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）指亲缘个体所产生的后代适合度下降，是进化生物学的核心研究主题之一。近交效应受种群遗传构成的影响，而种群遗传构成的动态变化由种群遗传瓶颈（genetic bottleneck）与遗传漂变（genetic drift）的历史事件所驱动。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较低，我们仍通过20年的谱系（pedigree）数据对个体近交程度进行测定，并检验其是否能预测个体间适合度的差异。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亚成体存活率，这表明近交衰退甚至会对这类高度近交的种群产生影响。当雏鸟的母本近交程度较低时，较强的近交仍会降低其存活率；但当母本近交程度较高时，较强的近交反而会提升雏鸟的存活率，这一结果与预期相悖。这一反直觉的交互效应无法通过多种潜在的混杂变量加以解释。我们提出了一种遗传机制：当高度近交的幼鸟拥有高度近交的亲本时，其继承的基因型是经过验证的，因此会获得适合度优势，这一机制可以解释上述交互效应。我们所发现的正反两方面效应表明，即便对于已经高度近交的种群而言，持续的近交仍会对个体适合度产生重要影响。