Fig. 2

Model for GSI: expected compatibility interactions and phenotypes generated in a full sib diallel between two plants with (left-hand column) one S-allele in common, and (right-hand column) no S-allele in common. (+/+ denotes fully compatible cross, +/− semicompatible cross and −/–incompatible cross). Reciprocal differences in incompatibility between types are indicated by shaded cells. A and B: When two plants with one S allele in common are crossed, the plants are fully semicompatible. If the F1 are crossed, then two phenotypic groups are obtained, independently of the direction of the cross (A × B or B × A). If all members of the F1 generation are crossed reciprocally (A × B + B × A), they form four phenotypes, two in each direction of the cross. Two of these four phenotypes are different from each other (one in A × B and one in B × A) and two are similar. Consequently, we would expect with the results of such reciprocal crosses to find three phenotypes with a frequency of 2:1:1. C and D: When plants that do not have an allele in common are crossed, the plants are fully or semi compatible. Crosses of the F1 of these plants, independent of direction of the cross (C × D or D × C) form four groups of phenotypes with a frequency of 1:1:1:1. This same result is obtained with the progeny of the two reciprocals of such crosses.