Agronomy Department, National Chung Hsing University, Taichung, Taiwan 40227, ROC
A group of weak offtypes were obtained as segregants in the selfed ([-1zF5) progeny of a normal T65-like F4 plant, which was derived from an early-heading F3 offtype (E' type; Tsai 1989). All these lines were isogenic, descending from a B9 heterozygote carrying Ef-lx/ef-1 in the genetic background of T65 (Taichung 65). The F5 population segregated into 188 normal: 57 weak plants showing a good fit to the 3:1 ratio. The weak plants showed chlorosis starting from the fourth leaf stage and had few tillers, headed about 25 days later than T65 and set some seeds. When an F7 weak plant, 8-2W, was crossed with T65, the F1 was normal and the F2 segregated into 3 normal: 1 weak type (Table 1). But some line supposedly homozygous for the weakness gene produced a few normal segregants with a frequency of 1-3 percent (Tsai 1989).
It was assumed that the weakness gene came from incorporation of a transposon (t) into a locus controlling vegetative growth, which is symbolized B. Gene Bt acts as a recessive gene (=b) and its homozygotes show weakness. In heterozygotes B/Bt, as t moves to other loci with frequency q, gametes with B and Bt would occur with frequencies 1/2(1+q) and 1/2(1-q), respectively. On this assumption, weak F2 plants (Bt/Bt) are expected to arise with frequency 1/4(1-q)2=0.25-0.5q+0.25q2, and the frequency of normal plants will be 1/4(3-q)(1+q)=0.75+0.5q-0.25q2. Then, q is obtainable as 1-2SQRT(a) where a is the frequency of weak plants.
The moving rate (q) differed according to lines, reducing the frequency of weak segregants to a value less than 1/4. It may also differ from male to female side and according to other conditions, but such variations may be neglected until they become known.
The frequencies of weak segregants observed in different populations are presented in Table 1. F7 lines 8-2N and 9-4N-1 showed 3 normal: 1 weak segre- gation. The cross of T65 with a weak plant (8-2W) also gave a 3 normal: 1 weak F2 ratio. F7 lines 9-4N-2 and 9-4N-3 segregated into three types, T65-like, late- heading and weak in a 9:3:4 ratio, although in the former line weak segregants (167/763) were fewer than 1/4 of total plants significantly. It is suggested that the late-heading type has E\2\t/E\2\t which is allelic to ef-2 (Tsai 1989, 1990). The late-heading segregants also tended to be deficient although the deviations were not significant. The segregation pattern fitting or close to 9:3:4 ratio suggests that the E\2\t locus for late-heading and Bt locus for weakness are independent. In the
Table 1. Segregation patterns for weak plants observed in selfed progenies of heterozygotes and F2 populations
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No. of segregants
Line/Cross (Genotype =========================== X2 from 3:1
assumed)a T65-like Lateb Weak Total Normal:Weak
normal heading type
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F7,8-N &9-4N-1 (pooled;B/Bt) 300 91 391 0.6ns
T65X8-2W,F2 (B/BXBt/Bt) 150 42 192 1.0ns
F7,9-4N-2 (E2/E2tB/Bt) 466 130 167 763 4.0*(q=0.064)c
F7,9-4N-3 (E2/E2tB/Bt) 161 57 66 284 0.5ns
T65(1)1f-1x8-2W,F2 (e2/e2 B/Bx 127 38 32 197 8.0**(q=0.194)c
E2/E2 Bt/Bt)
(F7)9-4N-2Lx8-2W,F2 (e2/e2 B/Bx 113 31 39 183 1.3ns
E2/E2 Bt/Bt)
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a-Abbreviated gene symbols: E2=Ef-2, e2=ef-2.
b-Gene for late heading: known to be allelic to ef-2 (Tsai 1989, 1990),
c-q: Moving rate of t at gametic level, explanation in text.
ns-non significant,*P<0.05,** P<0.01.
same manner, the F2 from a cross T65(1)lf-1 (with ef-2) X 8-2W (with Bt)
segregated into T65-like: late-heading: weak type in a near 9:3:4 ratio
although the weak plants were deficient. When a late-heading segregant,
9-4N-2L was crossed with a weak plant (8-2W), the F2 also showed a 9:3:4
ratio (Table 1).The weak offtypes have occurred in the progeny of heading-time variants which appeared to have mutant alleles at the Ef-1 locus (Tsai 1990). It was then suspected if the weakness gene was located near the Ef-1 locus. Gene pgl (pale green leaf) is linked with Ef-1 on chromosome 10. To examine whether or not the Bt gene is linked with pgl, a weak plant (8-2W, used as female) was crossed with an isogenic line T65(9)pgl derived from 9 backcrosses with T65. The parental genotypes in this cross are expected to be B/B pgl/pgl for T65(9)pgl and Bt/Bt +/+ for 8-2W. The F1 was a normal T65-type, and the F2 segregated into 113 normal: 56 pgl: 22 weak types. It may be assumed that gene pgl is not expressed in weak plants. The normal:weak ratio (113+56:22) deviates from 3:1 significantly. According to the above-given model, the frequency of weak plants, 22/191 = 0.1152, gives q = 0.321. The normal : pgl ratio, 113: 56, also deviates ftom 3:1 (X2 = 6.0, P<0.05) owing to an excess of the pgl type. This suggests that B and pgl are linked in repulsion phase. Letting their recombination value be p, the frequencies of normal and pgl plants are expected to be 1/4(2 +p2) : 1/4(1-p2) . However, the F2 segregation is distorted strongly as the result of deficiency of weak plants. The simple repulsion formulas do not give a reliable estimate of p (p=0.08+/-0.68 is useless on account of a large standard error). Taking q=0.321 into account, the Frequency of gametes with B will be 1/2(1+q)+0.6605 and that of gametes with Bt will be 1/2(1-q)=0.3395. From these gametic frequencies, the frequencies of F2 phenotypes are expected as follows:
(Phenotype) (Frequency) (Obs. no) (Exp. no, p=O.32)
Normal 0.449+0.442p+0.012p2 113 112.99
pgl 0.436-0.442p-0.012p2 56 56.04
Weak 0.115 22 21.97
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1.0 191
The maximum likelihood estimate of p was 0.32+/-0.071, and the observed
numbers fitted the expected ones well.If the B/Bt locus is linked with pgl, it will be linked also with Ef-1. To examine this linkage, T65(20)Ea with Ef-1a was crossed with 8 -2W, where the parental genotypes will be Ef-1a/Ef-1a B/B X ef-1/ef-1 Bt/Bt.The F1 plants were a normal early-heading type, and the F2 segregated into 124 early-heading: 47 T65-like: 29 weak types. The weak plants are fewer than the number expected ftom 3:1 segregation, and q=0.238 is obtained. Assuming that Ef-1 and B are linked in coupling, the three F2 phenotypes are expected as follows:
(Phenotype) (Frequency, q=0.238) (Frequency, q=0) (Obs. no) Early-heading 0.855-0.472p+0.089p2 0.75-0.5p+0.25p2 124 T65-like 0.472p-0.089p2 0.5p-0.25p2 47 Weak 0.145 0.25 29In this cross, however, the ratio of early-heading: T65-like is close to 3:1 (X2=0.56, P>0.5) and the estimate of p from the above formulas is about 0.5. The expected linkage was not confirmed.
The linkage between Ef-1 and pg1 was computed from the F2 of T65(20)Ea X T65(9)pgl by using the ordinary coupling formula. The F2 data were 304 early- heading:68 T65-like:48 early pgl:69 T65-like pgl, and P=0.27+/-0.04 was obtained. This value is close to 28% reported previously (RGN 4, 1987, p. 23). Accordingly, the order of the three loci on chromosome 10 is expected as follows: B-(0.32)-pgl-(0.27)-Ef-1.
References
Tsai, K. H., 1989. Weak, early-heading and late-heading variants found in selfed progenies of a backcross derivative of Taichung 65 rice. In Proc. 6th Internl. Congress of SABRAO, S. Iyama and G. Takeda (eds.), p. 293-296. Organ. Comm. SABRAO, Tokyo.
____, 1990. Unusual mode of inheritance of heading time and occurence of weak plants observed in rice. Jpn. J. Breed. 40: 133-146.