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...