21. A new leaf stripe gene st-5, its linkage with d-2 and the location of gene P in the second linkage group

Years ago, I found a spontaneous mutant with striped leaves in a linkage tester H-61 grown in the experimental field. This mutant line, called H-61-s, showed fine white stripes in the leaf blades and leaf sheaths throughout the growing period, and also in lemma and palea. When it was crossed with normal strains, the F\1\ plants were normal and the F\2\ segregated into 3 normal:1striped type (957:312), showing that the mutant gene was recessive.

H-61-s was then crossed with four linkage tester strains carrying genes for leaf stripes, fs-1 (linkage group I), st-1 (I), st-2 (VI+IX), and st-4 (II). All the F\1\ plants showed normal chlorophyll development, demonstrating that the mutant gene was not allelic to any of these striped leaf genes. In the cross with tester A-26 Ebisu, this gene was found to be linked with d-2 for Ebisu dwarfism, with a recombination value of 27.8+=1.6% (Table 1). This implies that the new gene belongs to linkage group II, so that it differs from st-3 of linkage group XI or from fs-2 of linkage group III. Accordingly, it is symbolized st-5.

The linkage relations of st-5 with genes of the second linkage group, Plⁱ (purple leaf), P (purple apiculus), and lg (liguleless), were tested, but no linkage was detected with any of these genes perhaps because these genes are located distant from st-5. For determining the location of st-5, its relation with d-3 should be examined. Gene d-3 which expresses "bunketsu-waito" is the only gene so far known to show linkage with both d-2 and Pl, which segregate independently of each other.

From the data of these experiments, the location of gene P (purple apiculus), which had remained unmapped, was also investigated. The P gene described by Takahashi (1957) is known to be present in most of Indica and Japonica varieties, but H-61-s has carried its recessive allele, p. The recombination value between P and lg was found to be 34.5% (Table 2). No linkage between P and d-2 was detected. The linkage relations between Plⁱ (purple coloration of leaves, in Indica varieties) and P or another marker in the F\2\ are shown by a modification of 9:39:3:13. This is because most rice varieties carry I-Pl-6, an inhibitor for Pl (independent of Pl although its linkage relations remain unknown); this inhibitor makes the carrier of Plⁱ green and often brings about a 3 purple : 13 green ratio in the F\2\ of purple/green crosses (Kinoshita and Maekawa 1986).

In the F\2\ which segregated for both P and Plⁱ, the P locus was found to be tightly linked with the Pl locus (Table 2), although their distance had been recognized to be 2.7% (RGN 4, p. 16). The distance between Plⁱ and lg was found to be 35.4%, slightly greater than that between P and lg although the difference is by no means significant (Table 2). Accordingly, it may be assumed that the P locus is close to the Pl locus at its proximal side (in the direction of lg).

Table 1. Test of linkages between st-5 and other genes in the second linkage group

Table 2. Recombination values estimated between P, lg, d-2 and plⁱ in the second linkage group

References

Kinoshita, T. and M. Maekawa, 1986. Inheritance of purple leaf color found in Indica rice. Genetical studies on rice plant, 94. J. Fac. Agr. Hokkaido Univ. 62: 453-466.

Takahashi, M., 1957. Analysis of apiculus color genes essential to anthocyanin coloration in rice. J. Fac. Agr. Hokkido Univ. 50: 266-362.