Data from: Unexpected positive and negative effects of continuing inbreeding in one of the world’s most inbred wild animals

Inbreeding depression, the reduced fitness of offspring of related individuals, is a central theme in evolutionary biology. Inbreeding effects are influenced by the genetic makeup of a population, which is driven by any history of genetic bottlenecks and genetic drift. The Chatham Island black robin represents a case of extreme inbreeding following two severe population bottlenecks. We tested whether inbreeding measured by a 20-year pedigree predicted variation in fitness among individuals, despite the high mean level of inbreeding and low genetic diversity in this species. We found that paternal and maternal inbreeding reduced fledgling survival and individual inbreeding reduced juvenile survival, indicating that inbreeding depression affects even this highly inbred population. Close inbreeding also reduced survival for fledglings with less-inbred mothers, but unexpectedly improved survival for fledglings with highly inbred mothers. This counterintuitive interaction could not be explained by various potentially confounding variables. We propose a genetic mechanism, whereby a highly inbred chick with a highly inbred parent inherits a “proven” genotype and thus experiences a fitness advantage, which could explain the interaction. The positive and negative effects we found emphasize that continuing inbreeding can have important effects on individual fitness, even in populations that are already highly inbred.近交衰退（inbreeding depression），即亲缘个体所产后代的适合度降低，是进化生物学的核心议题之一。近交效应受种群遗传组成影响，而该组成由遗传瓶颈（genetic bottleneck）与遗传漂变（genetic drift）的历史进程所驱动。查塔姆岛黑知更鸟（Chatham Island black robin）便是经历两次严重种群瓶颈后出现极端近交的典型案例。尽管该物种的平均近交水平较高且遗传多样性极低，我们仍通过20年的系谱数据估算近交程度，以此检验其能否预测个体间的适合度差异。研究发现，父本与母本的近交均会降低离巢雏鸟的存活率，而个体自身的近交则会降低幼体存活率，这表明即便在该高度近交的种群中，近交衰退依然会产生影响。此外，对于母本近交程度较低的雏鸟而言，强近交同样会降低其存活率；但令人意外的是，对于母本高度近交的雏鸟，强近交却能提升其存活率。这种违反直觉的交互效应无法通过各类潜在混淆变量进行解释。我们提出一种遗传机制：若高度近交的雏鸟拥有高度近交的亲本，其将继承"经证实"的基因型，从而获得适合度优势，这或可解释该交互效应。我们发现的双向效应表明，即便在已高度近交的种群中，持续的近交仍会对个体适合度产生重要影响。