THE RSS SYSTEM OF UNIDIRECTIONAL CROSS-INCOMPATIBILITY IN MAIZE .1. GENETICS

Self-incompatibility has never been reported in maize, but a few examples of cross-incompatibility have been. One such example was noted in 1975 at our maize cytogenetics nursery at Ames, Iowa. When the derivatives of a specific line am(pa-pu)/a 1 sh2 (now called RSS females) were crossed with specific males, namely a 1 et/a 1 et or A 1 et/A 1 et (now called RSS males), the resultant ears showed reduced seed setting (< 25 seeds/ear). The reciprocal crosses between the same parents resulted in normal seed setting. Previous studies in this cross-incompatibility showed that the incompatibility reaction in this case is not controlled by the known incompatibility genes. In the present studies, we found that this unidirectional cross-incompatibility is controlled by three recessive loci. One locus, cif, controls the incompatibility reaction in the female parent; the other two, cim1 and cim2, control the incompatibility reaction in the pollen parent. The cross is incompatible only when the female parent is homozygously recessive for the cif locus and when the male parent is homozygously recessive for the cim1 and the cim2 loci. The reciprocal cross between these parents is compatible. The RSS phenotype (reduced seed set) can only be visualized when the female (RSS female) and male (RSS male) parents of the cross have these designated genotypes.