DOI: 10.5061/dryad.69p8cz8xh
title: Is the central-marginal hypothesis a general rule? Evidence from three distributions of an expanding mangrove species, Avicennia germinans (L.) L.

description: 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 2 of 3 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 physiological adaptation to context-specific environmental factors.
description: Genotype data - 1,083 <em>Avicennia germinans</em>
 (black mangrove) trees from 41 collection sites across the United States
 were genotyped at 12 nuclear microsatellite loci. Samples from East
 Florida (EFL) were collected in January 2015, from West Florida (WFL) in
 September-October 2015, and from Texas and Louisiana (TX-LA) in October
 2015. Samples for two sites (code: TB, SFL) were obtained from preserved
 leaves collected in 2011. Collection sites with fewer samples (n = 9–11)
 were collected opportunistically between 2015 and 2016. 
 Functional trait data - 342 <em>Avicennia
 germinans</em> trees from 33 collection sites in East Florida (n =
 18 sites), West Florida (n = 8 sites), and Texas and Louisiana (n = 7
 sites) were sampled to measure five functional traits of leaves (area,
 length, width, ratio length:width, and specific leaf area). These trees
 are a subset of the same trees in the accompanying genotype data
 set. Traits were measured for 10 leaves per tree and the mean value for
 each tree is presented in the data set.
description: Genotype data - column 1: geographic region (TX-LA, Texas and
 Lousiana; WFL, West Florida; SFL, South Florida; EFL, East Florida);
 column 2: site (identification code for collection site); column 3: ID
 (identification code for sampled individual); column 4: lat (latitude at
 central point for each collection site); column 5: c_m (collection site
 classified as range core or range margin); column 6-29: multi-locus
 microsatellite genotype (12-loci). *NOTE: blank cells are missing
 data Functional trait data - column 1: geographic
 region (TX-LA, Texas and Lousiana; WFL, West Florida; EFL, East Florida);
 column 2: site (identification code for collection site); column 3: lat
 (latitude at central point for each collection site); column 4: c_m
 (collection site classified as range core or range margin); column 5: area
 (leaf area, cm<sup>2</sup>); column 6: length (leaf length,
 cm); column 7: width (leaf width, cm); column 8: l_w (ratio of leaf
 length:width); column 9: dry (leaf dry weight, g); column 10: SLA
 (specific leaf area, cm<sup>2</sup>
 g<sup>-1</sup>)