Data from: Relative risks of inbreeding and outbreeding depression in the wild in endangered salmon

Conservation biologists routinely face the dilemma of keeping small, fragmented populations isolated, wherein inbreeding depression may ensue, or mixing such populations, which may exacerbate population declines via outbreeding depression. The joint evaluation of inbreeding and outbreeding risks in the wild cannot be readily conducted in endangered species, so a suggested 'safe' strategy is to mix ecologically and genetically similar populations. To evaluate this strategy, we carried out a reciprocal transplant experiment involving three neighbouring populations of endangered Atlantic salmon (Salmo salar) now bred in captivity and maintained in captive and wild environments. Pure, inbred, and outbred (first and second generation) cross types were released and recaptured in the wild to simultaneously test for local adaptation, inbreeding depression and outbreeding depression. We found little evidence of inbreeding depression after one generation of inbreeding and little evidence of either heterosis or outbreeding depression via genetic incompatibilities after one or two generations of outbreeding. A trend for outbreeding depression via the loss of local adaptation was documented in one of three populations. The effects of inbreeding were not significantly different from the effects of outbreeding. Hence, at the geographic scale evaluated (34-50 km), inbreeding for one generation and outbreeding over two generations may have similar effects on the persistence of small populations. The results further suggested that outbreeding outcomes may be highly variable or unpredictable at small genetic distances. Our work highlights the necessity of evaluating the relative costs of inbreeding and outbreeding in the conservation and management of endangered species on a case-by-case basis.保护生物学家时常面临两难抉择：要么维持小型破碎化种群的隔离状态，这可能引发近交衰退（inbreeding depression）；要么混合此类种群，却可能因远交衰退（outbreeding depression）加剧种群衰退。野生环境下近交与远交风险的联合评估难以在濒危物种中直接开展，因此有学者提出了一项“安全”策略：混合生态与遗传特征相似的种群。 为评估该策略，我们针对3个相邻的濒危大西洋鲑（Salmo salar）种群开展了互易移植实验——这些种群目前均通过人工繁育得到维持，分别饲养于圈养与野生环境中。我们将纯繁、近交以及远交（第一代和第二代）的杂交组合个体放归野外并进行重捕，以此同时检验本地适应（local adaptation）、近交衰退与远交衰退效应。 研究发现，仅经历一代近交的种群几乎未表现出近交衰退迹象；而经过一代或两代远交后，也几乎未检测到杂种优势（heterosis）或由遗传不相容性（genetic incompatibilities）引发的远交衰退。在3个种群中的1个种群内，观测到了因本地适应丧失而导致远交衰退的趋势。近交与远交的效应并未表现出显著差异。因此在本次评估的地理尺度（34~50千米）下，一代近交与两代远交对小型种群存续的影响可能相似。 研究结果进一步表明，在较小的遗传距离下，远交的结果可能具有高度变异性或不可预测性。本研究强调，在濒危物种的保护与管理中，需针对具体案例逐一评估近交与远交的相对成本。