Diversification or collapse of self-incompatibility haplotypes as a rescue process

In angiosperm self-incompatibility systems, pollen with an allele matching the pollen recipient at the self-incompatibility locus is rejected. Extreme allelic polymorphism is maintained by frequency-dependent selection favoring rare alleles. However, two challenges result in a \"chicken-egg\"problem for the spread of a new allele (a tightly linked haplotype in this case) under the widespread \"collaborative non-self recognition\" mechanism. A novel pollen-function mutation alone would merely grant compatibility with a nonexistent style-function allele: a neutral change at best. A novel pistil-function mutation alone could only be fertilized by pollen with a nonexistent pollen-function allele: a deleterious change that would eliminate all seed set. However, a pistil-function mutation complementary to a previously neutral pollen mutation may spread if it restores self-incompatibility to a self-compatible intermediate. We show that novel haplotypes can also drive elimination of existing ones ...