HIGH INBREEDING DEPRESSION, SELECTIVE INTERFERENCE AMONG LOCI, AND THE THRESHOLD SELFING RATE FOR PURGING RECESSIVE LETHAL MUTATIONS

The evolutionary dynamics of recessive or slightly dominant lethal mut ations in partially self-fertilizing plants are analyzed using two mod els. In the identity-equilibrium model, lethals occur at a finite numb er of unlinked loci among which genotype frequencies are independent i n mature plants. In the Kondrashov model, lethals occur at an infinite number of unlinked loci with identity disequilibrium produced by part ial selfing. If the genomic mutation rate to (nearly) recessive lethal alleles is sufficiently high, such that the mean number of lethals (o r lethal equivalents) per mature plant maintained at equilibrium under complete outcrossing exceeds 10, selective interference among loci cr eates a sharp discontinuity in the mean number of lethals maintained a s a function of the selfing rate. Virtually no purging of the lethals occurs unless the selfing rate closely approaches or exceeds a thresho ld selfing rate, at which there is a precipitous drop in the mean numb er of lethals maintained. Identity disequilibrium lowers the threshold selfing rate by increasing the ratio of variance to mean number of le thals per plant, increasing the opportunity for selection. This theory helps to explain observations on plant species that display very high inbreeding depression despite intermediate selfing rates.