Genetic rescue versus outbreeding depression in Vallisneria americana: Implications for mixing seed sources for restoration Brittany W. Marsden a,⇑ , Katharina A.M. Engelhardt b , Maile C. Neel c,d a Marine Estuarine Environmental Sciences, University of Maryland, 2102 Plant Sciences Building, College Park, MD 20742, United States b University of Maryland, Center for Environmental Science Appalachian Laboratory, 301 Braddock Road, Frostburg, MD 21532, United States c Department of Plant Science and Landscape Architecture, University of Maryland, 2102 Plant Sciences Building, College Park, MD 20742, United States d Department of Entomology, University of Maryland, 2116 Plant Sciences Building, College Park, MD 20742, United States article info Article history: Received 18 January 2013 Received in revised form 1 August 2013 Accepted 8 August 2013 Keywords: Chesapeake Bay Inbreeding Local adaptation Plant population restoration Relatedness Submersed aquatic vegetation abstract The selection of seed stock for restoration remains a complex issue. Using local stock reduces the chances of outbreeding depression or genetic dilution, whereas mixing sources may increase diversity and coun- teract inbreeding depression. Evaluation of these opposing approaches remains difficult when planning a restoration project but is needed to increase chances of long-term population persistence. We evaluated seed production and germination success of seeds from controlled reproductive crosses of the submersed aquatic plant Vallisneria americana (wild celery) collected from populations throughout the Chesapeake Bay. We assessed differences in seeds, capsules, and germination success in three types of crosses: (1) individuals within-populations, (2) among-populations but within-genetically differentiated regions, and (3) among-regions. We observed population level differences in within-population and among- region crosses. Levels of genetic relatedness among individuals, genetic diversity within populations, or differentiation across populations did not predict reproductive success. Our data show that mixing sources from different populations and regions has both benefits and drawbacks. Thus, minimizing the risks of outbreeding and inbreeding depression, presented as a mostly dichotomous issue in the restora- tion literature, is not an either-or issue in V. americana. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Two contradictory paradigms for selecting source materials cre- ate a major tension in restoration ecology. One approach argues for maintaining purity of local genetic stock by using propagules from one or a few sites in close proximity to a restoration site. The underlying hypothesis is that local stock is well adapted to envi- ronmental conditions of a site and will successfully establish with no risk of outbreeding depression from gene flow of non-local al- leles (McKay et al., 2005; Montalvo and Ellstrand, 2000, 2001). Risk of restoration failure, however, can be high when source popula- tions are small, have been isolated and drastically reduced in size, or have low diversity or low fitness due to inbreeding depression (Broadhurst et al., 2008; Weeks et al., 2011). The alternative approach is to increase diversity and counteract local inbreeding by introducing genotypes from foreign source populations or by mixing genotypes from multiple populations (Broadhurst et al., 2008). Proponents argue that stock from multiple sources promotes persistence if associated phenotypes are adapted to a broader range of environmental conditions than individuals from any single population and mating among them following restoration results in heterosis (Broadhurst et al., 2008; Fenster and Dudash, 1994; Hughes et al., 2008; Weeks et al., 2011). Immediate negative consequences of such plantings arise if phenotypes are poorly adapted to local conditions and cannot survive and establish. Long-term consequences arise if reproduc- tion between local and foreign stock is not possible or fitness of their offspring is compromised. Advocates of mixing propagules from many populations argue that benefits of increased diversity outweigh any potential negative consequences of outbreeding depression (Broadhurst et al., 2008) and argue that risks of out- breeding depression are overstated and unsubstantiated (Frankham et al., 2011; Weeks et al., 2011). Inbreeding and outbreeding depression are increasingly pre- sented as extreme dichotomous conditions. We argue here that de- grees of differentiation among populations and inbreeding within populations are continuous gradients that vary independently. Managing the risks of using local or disparate sources of restora- tion stock, therefore, needs to account for the genetic context of natural source populations. In general, it appears that mixing slightly differentiated, inbred populations can lead to increased 0006-3207/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biocon.2013.08.012 ⇑ Corresponding author. Address: University of Maryland, 2102 Plant Sciences Building, College Park, MD 20742, United States. Tel.: +1 202 603 8277. E-mail addresses: bwest1@umd.edu (B.W. Marsden), kengelhardt@umces.edu (K.A.M. Engelhardt), mneel@umd.edu (M.C. Neel). Biological Conservation 167 (2013) 203–214 Contents lists available at ScienceDirect Biological Conservation journal homepage: www.elsevier.com/locate/biocon