Department of Life Sciences and Biotechnology, University of Ferrara, 44121 Ferrara, Italy.
Centre for Ecological and Evolutionary Synthesis, Department of Biosciences, University of Oslo, Oslo 1066, Norway.
Proc Natl Acad Sci U S A. 2017 Nov 7;114(45):E9589-E9597. doi: 10.1073/pnas.1707279114. Epub 2017 Oct 24.
About 100 km east of Rome, in the central Apennine Mountains, a critically endangered population of ∼50 brown bears live in complete isolation. Mating outside this population is prevented by several 100 km of bear-free territories. We exploited this natural experiment to better understand the gene and genomic consequences of surviving at extremely small population size. We found that brown bear populations in Europe lost connectivity since Neolithic times, when farming communities expanded and forest burning was used for land clearance. In central Italy, this resulted in a 40-fold population decline. The overall genomic impact of this decline included the complete loss of variation in the mitochondrial genome and along long stretches of the nuclear genome. Several private and deleterious amino acid changes were fixed by random drift; predicted effects include energy deficit, muscle weakness, anomalies in cranial and skeletal development, and reduced aggressiveness. Despite this extreme loss of diversity, Apennine bear genomes show nonrandom peaks of high variation, possibly maintained by balancing selection, at genomic regions significantly enriched for genes associated with immune and olfactory systems. Challenging the paradigm of increased extinction risk in small populations, we suggest that random fixation of deleterious alleles () can be an important driver of divergence in isolation, () can be tolerated when balancing selection prevents random loss of variation at important genes, and () is followed by or results directly in favorable behavioral changes.
在罗马以东约 100 公里处的亚平宁山脉中部，生活着一个濒临灭绝的约 50 只棕熊种群，它们完全处于隔离状态。由于有数百公里的无熊区，该种群之外的交配受到了阻碍。我们利用这个自然实验，更好地了解了在极小种群规模下生存的基因和基因组后果。我们发现，自新石器时代以来，欧洲的棕熊种群就失去了联系，当时农业社区不断扩张，森林火灾被用于清理土地。在意大利中部，这导致了种群数量减少了 40 倍。这种下降的总体基因组影响包括线粒体基因组和核基因组的大片段完全丧失变异。几个私人的和有害的氨基酸变化是由随机漂变固定的；预测的影响包括能量不足、肌肉无力、颅面和骨骼发育异常以及攻击性降低。尽管多样性的极度丧失，但是亚平宁山脉熊的基因组显示出高变异性的非随机峰，这些峰可能是由平衡选择维持的，在与免疫和嗅觉系统相关的基因中显著富集的基因组区域。我们挑战了小种群灭绝风险增加的范式，认为有害等位基因的随机固定（）可能是隔离中分化的一个重要驱动因素，（）在平衡选择防止重要基因的随机变异丧失时可以被容忍，并且（）之后或者直接导致有利的行为变化。
