Unidirectional cross-incompatibility in maize - a new system --Abdul Rashid and Peter A. Peterson A heritable unidirectional cross-incompatibility was observed in 1975 (Sukhapinda and Peterson, Can. J. Genet. Cytol. 25:270, 1983). The nature of this incompatibility is that a specific male genotype is "largely" rejected by a specific female genotype.

The incompatible female is a-m(pa-pu)/a sh2 while the male parent in this incompatible pair is a et/a et. A partial seed set (less than 25 seeds per ear) is observed in this cross but in reciprocal crosses the seed setting is normal. The genetics of this partial cross-incompatibility was studied and the results indicate that three independently segregating recessive genes are involved. One gene pair named cif (cross-incompatible female) is expressed in the female and when in the homozygous recessive state, the plant is an incompatible female. When used with the appropriate male the other two genes cim1 (cross-incompatible male) and cim2 are expressed in the male. When both of the cim genes are in the homozygous recessive state, the result is a partial seed set. The putative genotype of the incompatible cross is cif/cif x cim1/cim1, cim2/cim2.

Normal seed setting is dominant over reduced seed setting and this incompatibility is under the control of a sporophytic system of incompatibility. It has also been observed that the rejection of incompatible pollen occurs before fertilization. The mode of rejection of incompatible pollen is under study.

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