3. Geographical distribution of complementary recessive genes controlling hybrid breakdown of rice

Hybrid weakness was found in a cross between a Thai glutinous variety, Col. 15, and a Japanese nonglutinous variety, Sasanishiki (Okuno 1985). The genetic basis of this hybrid weakness can be elucidated by assuming a set of two independent loci. The segregants with one or no dominant genes, 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\ express the weakness.

Using B\1\F\3\ segregants with no dominant genes as a tester, one hundred Asian varieties were examined with respect to complementary genes at two independent loci. When the F\1\ plants between a\1\a\2\a\2\a\2\ and a given variety show vigorous growth, the variety will carry two pairs of dominant genes, A\1\A\1\A\2\A\2\. When the F\1\ plants show weakness, the variety will carry either of the dominant genes, A\1\A\1\ or A\2\A\2\. These can be distinguished by further test-crosses.

The geographical distribution of the genes controlling this hybrid breakdown, among Asian rice varieties, is shown in Table 1-Geographical distribution of complementary genes controlling hybrid breakdown. Most of Japanese varieties, and the native varieties from the Philippines and Indonesia as well, possessed one pair of dominant genes, being either A\1\A\1\a\2\a\2\ or a\1\a\1\A\2\A\2\. These types of varieties predominated also in India and Sri Lanka. However, varieties from Nepal all carried two pairs of dominant genes. Approximately half of the varieties from Burma and South China possessed two pairs of dominant genes.

Table 1. Geographical distribution of complementary genes controlling hybrid breakdown

Okuno, K. 1985. Complementary recessive genes controlling hybrid breakdown found in a varietal cross of rice. RGN 2: 52-54.