Hokuriku National Agricultural Experiment Station, Joetsu, Niigata-ken, 943-01 Japan
Two different types of hybrid weaknesses have been known in varietal crosses of O. sativa. One is due to complementary dominant genes which cause F1 weakness (Oka 1957; Amemiya and Akemine 1963), and the other is due to complementary recessive genes producing weak segregants in the F2, B1F1 and subsequent generations (Oka 1957; Sato et al. 1984). The F1 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 F1 plants with both Sasanishiki and Col. 15 (B1F1 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 (B1F2) were obtained. The B1F2 plants segretate into 1 normal : 3 weak types (Table 1). On the other hand, the backcrosses of the weak plants with Sasanishiki (B2F1 and B3F1) segregated into 1 normal : 1 weak types. The F2 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 A1A1a2a2 and a1a1A2A2 respectively, and segregants with one or zero dominant gene, such as A1a1a2a2, a1a1A2a2 and a1a1a2a2, 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 ============================================================================== F1 Sas.a/Col. 15 45 0 1:0 Col. 15/Sas. 105 0 1:0 B1F1 Sas./Col. 15//Sas. 30 12 3:1 0.29 Sas./Col. 15//Col.15 23 8 3:1 0.01 B2F1 Col.15/Sas.//2*Sas. 15 13 1:1 0.14 B3F1 Sas./Col. 15//3*Sas. 22 15 1:1 1.32 B1F2 Sas./Col. 15//Sas. 46 142 1:3 0.03 B1F3 Sas./Col.15//Sas. 46 83 54 1:2:1 2.28 F2 Sas./Col. 15 474 226 11:5 0.35 Col. 15/Sas. 326 165 11:5 1.27 ==============================================================================a Sas.=Sasanishiki
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 F1 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.