFLP framework map using recombinant
inbred lines in rice. Breeding Science 46: 279-284.
Nelson, JC, 1997. QGENE: software for marker-based genomic analysis and
breeding. Mol Breed 3: 239-245.