Going with the flow? Relative importance of riverine hydrologic connectivity versus tidal influence for spatial structure of genetic diversity and relatedness in a foundational submersed aquatic plant

Genetic connectivity in rivers is generally high and levels of genotypic and genetic diversity of riverine species are expected to accumulate in downstream locations. Genetic structure of marine and estuarine species is less predictable even though hydrologic connectivity is expected to be relatively high. These observations have been generated across different species and locations such that understanding effects hydrologic connectivity relative to tidal versus non-tidal environments in the same river remains incomplete. To control for species and location, we quantified diversity in 941 samples of Vallisneria americana Michx. (Hydrocharitaceae) collected from 36 sites along the species’ entire distribution in the tidal and non-tidal Potomac River of Maryland, Virginia, and the District of Columbia. Using ten microsatellite loci, we found 508 unique multilocus genotypes (MLGs), 36 of which were found multiple times across the riverscape, accounting for over 53% of the genotyped shoots. We found some evidence supporting connectivity throughout the river and stronger evidence that tidal regime drives genotypic and genetic structure within V. americana. Extensive clonality, including two MLGs spanning 230 and 152 river km, limits diversity in the non-tidal reaches and contrasts with very little evidence of clonal expansion (e.g., asexual reproduction) in non-tidal reaches. Genetic differentiation, structure, and pairwise relatedness of sampled shoots and MLGs also differed by tidal regime with the non-tidal Potomac having higher levels of relatedness and lower levels of genetic diversity. The differences in spatial distribution of genetic diversity suggest very different outlooks for V. americana adaptation and acclimation to current and future perturbations across tidal and non-tidal regions of the Potomac, which lead to different recommendations for restoration of the same species in the same river.

Methods
Leaves were collected in the field and DNA was extracted.  Microsatellites were amplified using PCR and detected on an Applied Biosystems Incorporated 3730 DNA analyzer.  Microsatellite alleles were called using GeneMapper and checked manually for accuracy.  Multilocus genotypes were identified using the program Genodive and the poppr package in the R statistical environment as described in the publication.