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.