Data from: Inbreeding depression and drift load in small populations at demographic disequilibrium

Inbreeding depression is a major driver of mating system evolution and has critical implications for population viability. Theoretical and empirical attention has been paid to predicting how inbreeding depression varies with population size. Lower inbreeding depression is predicted in small populations at equilibrium, primarily due to higher inbreeding rates facilitating purging and/or fixation of deleterious alleles (drift load), but predictions at demographic and genetic disequilibrium are less clear. In this study, we experimentally evaluate how lifetime inbreeding depression and drift load, estimated by heterosis, vary with census (Nc) and effective (estimated as genetic diversity, He) population size across six populations of the biennial Sabatia angularis as well as present novel models of inbreeding depression and heterosis under varying demographic scenarios at disequilibrium (fragmentation, bottlenecks, disturbances). Our experimental study reveals high average inbreeding depression and heterosis across populations. Across our small sample, heterosis declined with He, as predicted, whereas inbreeding depression did not vary with He and actually decreased with Nc. Our theoretical results demonstrate that inbreeding depression and heterosis levels can vary widely across populations at disequilibrium despite similar He and highlight that joint demographic and genetic dynamics are key to predicting patterns of genetic load in non-equilibrium systems.

近交衰退（inbreeding depression）是交配系统演化的主要驱动因素，对种群生存力具有关键影响。学界已从理论与实证层面开展研究，以预测近交衰退随种群规模的变化规律。处于平衡状态的小型种群中，近交衰退程度预计更低，这主要是因为较高的近交率会促进有害等位基因的清除和/或固定，即漂变负荷（drift load）；但在人口统计与遗传不平衡状态下的相关预测仍不明确。本研究以6个种群的二年生棱角花锚（Sabatia angularis）为研究对象，实验评估了以杂种优势（heterosis）估算的终生近交衰退与漂变负荷如何随普查种群规模（Nc）以及由遗传多样性（He）表征的有效种群规模发生变化；同时，本研究还提出了在不平衡人口统计情景（生境破碎化、种群瓶颈、干扰）下的近交衰退与杂种优势新模型。本实验研究结果显示，各种群的平均近交衰退与杂种优势水平均较高。在本次研究的小样本中，杂种优势随He的升高而下降，这与理论预测一致；而近交衰退并未随He发生显著变化，反而随Nc的升高而降低。本研究的理论结果表明，尽管不同种群的He值相近，但处于不平衡状态的种群间，近交衰退与杂种优势水平仍可能存在显著差异；同时，本研究强调，人口统计与遗传的联合动态是预测非平衡系统中遗传负荷分布模式的关键因素。