title: Costs of selfing prevent the spread of a self-compatibility mutation that causes reproductive assurance.
abstract: In flowering plants, shifts from outcrossing to partial or complete self-fertilization have occurred independently thousands of times, yet the underlying adaptive processes are difficult to discern. Selfing's ability to provide reproductive assurance when pollination is uncertain is an oft-cited ecological explanation for its evolution, but this benefit may be outweighed by costs diminishing its selective advantage over outcrossing. We directly studied the fitness effects of a self-compatibility mutation that was backcrossed into a self-incompatible (SI) population of Leavenworthia alabamica, illuminating the direction and magnitude of selection on the mating-system modifier. In array experiments conducted in two years, self-compatible (SC) plants produced 17-26% more seed, but this advantage was counteracted by extensive seed discounting-the replacement of high-quality outcrossed seeds by selfed seeds. Using a simple model and simulations, we demonstrate that SC mutations with these attributes rarely spread to high frequency in natural populations, unless inbreeding depression falls below a threshold value (0.57 ≤ δthreshold≤ 0.70) in SI populations. A combination of heavy seed discounting and inbreeding depression likely explains why outcrossing adaptations such as self-incompatibility are maintained generally, despite persistent input of selfing mutations, and frequent limits on outcross seed production in nature.
authors.affiliation: School of Biological Sciences, Washington State University, Pullman, Washington, 99164.
authors.forename: Nathan C
authors.initials: NC
authors.lastname: Layman
authors.affiliation: Department of Biology, Evolution and Ecology Group, Middle Tennessee State University, Murfreesboro, Tennessee, 37132.
authors.forename: M Thilina R
authors.initials: MT
authors.lastname: Fernando
authors.affiliation: Department of Biology, Evolution and Ecology Group, Middle Tennessee State University, Murfreesboro, Tennessee, 37132.
authors.forename: Christopher R
authors.initials: CR
authors.lastname: Herlihy
authors.affiliation: School of Biological Sciences, Washington State University, Pullman, Washington, 99164.
authors.forename: Jeremiah W
authors.initials: JW
authors.lastname: Busch
mesh_terms: D000407:Alabama; D000818:Animals; D019607:Brassicaceae / Q000235:genetics / Q000502:physiology*; D056084:Genetic Fitness; D007313:Insecta / Q000502:physiology; D009154:Mutation; D011058:Pollen; D054817:Pollination; D012639:Seeds / Q000254:growth & development; D057945:Self-Fertilization
keywords: Automatic selection; inbreeding depression; outcrossing; seed discounting; self-fertilization; self-incompatibility