High Inbreeding Depression, Selective Interference Among Loci, and the Threshold Selfing Rate for Purging Recessive Lethal Mutations

Abstract

The evolutionary dynamics of recessive or slightly dominant lethal mutations in partially self-fertilizing plants are analyzed using two models. In the identity-equilibrium model, lethals occur at a finite number of unlinked loci among which genotype frequencies are independent in mature plants. In the Kondrashov model, lethals occur at an infinite number of unlinked loci with identity disequilibrium produced by partial selfing. If the genomic mutation rate to (nearly) recessive lethal alleles is sufficiently high, such that the mean number of lethals (or lethal equivalents) per mature plant maintained at equilibrium under complete outcrossing exceeds 10, selective interference among loci creates a sharp discontinuity in the mean number of lethals maintained as a function of the selfing rate. Virtually no purging of the lethals occurs unless the selfing rate closely approaches or exceeds a threshold selfing rate, at which there is a precipitous drop in the mean number of lethals maintained. Identity disequilibrium lowers the threshold selfing rate by increasing the ratio of variance to mean number of lethals 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.

Gender‐specific inbreeding depression in a gynodioecious plant, Geranium maculatum (Geraniaceae)

“…Preliminary results from a genetic study using seeds have shown that females do indeed have a selfing rate close to 0% in both the MP and OT populations (C. Deen, University of Georgia, and S.-M. Chang, unpublished data), similar to what is seen in many other gynodioecious species (e.g., Collin and Shykoff, 1993;Kohn and Biardi, 1995;Gibson and Wheelwright, 1996). However, use of mature seeds would underestimate the primary selfing rates (i.e., sefling rates for all fertilized ovules) if inbreeding depression is observed at stages prior to seed maturation, as shown in this study (Lande et al, 1994;Husband and Schemske, 1996). These values are on the low end of the selfing rate distribution even among outcrossing species (Vogler and Kalisz, 2001).…”

“…Lande et al (1994) showed theoretically that, when selfing rate is low, high rates of lethal mutations may kill nearly all selfed progeny and leave the population with only outcrossed progeny, a phenomenon called selective interference (Ganders, 1972). Lande et al (1994) showed theoretically that, when selfing rate is low, high rates of lethal mutations may kill nearly all selfed progeny and leave the population with only outcrossed progeny, a phenomenon called selective interference (Ganders, 1972).…”

Outbreeding Depression, but No Inbreeding Depression in Haplodiploid Ambrosia Beetles With Regular Sibling Mating

“…However, X. germanus is probably the species with the highest degree of continuous sibling mating in which inbreeding effects ever have been examined. Theoretical work has shown that purging may not occur until a critical threshold selfing/inbreeding rate is reached (Lande et al 1994).…”

Evolution of Outcrossing in Experimental Populations of Caenorhabditis elegans

“…For instance, with segregation of partially-dominant or recessive deleterious alleles, male maintenance is favored because it mitigates inbreeding depression [1], [5], [24], [25]. In populations without genetic diversity however, selfing is expected to be more efficient at purging new deleterious alleles than outcrossing since more homozygotes are readily produced [26], [27], [28]. In C. elegans inbreeding depression should not however explain male maintenance because crosses among wild isolates have failed to show it [11], [29], and males have only a minor role in buffering partially-dominant deleterious mutations in laboratory populations [30], [31] (but see [2]).…”