24. Occurrence of weak plants in backcrossed progeny of Taichung 65 and their genetic behavior

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

Several isoalleles advancing heading time have been found at the Ef-1 locus (fgl linkage group). One of them, Ef-1^X (abbr. E^X), was obtained as a mutant in the progeny of X-rayed seed of Taichung 65 (abbr. T65). It had a lower heading-advancing effect than other isoalleles at the same locus (Tsai 1976). To look more closely into the action of this allele, a B\5\F\2\ line of T65 carrying it, T65(5)E^X, was backcrossed by T65 several times more. The B\9\F\2\ population segregated into 74 early (E-type) and 19 late (T65-type) plants. The F\3\ to F\5\ lines were then raised from several F\2\ plants of E- and T65-types. Unexpectedly, and F\5\ line from a plant of T65-type (84-1-2-49) segregated into normal and weak plants in a 188:57 ratio. The weak plant showed chlorosis starting from the fourth leaf and had few tillers, but survivied and produced panicles about 10 days later than the heading time of T65 and set some seeds (Fig. 1). Among the 188 normal plants, three showed an early heading time like the E^X line, while the rest showed the heading time of T65-type. In the subsequent experiments, the normal and weak F\5\ plants are designated as "49N" and "49W", respectively.

The F\6\ lines derived from 5 (49N) plants, one early-heading normal plant, and 9 (49W) plants were observed in the next season. One line from a (49N) plant was fixed for normal stature. Four lines from other (49N) plants each showed a 3 normal: 1 weak segregation, although in some of them weak segregants were deficient. The F\6\ line from a normal early-heading F\5\ plant segregated into 136 normal E-type : 22 dwarf E-type plants. The dwarf plants (30-40 cm in height) resembled another dwarf type found previously in the progeny of an early-heading offtype which occurred from a cross of late-heading parents (Tsai 1986).

The following segregation pattern was observed in the F\6\ lines from 9 (49W) plants. Seven of the 9 lines were fixed for weakness. The eight line (8-2) segregated into one normal (T65-type, 8-2N) and 99 weak (8-2W) plants. The last line (9-4) segregated into 6 normal (9-4N) and 194 weak (9-4W) plants. Reciprocal crosses were made between T65 and an (8-2W) plant. The F\1\ plants were all normal, and the F\2\ segregated into 3 normal:1 weak type (Table 1).

An F\7\ line derived from an (8-2N) plant segregated into normal and weak types in a 140:55 ratio (Table 1). The progeny lines from (9-4N) plants also showed a 3 normal:1weak segregation, although weak plants were deficient in some cases. In two lines, the normal plants segregated into two heading types, one being T- 65 like and the other a late-heading type.

These observations suggest that the weak mutants have a recessive gene which is located at or near the Ef-1 locus. But its genetic behavior is unusual. The occurrence of non-parental dwarf type and late-heading types in the selfed progeny of normal segregants, as well as the reversion to normal of weak plants, indicates that the gene is unstable. The behavior of this gene may be favorably explained by assuming an autonomous transposon involved in the gene system.

Fig.1. Weak segregants found in a B⁹F⁵ progeny form a line with Ef-1^X backcrossed by Taichung 65.

Table 1. Segregation patterns observed in F\2\ populations from crosses between normal and weak types and in the selfed progenies of weak plants

Tsai, K.H., 1976. Studies on earliness genes in rice, with special reference to analysis of isoalleles at the E locus. Jpn. J. Genet. 51: 115-128.

Tsai, K.H., 1986. Gene loci and alleles controlling the duration of basic vegetative growth of rice. In Rice Genetics, pp. 339-349. IRRI, Manila.