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）指亲缘个体所产后代的适合度降低，是进化生物学的核心议题。近交效应受种群遗传组成的影响，而种群遗传组成的动态由其经历的遗传瓶颈（genetic bottleneck）与遗传漂变（genetic drift）历史所驱动。查塔姆群岛黑知更鸟便是经历两次严重种群瓶颈后出现极端近交的典型案例。本研究基于20年的谱系（pedigree）记录估算个体近交程度，旨在检验尽管该物种平均近交水平较高、遗传多样性较低，近交程度是否能够预测个体间的适合度差异。研究结果显示，父本与母本的近交均会降低雏鸟存活率，而个体自身的近交则会降低幼鸟存活率，这表明近交衰退甚至会对这类高度近交的种群产生影响。个体自身的高度近交同样会降低母本近交程度较低的雏鸟的存活率，但却出乎意料地提升了母本近交程度较高的雏鸟的存活率。这种违背直觉的互作效应无法通过多种潜在混淆变量予以解释。本研究提出一种遗传机制：若高度近交的雏鸟拥有高度近交的亲本，则其可继承“经过验证”的基因型，从而获得适合度优势，这一机制能够解释上述互作效应。本研究发现的正反两方面效应表明，即便对于已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。