Lande Russell, Schemske Douglas W, Schultz Stewart T
Department of Biology, University of Oregon, Eugene, Oregon, 97403.
Department of Botany, University of Washington, Seattle, Washington, 98195.
Evolution. 1994 Aug;48(4):965-978. doi: 10.1111/j.1558-5646.1994.tb05286.x.
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.
利用两个模型分析了部分自花授粉植物中隐性或轻度显性致死突变的进化动力学。在恒等平衡模型中，致死突变发生在有限数量的不连锁基因座上，在成熟植株中基因型频率是独立的。在康德拉肖夫模型中，致死突变发生在无限数量的不连锁基因座上，部分自交产生恒等不平衡。如果向（近乎）隐性致死等位基因的基因组突变率足够高，以至于在完全异交情况下平衡时每株成熟植株的致死突变（或致死当量）平均数超过10，基因座间的选择干扰会导致作为自交率函数的平衡时致死突变平均数出现急剧间断。除非自交率接近或超过一个临界自交率，否则几乎不会发生致死突变的清除，在临界自交率时，平衡时致死突变的平均数会急剧下降。恒等不平衡通过增加每株植物致死突变的方差与平均数之比，增加选择机会，从而降低临界自交率。该理论有助于解释尽管自交率处于中等水平，但仍表现出非常高的近交衰退的植物物种的观察结果。
