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 selling rate. Virtually no purging of the lethals occurs unless the selling rate closely approaches or exceeds a threshold selfing rate, at which there is a precipitous drop in the mean number oflethals maintained. Identity disequilibrium lowers the threshold selling rate by increasing the ratio of variance to mean number of lethaIs 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.

Mutation–selection balance in mixed mating populations

“…The difference is largest when deleterious alleles are substantially recessive (h < 0.2) and the mutation rate is high. This deviation is a consequence of 'selective interference' (Lande et al, 1994), wherein identity disequilibria among loci reduces the efficiency of selection. For low outcrossing rates (high selfing), the mutational load predicted by OC74 consistently overestimates the Q predicted by K85 (Figure 3).…”

“…Consider selective interference, which is responsible for the large difference between single and multi-locus predictions in Figure 2. This effect is often associated with deleterious mutations of large effect because Lande et al (1994) focused on embryonic lethals in their study of interference. Their parameter sets were clearly motivated by empirical studies, but it is worth noting that selective interference is not a consequence of high selection coefficients.…”

“…Selection dramatically alters the proportions, and as a consequence, the outbred cohort produces the great majority of individuals in the next generation. The increased efficiency of selection against deleterious mutations within inbred cohorts becomes inconsequential because these cohorts make a very limited contribution to the next generation (Ganders, 1972;Lande et al, 1994). In this case, it is the aggregate effect of many mutations and not the exposure of a single recessive lethal that limits the contribution of inbred cohorts.…”

The effects of embryo competition with mixed mating on the genetic load in plants

“…The purging of recessive mutations is most effective when there is high variance in fitness among selfed zygotes. Lande et al (1994) have demonstrated that when load is high, viability of selfed zygotes is uniformly near zero, and thus there is little opportunity for purging. Essentially, with high load, selfed seeds are inviable, and embryo competition will have little effect on the frequency of recessive mutations.…”

“…Fitness interactions are multiplicative across loci, and it is assumed throughout that mutations are sufficiently rare that mean fitness of selfs is greater than zero (see Lande et a!., 1994 …”