Gametophytic self-incompatibility re-examined: a reply

In principle, as has long been known, there are two ways by which the stigmas or styles of the flowers of self-incompatible species could discriminate between incompatible and compatible pollen, for either the growth of the former would be inhibited (the oppositional hypothesis) or that of the latter stimulated (the complementary hypothesis). In practice, however, the genetics of gametophytic systems all but rules out the second possibility, largely because of the number of alleles involved. Furthermore, nearly all of the more recent evidence concerning the behaviour of pollen on the stigma or style, both of the observational and the biochemical kind, is inconsistent with the complementary hypothesis. In a recent paper, however, Mulcahy and Mulcahy (1983) have argued that the oppositional hypothesis is unable to provide a satisfactory explanation of several more recent observations concerning gametophytic systems and have proposed a new, multigenic model that is based on the complementary hypothesis of discrimination. A review of the evidence regarded by the Mulcahys as inconsistent with the oppositional hypothesis reveals that this is either not relevant to the question of discrimination or has been misunderstood. Furthermore, the Mulcahys' model is inconsistent with the empirical evidence concerning multi-locus systems and is formally consistent with one-locus systems only if it is assumed that the constituent loci of the S-supergene are very tightly or completely linked, an assumption not made by the Mulcahys. Their perception of the evidence, therefore, is incomplete and their model ill-founded. Hence the Mulcahys' claim to have put forward a new model of gametophytic self-incompatibility which is consistent with the empirical evidence cannot be sustained.