A Model for the Evolution of Distyly

Abstract

Reasons are given for believing that morphological distyly is unlikely to evolve before the incompatibility system. The first problem studied is therefore the evolution of incompatibility of the type found in distyly, with two pollen and two stigma types. It is argued that the first step, starting from self-compatibility, is a mutation to a new pollen type, such that the mutant cannot self-fertilize or fertilize other individuals. Conditions for the spread of such a mutation (assumed recessive) are given. Next, a dominant mutation to a new stigma type, incompatible with the original pollen type but compatible with the new type, is studied. Such a mutation is almost certain to be eliminated if it occurs first, but can spread in a population polymorphic for the pollen mutation provided that it occurs at a linked locus. With tight linkage, this model generates a population with two incompatibility types, one dominant to the other, with only rare recombinant types. It is shown by computer runs that modifiers causing tighter linkage will spread in such populations. The next problem studied is the evolution of differences in anther and stigma positions. These are assumed to reduce the selfing rate. It is argued that anther position is unlikely to change first, as this will reduce male fertility by as much as the reduction in the chance of pollen transfer to the stigmas of other flowers; a changed stigma position will probably affect female fertility less than this, assuming that excess pollen is initially present and that all ovules can be fertilized unless the amount of compatible pollen received falls below some threshold level. If the incompatibility system is perfect, a new stigma position can be selected for, but linkage to the incompatibility locus does not affect the results. If there is partial self-compatibility and the more self-compatible form is dominant, linkage is very important for the spread of dominant mutations affecting stigma or anther position; an unlinked mutation is frequently either eliminated or fixed under conditions in which a linked gene will establish polymorphism. Computer runs show that there is selection for tighter linkage between morphology mutations and the incompatibility locus, in situations where both are polymorphic. When a recessive mutation making the final morphological change is introduced, it spreads faster the tighter its linkage to the other loci. There is again strong selection for tighter linkage, and at equilibrium with tight linkage there are approximately equal frequencies of two phenotypes, one homozygous for all the recessive alleles, and one heterozygous at all the loci. The more self-compatible form is dominant and so usually is the form with the new stigma position. This agrees with many distyled species. Species in which the more self-compatible form is recessive cannot be explained by this model, but can be accounted for if the gene affecting the stigma reaction also controls its position. This case is also studied. It has properties similar to the other case; in particular, there is a strong linkage constraint for mutations with this type of effect when introduced into a population segregating for a pollen-reaction gene. This model can also explain species with differences in stigma position only, and with two equally frequent forms, each corresponding to one incompatibility type. Such populations are not easy to explain by the previous model.