Willis John H
Department of Biology, University of Oregon, Eugene, Oregon, 97403.
Evolution. 1999 Dec;53(6):1678-1691. doi: 10.1111/j.1558-5646.1999.tb04553.x.
Severe inbreeding depression is routinely observed in outcrossing species. If inbreeding load is due largely to deleterious alleles of large effect, such as recessive lethals or steriles, then most of it is expected to be purged during brief periods of inbreeding. In contrast, if inbreeding depression is due to the cumulative effects of many deleterious alleles of small effect, then it will be maintained in the face of periodic inbreeding. Whether or not inbreeding depression can be purged with inbreeding in the short term has important implications for the evolution of mating systems and the probability that a small population will go extinct. In this paper I evaluate the extent to which the tremendous inbreeding load in a primarily outcrossing population of the wildflower, Mimulus guttatus, is due to alleles of large effect. To do this, I first constructed a large outbred "ancestral" population by randomly mating plants collected as seeds from a natural population. From this population I formed 1200 lines that were maintained by self-fertilization and single seedling descent: after five generations of selling, 335 lines had survived the inbreeding process. Selection during the line formation is expected to have largely purged alleles of large effect from the collection of highly inbred lines. Because alleles with minor effects on fitness should have been effectively neutral, the inbreeding depression due to this class of genes should have been unchanged. The inbred lines were intercrossed to form a large, outcrossed "purged" population. Finally, I estimated the fitness of outbred and selfed progeny from the ancestral and purged populations to determine the contribution of major deleterious alleles on inbreeding depression. I found that although the average fitness of the outcrossed progeny nearly doubled following purging, the limited decline in inbreeding depression and limited increase in inbred fitness indicates that alleles of large effect are not the principle cause of inbreeding depression in this population. In aggregate, the data suggest that lethals and steriles make a minority contribution to inbreeding depression and that the increased outbred fitness is due primarily to adaptation to greenhouse conditions.
在异交物种中经常观察到严重的近交衰退。如果近交负荷主要是由于具有较大效应的有害等位基因，如隐性致死基因或不育基因，那么预计其中大部分会在短暂的近交期内被清除。相反，如果近交衰退是由于许多具有较小效应的有害等位基因的累积效应，那么它将在周期性近交的情况下得以维持。近交衰退能否在短期内通过近交被清除，这对于交配系统的进化以及小种群灭绝的概率具有重要意义。在本文中，我评估了野生花卉沟酸浆一个主要异交种群中巨大的近交负荷在多大程度上是由具有较大效应的等位基因所致。为此，我首先通过将从自然种群收集的种子培育出的植株随机交配，构建了一个大型的远交“祖先”种群。从这个种群中，我形成了1200个通过自交和单株后代繁殖来维持的品系：经过五代自交后，335个品系在近交过程中存活下来。预计在品系形成过程中的选择已经从高度近交的品系集合中基本清除了具有较大效应的等位基因。由于对适合度影响较小的等位基因应该基本上是中性的，因此这类基因导致的近交衰退应该没有变化。将这些近交系进行杂交，形成一个大型的、异交的“清除”种群。最后，我估计了祖先种群和清除种群中远交和自交后代的适合度，以确定主要有害等位基因对近交衰退的贡献。我发现，尽管清除后异交后代的平均适合度几乎翻了一番，但近交衰退的有限下降和近交适合度的有限增加表明，具有较大效应的等位基因不是该种群近交衰退的主要原因。总体而言，数据表明致死基因和不育基因对近交衰退的贡献较小，而异交适合度的增加主要是由于对温室条件的适应。
