Decades of nutrient and sediment runoff from the surrounding landscape have compromised water quality in the Chesapeake Bay, reducing extent of and connectivity among patches of the foundation aquatic plant species Vallisneria americana. Theory suggests that the magnitude and duration of declines could cause loss of genetic diversity to the point that acclimation and adaptation potentials are limited. We tested effects of population declines by quantifying the number of multilocus genotypes (MLGs), allelic richness, and inbreeding based on 10 microsatellite loci assayed in 1,417 samples from 37 sites in the central, northern, and northeastern Bay. We calculated the proportion of samples yielding unique MLGs (GD) to gain insight into relative amounts of sexual versus asexual reproduction. We used the structure of diversity among sites as quantified by correspondence analysis to understand connectivity among sites. Together these analyses give insight into whether past environmental effects have compromised the capacity of V. americana to adapt, acclimate, or disperse with sea level rise and changes in connectivity.
Results/Conclusions
The 1,417 samples were assigned to 1,230 MLGs, indicating ~87% of samples resulted from sexual reproduction. Twenty-six sites had GD>0.8. Sites with GD<0.8 (from 0.17-0.77) were all from the central Bay where Vallisneria occurs in isolated tributaries. Two sites with the lowest GD were in the highest salinities. Multisample MLGs comprise 35% percent of central Bay samples but only 8% and 12% in the northern and northeastern Bay, respectively. We found 114 alleles, with an average of 5.4 alleles/locus within sites. The three regions did not differ in allelic richness. We found evidence of apparent inbreeding at only one site and suggest it was due to a Wahlund effect. The first two correspondence analysis axes explained 38.1% of the variance in allele frequencies among sites with axis 1 separating most central Bay sites from the others and axis 2 separating northern and eastern Bay sites. The results indicated high gene flow among most central sites and among northern sites, but low gene flow among the three regions and among northeastern Bay sites. In general, diversity is moderate to high, especially considering the impacts human land uses have caused to the Bay. Lower diversity and more prevalent clonal growth suggest the need to examine the central Bay locations further to understand acclimation and adaptation potential. Restricted gene flow between regions suggests sourcing restoration material from within regions is prudent.