Mixing genetically differentiated populations successfully boosts diversity of an endangered carnivore

Biodiversity decline and genetic erosion are among the most challenging
conservation issues. Genetic admixture, the mixing of two or more
genetically differentiated populations, can increase genetic diversity of
admixed individuals. However, genetic admixture for conservation purposes
is rare due to concerns over outbreeding depression, loss of local
adaptations and scepticism regarding the benefits of mixing populations.
We used an introduced population of Tasmanian devils (Sarcophilus
harrisii) descended from two genetically differentiated source populations
to illustrate the benefits of genetic admixture for translocation
programs. Devils are endangered due to an infectious cancer causing 80%
population declines across their range since disease emergence in 1996.
Devil populations are known to be structured on an east-west cline across
Tasmania. As part of their conservation management, devils were introduced
to Maria Island, Tasmania in an assisted colonisation in 2012 with
supplementations in 2013 and 2017. Of the released individuals (N = 34),
23 were western (13M; 10F), four were eastern (4F), and seven were of
mixed origin (2M; 5F). The genetic composition of devils born on Maria
Island (N = 185) was examined using 927 SNP loci. Using a reconstructed
pedigree for each individual, we examined the proportion of founding
origin (east, west or mixed) and used Shannon’s Diversity Index to
quantify the evenness of founder origin. Individuals with mixed origins (N
= 102) had higher genetic diversity than purebred individuals (N = 83),
and increased evenness of founder origin was positively correlated with
genetic diversity. Increased genetic diversity had no influence on
reproductive success. For a genetically depauperate species, mixing
individuals descended from differentiated populations, resulted in
increased diversity in subsequent generations. This finding permits
conservation managers to select individuals for translocation that produce
offspring with higher genetic diversity, creating high-diversity source
populations that can be used when augmenting other declining wild
populations.

生物多样性下降与遗传侵蚀是最具挑战性的保护问题之一。遗传混合（genetic admixture）指两个或多个遗传分化群体的混合，可提高混合个体的遗传多样性。然而，出于对远交衰退（outbreeding depression）、本地适应性丧失以及对群体混合益处的质疑，用于保护目的的遗传混合却较为罕见。我们以源自两个遗传分化源种群的引入塔斯马尼亚袋獾（Sarcophilus harrisii）种群为例，阐明遗传混合在易位计划（translocation programs）中的益处。袋獾因一种传染性癌症而濒临灭绝，自1996年疾病出现以来，其分布范围内的种群数量下降了80%。已知塔斯马尼亚岛的袋獾种群呈东西向渐变群结构。作为保护管理的一部分，袋獾于2012年通过辅助定居被引入塔斯马尼亚州的玛丽亚岛，并在2013年和2017年进行了补充。在释放的34只个体中，23只来自西部（13雄；10雌），4只来自东部（4雌），7只具有混合起源（2雄；5雌）。利用927个SNP位点（SNP loci）分析了玛丽亚岛出生的185只袋獾的遗传组成。通过重建每个个体的系谱，我们分析了创始起源（东、西或混合）的比例，并使用香农多样性指数（Shannon’s Diversity Index）量化创始起源的均匀度。混合起源个体（102只）的遗传多样性高于纯种个体（83只），且创始起源均匀度的增加与遗传多样性呈正相关。对于遗传贫乏物种（genetically depauperate species）而言，混合源自分化种群的个体可提高后代的遗传多样性。这一发现使保护管理者能够选择易位个体以产生遗传多样性更高的后代，从而建立高多样性源种群，用于补充其他衰退的野生种群。