Denoting the mean viability of selfed zygotes caused by embryonic lethals as [Mathematical Expression Omitted] and that of outcrossed zygotes as [Mathematical Expression Omitted], the component of inbreeding depression caused by embryonic lethal mutations is defined as
An unrealistic feature of this model is that for completely recessive lethal mutations under low selfing rates, the mean number of lethals per plant becomes infinite.
We derive a version of Kondrashov's model for the case of homozygous lethal mutations, such that wild-type homozygotes, mutant heterozygotes, and mutant homozygotes at each locus have viabilities of 1, 1 - h, and 0 respectively.
Time scales for approaching equilibrium varied from infinity for completely recessive lethal mutations with selfing rates below the threshold, to a few hundred generations for slightly dominant lethals under random mating, and several generations under complete selfing.
Thus, when 2n[square root of [Mu]] [greater than] 10 selective interference among loci creates a threshold selfing rate necessary for purging recessive lethal mutations.
Slight dominance of lethal mutations substantially reduces the equilibrium load in comparison to that for full recessivity, as seen in figures 2 and 3.
Regardless of the average dominance of lethal mutations, for selfing rates below the threshold, nearly all selfed zygotes are lethal.
Solid lines in figure 2 show that in the Kondrashov model the mean number of lethals per plant blows up for completely recessive lethal mutations, not only under random mating, but also in any situation where the inbreeding depression is very high and the secondary selfing rate is very low.
Figure 5 demonstrates that for completely recessive lethal mutations the mean number of lethal alleles maintained at low selfing rates increases sharply as the number of loci is increased, confirming that the blow up of the mean number of completely recessive lethals per plant in the Kondrashov model is caused by the assumption of an infinite number of loci.
Observations on plant species that maintain high early inbreeding depression under intermediate primary selfing rates are difficult to explain by previous theory that nearly recessive lethal mutations are effectively purged from populations with selfing rates of a few or several percent (Lande and Schemske 1985; Charlesworth and Charlesworth 1987).