Husband Brian C, Schemske Douglas W
Department of Botany, University of Washington, Seattle, Washington, 98195.
Evolution. 1996 Feb;50(1):54-70. doi: 10.1111/j.1558-5646.1996.tb04472.x.
Estimates of inbreeding depression obtained from the literature were used to evaluate the association between inbreeding depression and the degree of self-fertilization in natural plant populations. Theoretical models predict that the magnitude of inbreeding depression will decrease with inbreeding as deleterious recessive alleles are expressed and purged through selection. If selection acts differentially among life history stages and deleterious effects are uncorrelated among stages, then the timing of inbreeding depression may also evolve with inbreeding. Estimates of cumulative inbreeding depression and stage-specific inbreeding depression (four stages: seed production of parent, germination, juvenile survival, and growth/reproduction) were compiled for 79 populations (using means of replicates, N = 62) comprising 54 species from 23 families of vascular plants. Where available, data on the mating system also were collected and used as a measure of inbreeding history. A significant negative correlation was found between cumulative inbreeding depression and the primary selfing rate for the combined sample of angiosperms (N = 35) and gymnosperms (N = 9); the correlation was significant for angiosperms but not gymnosperms examined separately. The average inbreeding depression in predominantly selfing species (δ = 0.23) was significantly less (43%) than that in predominantly outcrossing species (δ = 0.53). These results support the theoretical prediction that selfing reduces the magnitude of inbreeding depression. Most self-fertilizing species expressed the majority of their inbreeding depression late in the life cycle, at the stage of growth/reproduction (14 of 18 species), whereas outcrossing species expressed much of their inbreeding depression either early, at seed production (17 of 40 species), or late (19 species). For species with four life stages examined, selfing and outcrossing species differed in the magnitude of inbreeding depression at the stage of seed production (selfing δ = 0.05, N = 11; outcrossing δ = 0.32, N = 31), germination (selfing δ = 0.02, outcrossing δ = 0.12), and survival to reproduction (selfing δ = 0.04, outcrossing δ = 0.15), but not at growth and reproduction (selfing δ = 0.21, outcrossing δ = 0.27); inbreeding depression in selfers relative to outcrossers increased from early to late life stages. These results support the hypothesis that most early acting inbreeding depression is due to recessive lethals and can be purged through inbreeding, whereas much of the late-acting inbreeding depression is due to weakly deleterious mutations and is very difficult to purge, even under extreme inbreeding.
从文献中获得的近亲繁殖衰退估计值，被用于评估自然植物种群中近亲繁殖衰退与自花受精程度之间的关联。理论模型预测，随着有害隐性等位基因通过选择得以表达和清除，近亲繁殖衰退的程度会随着近亲繁殖而降低。如果选择在生活史阶段中发挥不同作用，且各阶段的有害影响不相关，那么近亲繁殖衰退的发生时间也可能会随着近亲繁殖而演变。我们针对79个种群（使用重复样本的均值，N = 62）编制了累积近亲繁殖衰退和特定阶段近亲繁殖衰退（四个阶段：亲本种子生产、发芽、幼年存活以及生长/繁殖）的估计值，这些种群包含来自维管植物23个科的54个物种。在可行的情况下，还收集了交配系统的数据，并将其用作近亲繁殖历史的一种衡量指标。在被子植物（N = 35）和裸子植物（N = 9）的综合样本中，发现累积近亲繁殖衰退与主要自交率之间存在显著的负相关；对于分别研究的被子植物而言，这种相关性显著，但裸子植物并非如此。主要自花授粉物种的平均近亲繁殖衰退（δ = 0.23）显著低于（低43%）主要异花授粉物种（δ = 0.53）。这些结果支持了自交降低近亲繁殖衰退程度的理论预测。大多数自花受精物种在生命周期后期，即生长/繁殖阶段（18个物种中的14个）表现出其大部分的近亲繁殖衰退，而异花授粉物种则在早期，即种子生产阶段（40个物种中的17个）或后期（19个物种）表现出其大部分的近亲繁殖衰退。对于研究了四个生活阶段的物种，自花授粉和异花授粉物种在种子生产阶段（自花授粉δ = 0.05，N = 11；异花授粉δ = 0.32，N = 31）、发芽阶段（自花授粉δ = 0.02，异花授粉δ = 0.12）以及存活至繁殖阶段（自花授粉δ = 0.04，异花授粉δ = 0.15）的近亲繁殖衰退程度存在差异，但在生长和繁殖阶段（自花授粉δ = 0.21，异花授粉δ = 0.27）则没有差异；自花授粉物种相对于异花授粉物种的近亲繁殖衰退从生命早期到晚期逐渐增加。这些结果支持了这样一种假设，即大多数早期起作用的近亲繁殖衰退是由隐性致死基因引起的，并且可以通过近亲繁殖清除，而许多晚期起作用的近亲繁殖衰退是由弱有害突变引起的，即使在极端近亲繁殖的情况下也很难清除。
