INHERITANCE OF PATHOGENICITY AND UREDIOSPORE COLOR IN CROSSES BETWEEN PHYSIOLOGIC RACES OF OAT STEM RUST

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Abstract
In the F1 generation of crosses between physiologic races 7 (orange) and 11 (red) of Puccinia graminis Pers. var. Avenae Erikss. and Henn., the medium sized uredia (type 2 infection) formed by race 11 on the oat variety White Tartar were dominant to the large (type 4) uredia of race 7, and the small (type 1) uredia of race 7 on the variety Richland were dominant to the type 4 uredia formed by race 11. On the varieties Sevnothree and Joanette Strain, the F1 hybrids produced the same type of infection as did the "maternal" parent race, that is, hybrids from race 7 × race 11 crosses produced a type 4 infection like race 7, and reciprocal hybrids produced a type 1 infection like race 11. These facts led to the suggestion that the cytoplasm of the maternal parent race influenced the infection type of the F1 hybrid on these two oat varieties.A study of the F2 generation of the cross race 11 × race 7 showed that on the varieties White Tartar and Richland the dominant and recessive infection types appeared in a ratio of 9:7, which suggests that their inheritance is governed by two pairs of complementary genes. The distribution of physiologic races in F2 conforms to this assumption and indicates that the genes governing infection types on these two varieties associate at random to produce physiologic races 1, 11, 3, and 4. These races occurred in F2 in a ratio of 31: 20: 22: 12, as compared to an expected ratio of 27: 21: 21: 16. On the variety Sevnothree, 84 of 85 F2 cultures produced type 1 uredia, in this way resembling the maternal parent, race 1, and the maternal grandparent, race 11. One F2 culture, only produced the type 4 uredia characteristic of the paternal grandparent, race 7. It is concluded from this study, and from crosses between races 1 and 2, that the maternal (cytoplasmic) influence evidenced in the F1 generation persists m F2 and F3.In F1, the red urediospore color of race 11 was dominant to the orange color of race 7. The distribution in these two color classes in F2 and F3 suggests that the inheritance of urediospore color is governed by a single pair of genes.

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