28. Distribution of Ef-1 alleles in tropical rice cultivars and common wild rice strains

Department of Agronomy, National Chung Hsing University, Taichung, Taiwan 40227, ROC

Gene Ef-1 (abr. E) for early heading was found in various Japanese rice varieties (Tsai 1973, 1986b, 1991; Tsai and Oka 1966; Sato and Hayashi 1985). Recently, this gene was found in certain Indica cultivars (Tsai 1986b) and in a philippine variety of upland Japonica type, Inakupa (Tsai 1986a). Furthermore, similar genes were found in an Indica variety, IR4227 (IR4227-18-3-2) and some Indian Aman varieties (Tsai 1987).

The heading date of IR4227 is about 26 days later than that of T65, but their F1 plants were 5-8 days earlier heading than of T65. The B1F1 of IR4227 with T65 produced segregants like Ef-1 carriers (Tsai 1987). One of such early- heading plants was backcrossed with T65 three times, and an early line (3)IR 4227E was obtained. The F2 of its cross with T65(20)E^a carrying Ef-1^a showed heading dates within the parental range (Table 1). There were a few relatively late segregants whose frequency was less than 1/16, but their heading dates were still earlier than that of T65. This suggests that (3)IR4227E and T65(20)E^a have similar alleles at the Ef-1 locus, and their interaction in heterozygotes produces such relatively late-heading segregants.

A long-grain variety from Brazil, BR-IRGA409 shows a heading date about 10 days later than that of T65. A radiation-induced mutant from this variety, N115 showed almost the same heading date as of T65(20)E^a. Their F1 plants were backcrossed with T65 three times, and an early-heading line (3)M15E was fixed. The F2 from T65(20)E^a X (3)M15E showed almost the same heading dates as of the parents (Table 1). This indicates that (3)M15E has an Ef-1 allele.

The Aman varieties from India are highly sensitive to daylength. They produce no panicles in the first crop-season at Taichung and are about 15 days later than T65 in the second crop-season. The reciprocal crosses between T65 and an Aman variety produced F1 plants about 15 days earlier-heading than T65 in the first crop-season and similar to T65 in the second crop-season. In the F2 were found segregants similar to Ef-1 carriers (Tsai 1987). These early-heading plants were backcrossed with T65 five times to obtain (5)Aman-E-3, -4, and -7, which were 2-6 days earlier-heading than T65(20)E^a. When they were crossed with the latter, the F1 showed parent-like phenotypes, and the heading dates of the F2 plants were within the parantal range (Table 2). This indicates that the Aman-E lines have Ef-1 alleles. Genes for photo-sensitivity were also isolated from IR4227-18-3-2 and Aman varieties, and the breeding of isogenic lines with those genes is under way.

An Indica variety from mainland China, Nantsao 33, is about one-month earlier heading than that of T65. This variety was found to carry a late-heading gene ef-4(t) and an early heading gene (Tsai 1991). The early-heading gene was introduced into T65, and three backcrosses of the hybrid produced a semi-isogenic line (3)TK45E. Its cross with T65(20)E^a showed heading-time variations (Table

As mentioned above, rice cultivars so far examined all had Ef-1 alleles. Their wild progenitor, O. rufipogon, was then examined in respect to its genes for heading time. Of three strains used, W593 and W1623 produced no panicles in the first crop-season at Taichung, showing their photoperiod sensitivity. In the second crop-season, they headed several days later than T65. When these wild strains were crossed with T65, the heading dates of the F1 plants were similar to, or a few days earlier than that of T65 (Table 3). In the F2 or B1F1 populations, early-heading plants like Ef-1 carriers were found. These plants were backcrossed with T65 4 to 5 times, and fixed early-heading lines (4)W593E, (5)W630E, and (4)W1623E were obtained. When these lines were crossed with T65(20)E^a, the heading dates of the F2 plants were within the parental range (Table 2). Although some segregants were earlier than the parents, no late-heading segregants like T65 were obtained. The results of these experiments suggest that all the lines have similar alleles at the Ef-1 locus. This indicates the presence of Ef-1 in the wild strains.

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