The expression of self-incompatibility in angiosperms is bimodal

Self-incompatibility is expressed by nearly one half of all angiosperms. A large proportion of the remaining species are self-compatible, and they either outcross using various contrivances or self-fertilize to some extent. Because of the common occurrence of populations and individuals with intermediate levels of self-incompatibility, categorization of the expression of self-incompatibility as an approximately binary trait has become controversial. We collect a widely reported index (ISI) used to asses the strength and variation of self-incompatibility from over 1200 angiosperm taxa. Its distribution is bimodal and positively associated with outcrossing rate, albeit with a weak relationship within self-compatible taxa. A substantial fraction of species have intermediate mean values of ISI. Their occurrence can be caused by segregating ephemeral self-compatible mutations, averaging artifacts, and experimental biases, in addition to the often invoked stabilizing selection acting on the e...