Is the central-marginal hypothesis a general rule? Evidence from three distributions of an expanding mangrove species, Avicennia germinans (L.)

Kennedy, John Paul, Preziosi, Richard ORCID: https://orcid.org/0000-0003-0468-6655, Rowntree, Jennifer ORCID: https://orcid.org/0000-0001-8249-8057 and Feller, Ilka C (2020) Is the central-marginal hypothesis a general rule? Evidence from three distributions of an expanding mangrove species, Avicennia germinans (L.). Molecular Ecology, 29 (4). pp. 704-719. ISSN 0962-1083

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Abstract

The central-marginal hypothesis (CMH) posits that range margins exhibit less genetic diversity 16 and greater inter-population genetic differentiation compared to range cores. CMH predictions 17 are based on long-held ‘abundant-centre’ assumptions of a decline in ecological conditions and 18 abundances towards range margins. Although much empirical research has confirmed CMH, 19 exceptions remain almost as common. We contend that mangroves provide a model system to 20 test CMH that alleviates common confounding factors and may help clarify this lack of 21 consensus. Here, we document changes in black mangrove (Avicennia germinans) population 22 genetics with 12 nuclear microsatellite loci along three replicate coastlines in the United States 23 (only 2 of 3 conform to underlying ‘abundant-centre’ assumptions). We then test an implicit 24 prediction of CMH (reduced genetic diversity may constrain adaptation at range margins) by 25 measuring functional traits of leaves associated with cold tolerance, the climatic factor that 26 controls these mangrove distributional limits. CMH predictions were confirmed only along the 27 coastlines that conform to ‘abundant-centre’ assumptions and, in contrast to theory, range margin 28 A. germinans exhibited functional traits consistent with greater cold tolerance compared to range 29 cores. These findings support previous accounts that CMH may not be a general rule across 30 species and that reduced neutral genetic diversity at range margins may not be a constraint to 31 shifts in functional trait variation along climatic gradients.