34. Varying frequencies of occurrence of offtypes and unusual segregation patterns observed in crosses of isogenic Taichung 65 lines

Kuo-Hai TSAI

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

Gene Ef-1 (chromosome 10) was isolated from early-maturing varieties of China and Japan. Isogenic lines of Taichung 65 (T65) with this gene were established after 7 to 20 backcrosses. This gene is widely distributed not only in early-maturing but in late-maturing varieties of southern origins also. Its recessive allele ef-1 carried by T65 and similar Taiwan Japonicas is rather rare. The floral initiation of Ef-1 carriers is about 10 days earlier than that of ef-1 carriers. Several isoalleles have been detected through comparison of isolines with Ef-1 of different origins, e.g., Ef-1a (Ea, from Tatung-tsailai), Ef-1b (Eb, from Bozu 5), Ef-1x (Ex, X-ray induced), EF-1γ (Eγ, gamma-ray induced), etc., whose carriers differ in heading date slightly but significantly. Their segregation in hybrids was demonstrated by an increase in F2 variance and the occurrence of ef-1 types with a frequency less than 1% (Tsai 1976). It was suggested that the Ef-1 locus would have a compound structure and unequal crossovers within locus would bring about a few T65-like offtypes.

Gene m-Ef-1 (chromosome 7) was also obtained from Chinese and Japanese early-maturing varieties. This recessive gene enhances floral initiation about three weeks through interaction with Ef-1 in T65 isogenic lines, but when combined with ef-1, its effect is much reduced and almost silent when the carriers are grown in the summer crop of Taichung. The m-Ef-1 locus sometimes produces offtypes with frequencies higher than 1%. The selfed progenies of such offtypes produced offtypes again with much higher frequencies (Table 1; Tsai 1984, 1987).

Crosses between ef-1 carriers of T65 type sometimes produced different offtypes, whose heading dates were several days earlier than Ef-1 carriers (E'-type), similar to that of Ef-1 carriers (E type), a few days earlier than ef-1 carriers (e'-type), and about 20 days later-heading than T65 (late type) with varying frequencies, some being very high (Table 2; Tsai 1990). The late type was similar to the carriers of an X-ray induced late-heading gene (ef-2, independent of ef-1, locus unknown), and its selfed progenies generally showed high frequencies of offtypes. The selfed progeny of an e' offtype fluctuated in heading date and plant height, the two characters being correlated (r=0.52). Unexpectedly, the late offtypes were found to have a late-heading gene allelic to ef-2.

            
Table 1.  F2 offtypes found in crosses of "Em" lines with Ef-1 and m-Ef-1
===============================================================================
                                  Phenotypea       No. of  Offtype  Supposed
Cross           Generation ======================  plants  freq.(%) gene change
                                 Em     E      e
===============================================================================
Eam(7)xEam(7)        F2         274     6             280    2.1     m->+m  
Eam(7)xEam(7)        F2         254     3             257    1.2     m->+m  
Eam(7)xEam(7)     F3 from E      12   103             115   10.4     +m->m
Eam(7)xEam(7)     F3 from E+m    60   102             162   37.0     +m->m
Eam(7)xEam(7)     F3 from Em     80    10              90   11.1     m->+m
Eam(7)xEam(20)       F2         332     8             340    2.4     m->+m
Eam(7)xEam(20)    F3 from E      8    28      3       39   28.2     +m->m
Eam(7)xEam(20)    F3 from E     13    67              80   16.3     +m->m
===============================================================================
a-Em: Ef-1 and m-Ef-1 carriers like, E: Ef-1 carriers like, e: T65 like.

          
Table 2.  Frequencies of offtypes in the progenies of ef-1 carriers and
others
===============================================================================
                                      Phenotypea               No. of  Offtype
Cross             Generation  =============================    Plants  freq.(%)
                                 E'   E     e'    e    late
===============================================================================
ea(7)xeb(15)      F2 from(F1)e        7         2448            2455       0.3
                  F3 from(F2)E       38           27     5        70      45.7
                  F6 from(F5)e   24  14     6            2        46      69.6
                  F6 from(F5)e'            14     25    16        55      74.5
                  F7 from(F6)e'      67    28      4              99      71.7
                  F7 from(F6)e'  13  24    27      9     4        77      64.9
AB60L-1xT65ef-2   F2 from(F1)late           1b         192       193       0.5
AB60L-3xT65ef-2   F3 from(F2)late                  2   118       120       1.7
                  F4 from(F3)e                   144    66       210      31.4
===============================================================================
a-E': Several days earlier than Ef-1 carriers, E: Ef-1 carriers like, e': A
few days earlier than T65, e: T65 like, late: About 20 days later than T65.
b-The selfed progenies of this plant produced various offtypes including
"weak-dwarf" plants.

Furthermore, the occurrence of weak plants was found from at least two sources, i.e., the selfed progeny of an early-heading offtype (e'-type; obtained from an F2 between a late offtype and an ef-2 carrier), and a selfed progeny of T65 X T65Ef-1x B9 backcross (which segregated into 188 normal and 57 weak plants). In the former case (1), the F2 segregated into 127 normal and 22 weak plants and the normal plants consisted of E', E and e types. The weak plants had a few tillers, poor root development and a reduced plant height (30-40 cm at maturity). In the latter case (2), the selfed progeny of a B9F5 plant segregated into normal and weak types. The selfed progenies of weak plants reverted to normal types with varying frequencies (Table 3). In both cases, a part of weak offtypes were strongly weak, called "weak-dwarf". The F2s between T65-like normal and weak lines showed 3 normal: 1 weak ratios, and the selfed progenies of some normal segregants segregated again into normal and weak types. But when selfing was repeated, the frequency of weak segregants tended to be reduced, some giving significant deviations from the 3: 1 ratio. To account for the offtype occurrence with high frequencies, a transposon was assumed to be involved (Tsai 1989, 1990).

The existence of transposon DNAs in the rice genome was confirmed by Dr. H. Hirochika of National Institute for Aerobiological Resources, Japan (pers. comm., 1991). The DNAs were detected at three independent sites and their probes were isolated.

  
Table 3. Occurrence of weak plants in the selfed progeny of an B9F5 plant
derived from T65 x T65Ef-1x and their behavior in selfed progenies and
crosses with T65
===============================================================================
                            Segregant phenotype and number   #of  Normal : Weak
Generation  Parental pheno.   E'  e  late  Weak Weak-dwarf plants X2 from 3:1
===============================================================================
F3         (F2)e              2  75                          77
F4         (F3)E'             1  91                          92
F5         (F4)e              3 185         57              245       0.30
F6         (F5)E'               136                 22      158      10.34**
F7         (F6)Weak               1         99              100
F7         (F6)Weak               6        194              200
F7         (F6)Normal           160         36              196       4.60*
F7         (F6)Normal           466  130   167              763       3.96*
F7         (F6)Normal           161   57    66              284       0.47
F8         (F7)Normal,late           426   107              533       6.89**
Cross, F2  T65x(F7)Weak         131         57              168       0.79
Cross, F2  (F7)WeakxT65         301         99              400       0.01
Cross, F2  (F6)Weak-dwarfxT65   214   60    46              320      19.67**
===============================================================================
*P<0.05,  ** P<0.01.

References

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., 1984. Unusual segregation patterns found at the m-Ef locus. RGN 1: 115-116.

Tsai, K. H., 1987. Interaction between an early flowering gene E and its emphasizing gene m in rice and their unusual segregation patterns. J. Agric. Assoc. China 137: 5-20. (in Chinese with English summary)

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.

Tsai, K. H., 1990. Unusual mode of inheritance of heading time and occurrence of weak plants observed in rice. Jpn. J. Breed. 40: 133-146.