25. Unusual segregation patterns found at the m-Ef locus

Department of Agronomy, National Chung Hsing University, Taichung, Taiwan 400, ROC

An early flowering isogenic line of T65 (Taichung 65) with gene Ef-l^a (formerly symbolized E^a, a hereinafter abbreviated as E^a), T65(7)E^a, was obtained from recurrent backrosses (7 times) of T65 with a native variety from northern China, Tatung-tsailai, used as the donor parent (Tsai 1961; Tsai and Oka 1965). Three isogenic lines with E^a plus m-Ef (formerly symbolized m^a, hereinafter abbreviated as m, since m^a and m^b were found to be identical), T65(7)E^am^a, T65(7)E^am¹, and T65(7)E^am², were also isolated from the backcrossing experiment. The three sib-lines showed a heading time much earlier (about 10 days) than that of T65E^a which was about 8 days earlier than T65 (with ef-1 and m⁺-Ef) in both the winter (first-crop) and summer (second-crop) seasons almost similarly; the m allele emphasizes the heading-promoting effect of E^a. However, the heading time of lines with m alone (without E^a or with ef-1) was earlier than that of T65 only a few days in winter, and did not differ from that of T65 in summer (Tsai and Oka 1966). The m locus was found to be linked with Rc (red pericarp), the recombination value being 23%; it belongs to the 4th linkage group (Tsai 1984).

The F\2\ of T65(7)E^a x T65(7)E^am^a, segregated for m into 1 early (E^am): 2 medium: 1 late (E^aM⁺) type, and the early type bred true in the F\3\. The F\2\ of T65(7)E^a x T65(7)E^am¹, 149 plants in total, also segregated similarly, but a part (6 of 28) of the early-flowering segregants did not breed true; the 6 F\3\ lines, which were expected to be homozygous for earliness (E^am), segregated into early and late types, the ratio being approximately 8:1. Among the F\4\ lines derived from 2 of the 6 segregating F\3\ lines, those from 25 early-flowering F\3\ plants bred true for earliness, but those from 5 late-flowering (E^am⁺ type) F\3\ plants showed a 1 early : 2 medium : 1 late ratio, suggesting that the late segregants from E^aM plants had the m⁺ allele.

The F\2\ of T65(7)E^am^a x T65(7)E^am², both parents being early tuypes, produced 6 late-flowering off-types (E^am⁺ type) out of 280 plants. The F\3\ lines from 3 of the F\2\ off-types segregated into 6 early and 51 late plants, and those from the other 3 F\2\ off-types segregated into 11 early and 47 late plants. Although the parental lines had no m⁺ allele, the F\2\ offtypes appeared as if they were heterozygous for the m locus (E^a/E^am/m⁺). About 2/3 of F\4\ lines derived from 3 early- and 16 late- flowering plants of an F\3\ segregating line showed a 1 early : 3 late ratio in each of them.

With regard to the occurrence of early- and late-flowering off-types due to some genic changes at the E^a locus, the present author has suggested intralocus recombination of subunits assuming that E^a was a complex locus, as the frequency of off-types was lower (1% or less; Tsar 1976). In the present case, however, the genic changes have a higher frequency and the m locus appears as if it is mutable. In families showing unusual segregation for heading time, a few morphological off-types (dwarfing and chlorophyll anomaly) were also found. It may be suggested that a controlling element or 'transposon' is attached to the m locus, as suggested in maize by McClintock (1951) and Fedoroff (1983). The evolutionary significance of the mutability of loci controlling heading time may also be noticed.

Fedoroff, N.V., 1983. Controlling elements in maize. In J. A. Shapiro (ed.), Mobile Genetic Elements, p.1-63. Acad. Press, New York.

McClintock, B., 1951. Chromosome organization and gene expression. Cold Spring Harbor Symp 16: 13-47.

Tsai, K. H., 1961. Breeding of glutinous and early isogenic lines from a rice variety, Taichung 65. J. Agric. Assoc. China, N.S. 35: 18-23.

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

----- , 1984, Physiology and genetics of heading property in rice, 1. Analysis of the emphatic gene of earliness. J. Agric. Assoc. China, (in press)

----- and H. I. Oka, 1965. Genetic studies of yielding capacity and adaptability in crop plants, 1. Characters of isogenic lines in rice. Bot. Bull. Acad, Sinica 6: 19-31.

----- and ----- , 1966. Ditto, 2. Analysis of genes controlling heading time in Taichung, 65 and other rice varieties. Bot. Bull. Acad. Sinica 7: 54-70.