Genetics and physiology of angiosperm incompatibility systems

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Abstract
In many angiosperms outbreeding is enforced by self-incompatibility systems which inhibit the function of self-pollen. The genetic control may be through one, two or three multiallelic loci, identity in pollen and pistil determining incompatibility. In the so-called sporophytic systems, the haploid male gametophyte behaves according to the genotype of its parent; in the gametophytic systems, according to its own genotype. Recognition in sporophytic systems takes place at or near the stigma surface. The control is probably through proteins derived from the parent and conveyed in the pollen wall; the receptor sites on the female side are on the outer surface of the stigma cells, separated from the cell membrane by a cuticularized pectocellulosic wall. Gametophytic systems are functionally more diverse, but in some examples it seems likely that the initial interaction is between factors held at the tube tip and materials secreted into the intercellular spaces of the transmitting tract. The self incompatibility responses form only part of the complex of pollen-stigma interactions. 'Recognitions' of another kind are involved, for example, in the enzyme activations that permit the pollen tube to penetrate the stigma cuticle.

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