The magnitude of inbreeding depression (ID) varies unpredictably among environments. ID often increases in stressful environments suggesting that these expose more deleterious alleles to selection or increase their effects. More simply, ID could increase under conditions that amplify phenotypic variation (CV²), e.g., by accentuating size hierarchies among plants. These mechanisms are difficult to distinguish when stress increases both ID and phenotypic variation. We grew in- and outbred progeny of Mimulus guttatus under six abiotic stress treatments (control, waterlogging, drought, nutrient deficiency, copper addition, and clipping) with and without competition by the grass Poa palustris. ID differed greatly among stress treatments with δ varying from 7% (control) to 61% (waterlogging) but did not consistently increase with stress intensity. Poa competition increased ID under nutrient deficiency but not other stresses. Analyzing effects of initial size on performance of outbred plants suggests that under some conditions (low N, clipping) competition increased ID by amplifying initial size differences. In other cases (e.g., high ID under waterlogging), particular environments amplified the deleterious genetic effects of inbreeding suggesting differential gene expression. Interestingly, conditions that increased the phenotypic variability of inbred progeny regularly increased ID whereas variability among outbred progeny showed no relationship to ID. Our study reconciles the stress- and phenotypic variability hypotheses by demonstrating how specific conditions (rather than stress per se) act to increase ID. Analyzing CV² separately in inbred and outbred progeny while including effects of initial plant size improve our ability to predict how ID and gene expression vary across environments.

近交衰退 (ID) 的严重程度在环境之间变化不可预测。ID 在压力环境中通常会增加，这表明这些环境会将更多有害的等位基因暴露于选择或增加其影响。更简单地说，ID 可以在放大表型变异 (CV²) 的条件下增加，例如，通过强调植物之间的大小等级。当压力增加 ID 和表型变异时，这些机制很难区分。我们长大的入口和远交后代沟酸浆属鱯按六个非生物胁迫处理（控制，水涝，干旱，营养缺乏，铜此外，裁剪），有和没有通过草竞争泽地早熟禾。δ应力处理之间的 ID 差异很大从 7%（控制）到 61%（涝渍）不等，但并没有随着压力强度的增加而持续增加。早熟禾竞争增加营养缺乏下的 ID，但不增加其他胁迫。分析初始尺寸对远交植物性能的影响表明，在某些条件下（低氮，剪裁），竞争通过放大初始尺寸差异来增加 ID。在其他情况下（例如，涝渍下的高 ID），特定环境放大了近亲繁殖的有害遗传效应，表明基因表达差异。有趣的是，增加近交后代表型变异性的条件通常会增加 ID，而远交后代之间的变异性与 ID 无关。我们的研究通过展示特定条件（而不是压力本身）如何增加 ID 来协调压力和表型变异性假设。