Comparing genetic diversity, relatedness, and growth performance in Vallisneria americana collected from three rivers

Background/Question/Methods: Vallisneria americana is an important and broadly distributed submersed aquatic plant that has undergone dramatic declines in abundance and is the target of many restoration initiatives. To appropriately manage for resilience in the face of climate change, additional knowledge on the capacity of remaining populations to either adapt through natural selection or acclimate to emerging conditions is urgently needed. We quantified the genetic diversity in V. americana populations sampled from five sites from three rivers – the Potomac River (MD), the Hudson River (NY), and the Kennebec River (ME) and assessed the structure of genetic diversity within each river using individual based networks of relatedness. Common garden experiments were then used to evaluate local adaptation or acclimation potential. Three temperature and photoperiod treatments simulated local conditions for Potomac (n=94), Hudson (n=94), and Kennebec (n=94) V. americana. A fourth treatment simulated a future Potomac River global warming scenario (n=94). Morphological and life history traits of each plant were evaluated using generalized linear models (GLMs) for evidence of local adaptation.

Results/Conclusions:  Out of 440 sampled V. americana shoots, 421 were successfully assigned to one of 314 unique multilocus genotypes (MLGs). Sites varied in terms of genotypic diversity (0.27-1.00), allelic diversity (2.8-5.5), and observed heterozygosity (0.36-0.60). The Potomac had higher allelic diversity while the Hudson had higher pairwise relatedness. Five MLGs were found across multiple sites, including two in the Hudson and three in the Kennebec. Individual network analysis of pairwise relatedness revealed no common patterns in the distribution of genetic diversity within rivers. Evaluation of morphological and life history traits only indicated evidence of local adaptation in plants from the Potomac. Few differences were observed between local versus foreign sourced plants, suggesting potential for acclimation. However, V. americana grown under global warming conditions frequently had reduced growth and reproductive performance. Overall, limited evidence of local adaptation and high acclimation to different conditions suggest that V. americana have high potential for resilience in the face of change, so long as temperatures do not exceed thermal tolerances. However, some sites may lack the genetic diversity needed for long term evolutionary potential. Moreover, regional differences mean that data from one river cannot be used to inform restoration and management decisions in other rivers.