7. Complementary recessive genes controlling hybrid breakdown found in a varietal cross of rice

Two different types of hybrid weaknesses have been known in varietal crosses of O. sativa. One is due to complementary dominant genes which cause F\1\ weakness (Oka 1957; Amemiya and Akemine 1963), and the other is due to complementary recessive genes producing weak segregants in the F\2\, B\1\F\1\ and subsequent generations (Oka 1957; Sato et al. 1984). The F\1\ weakness is also found in crosses between strains of O. glaberrima and O. breviligulata (Chu and Oka). The case reported in this paper belongs to the latter type.

Thirty glutinous cultivars from different Asian countries were used for crosses to introduce the wx gene into a Japanese non-glutinous cultivar, Sasanishiki. When a Thai glutinous variety, Col. (Collection) 15 was crossed, the F\1\ plants with both Sasanishiki and Col. 15 (B\1\F\1\ segregated into 3 normal : 1 weak types. The weak plants were not distinguishable at the seedling stage, but became yellow at the beginning of tillering stage an their growth was retarded. However, they produced panicles when protected, and their progenies (B\1\F\2\) were obtained. The B\1\F\2\ plants segretate into 1 normal : 3 weak types (Table 1). On the other hand, the backcrosses of the weak plants with Sasanishiki (B\2\F\1\ and B\3\F\1\) segregated into 1 normal : 1 weak types. The F\2\ ratio was 11 normal : 5 weak. There was no difference between reciprocal crosses (Table 1).

The genetic basis of this hybrid weakness can be elucidated by assuming a set of two independent loci in the same manner as postulated by Oka (1957). Under this postulate, the parental genotypes would be A\1\A\1\a\2\a\2\ and a\1\a\1\A\2\A\2\ respectively, and segregants with one or zero dominant gene, such as A\1\a\1\a\2\a\2\, a\1\a\1\A\2\a\2\ and a\1\a\1\a\2\a\2\, would express the weakness. The symbols for these weakness genes have not been assigned as yet.

Table 1. Segregation ratios for hybrid weakness observed.

==============================================================================
Generation  Cross combination       Number of plants            Ratio     x2  
                                ----------------------          expected
                               Normal  Segregating   Weak
==============================================================================
F\1\        Sas.a/Col. 15         45                   0         1:0
            Col. 15/Sas.         105                   0         1:0
B\1\F\1\    Sas./Col. 15//Sas.    30                  12         3:1      0.29
            Sas./Col. 15//Col.15  23                   8         3:1      0.01
B\2\F\1\    Col.15/Sas.//2*Sas.   15                  13         1:1      0.14
B\3\F\1\    Sas./Col. 15//3*Sas.  22                  15         1:1      1.32
B\1\F\2\    Sas./Col. 15//Sas.    46                  142        1:3      0.03
B\1\F\3\    Sas./Col.15//Sas.     46      83           54      1:2:1      2.28
F\2\        Sas./Col. 15         474                  226       11:5      0.35
            Col. 15/Sas.         326                  165       11:5      1.27
==============================================================================

References

Amemiya, A. and H. Akemine, 1963. Biochemical genetic studies on the root growth inhibiting complementary lethal genes of rice plant. Bull. Nat. Inst. Agr. Sci. D10:139-226 (in Jap. with Eng. summary).

Chu, Y.E. and H.I. Oka, 1972. The distribution and effects of genes causing F\1\ weakness in Oryza breviligulata and O. glaberrima. Genetics 70: 163-173.

Oka, H.I., 1957. Phylogenetic differentiation of cultivated rice, XV. Complementary lethal genes in rice. Jpn. J. Genet. 32:83-87.

Sato, Y.I., S. Matusuura and K. Hayashi, 1984. The genetic basis of hybrid chlorosis found in a cross between two Japanese native cultivars. RGN 1:106- 107.