Habitat fragmentation from urban development leaves species vulnerable to inbreeding depression and genomic erosion. Restoring gene flow can reduce inbreeding and preserve genetic diversity, but a common concern is that genomic incompatibilities may lead to outbreeding depression. The introduction of deleterious genetic load is less commonly considered. The endangered Pacific pocket mouse (Perognathus longimembris pacificus) persists in three isolated populations in southern California. Mitochondrial and microsatellite data indicated that effective population sizes were extremely small (N_e< 50), and continued declines prompted a conservation breeding program founded by individuals from each population. We tracked genetic diversity and individual fitness (measured by reproductive success) in a captive setting over six generations of admixture. Although we observed an increase in fitness in the F1 and F2 generations relative to the founding populations, inbreeding depression alone did not explain the fitness patterns, and outbreeding depression was difficult to rule out as reproductive success waned after F2. However, reproductive success was consistently higher in admixed individuals than founders from Dana Point, the smallest population with the lowest heterozygosity. Across generations, we saw a strong negative correlation between individual reproductive success and ancestry to Dana Point, leading to a rapid decrease of Dana Point alleles. Although the genomic underpinnings remain to be determined, reduced fitness associated with Dana Point ancestry is consistent with high deleterious genetic load in this population, and thus any facilitated migration should be unidirectional. Our findings highlight that, even in the absence of outbreeding depression, there may be a cost to restoration of gene flow if populations harbor high genetic load.

城市发展造成的生境破碎化使物种容易遭受近亲衰退和基因组侵蚀。恢复基因流动可以减少近交并保持遗传多样性，但是一个共同的担忧是基因组不相容性可能导致近交衰退。引入有害遗传负荷的考虑较少。濒临灭绝的太平洋袖珍老鼠（Perognathus longimembris pacificus）在加利福尼亚南部的三个孤立种群中持续存在。线粒体和微卫星数据表明有效种群规模非常小（N _e<50），并且持续下降促使每个种群的个体建立了一个保护育种计划。我们在六代外加剂的圈养环境中追踪了遗传多样性和个体适应性（以繁殖成功率衡量）。尽管我们观察到F1和F2代的适应性相对于建群人群有所增加，但仅近亲消沉并不能解释其适应性模式，并且由于F2之后的生殖成功减弱，难以排除近亲消沉。然而，在混合个体中，生殖成功一直高于来自Dana Point（杂合性最低，最小的人群）的创始人。在几代人当中，我们看到了个人生殖成功与Dana Point祖先之间的强烈负相关，导致Dana Point等位基因迅速减少。尽管基因组基础尚待确定，但与Dana Point血统有关的适应性降低与该人群中有害的遗传高负荷相一致，因此，任何便利的迁移都应是单向的。我们的研究结果突出表明，即使没有远亲繁殖的抑郁症，如果种群具有很高的遗传负荷，恢复基因流也可能会付出一定的代价。