The central-marginal hypothesis (CMH) posits that range margins exhibit less genetic diversity and greater inter-population genetic differentiation compared to range cores. CMH predictions are based on long-held "abundant-centre" assumptions of a decline in ecological conditions and abundances towards range margins. Although much empirical research has confirmed CMH, exceptions remain almost as common. We contend that mangroves provide a model system to test CMH that alleviates common confounding factors and may help clarify this lack of consensus. Here, we document changes in black mangrove (Avicennia germinans) population genetics with 12 nuclear microsatellite loci along three replicate coastlines in the United States (only two of three conform to underlying "abundant-centre" assumptions). We then test an implicit prediction of CMH (reduced genetic diversity may constrain adaptation at range margins) by measuring functional traits of leaves associated with cold tolerance, the climatic factor that controls these mangrove distributional limits. CMH predictions were confirmed only along the coastlines that conform to "abundant-centre" assumptions and, in contrast to theory, range margin A. germinans exhibited functional traits consistent with greater cold tolerance compared to range cores. These findings support previous accounts that CMH may not be a general rule across species and that reduced neutral genetic diversity at range margins may not be a constraint to shifts in functional trait variation along climatic gradients.

中文翻译：

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中央边缘假设是一般规则吗？来自红树林物种Avicennia的起源和分布的三个分布的证据。

与范围核心相比，范围边缘的中央边缘假设（CMH）假设具有较少的遗传多样性和更大的种群间遗传分化。CMH的预测基于长期存在的“丰富中心”假设，即生态条件下降和向范围边界的丰度。尽管许多经验研究都证实了CMH，但例外情况几乎仍然很常见。我们认为，红树林提供了一种测试CMH的模型系统，该模型系统可以减轻常见的混杂因素，并可能有助于澄清这种缺乏共识的情况。在这里，我们记录了在美国三个重复海岸线上，具有12个核微卫星基因座的黑红树林（Avicennia Germinans）种群遗传学的变化（三个样本中只有两个符合基本的“丰富中心”假设）。然后，我们通过测量与耐寒性相关的叶片功能性状（控制这些红树林分布限制的气候因素），来测试CMH的隐含预测（减少的遗传多样性可能会在范围边缘限制适应性）。CMH的预测仅在符合“丰富中心”假设的海岸线上得到确认，并且与理论相反，幅度裕度A.细菌菌种表现出的功能性状与范围核心相比具有更高的耐寒性。这些发现支持了以前的观点，即CMH可能不是跨物种的通用规则，并且范围边缘处中性遗传多样性的降低可能不是对功能性状沿气候梯度变化的限制。