Maintenance of high inbreeding depression in selfing populations: Two-stage effect of early- and late-acting mutations

High estimates of inbreeding depression have been obtained in many plant populations with high selfing rates. However, deleterious mutations might be purged from such populations as a result of selfing. I developed a simulation model assuming the presence of mutations at two sets of loci, namely, early- and late-acting loci, and the selective abortion of embryos coupled with ovule overproduction. In the model, early-acting loci are expressed during embryo initiation, and less vigorous embryos are aborted. Late-acting loci are expressed after selective abortion ends; the surviving embryos (seeds) compete, and some of them form the next generation. If mutations are allowed to occur in both early- and late-acting loci, both types increase in frequency in populations with high selfing rates. However, this phenomenon does not occur if mutations occur only in the early- or only in the late-acting loci. Consistent results are observed even if the total number of loci in which mutations are allowed to occur is the same for simulations with both early- and late-acting loci, only early-acting loci, or only late-acting loci, indicating that the presence of both types of loci is the causal factor. Thus, the two-stage effect, or occurrence of both early- and late-acting mutations, promotes the maintenance of these mutations in populations with high selfing rates.