Investigating causes and implications of morphological variation in a native pondweed (Stuckenia pectinata) of San Francisco Estuary

Morphological variation in widely distributed plants may be driven by phenotypic plasticity or underlying genetic differences. In the brackish to fresh open waters of San Francisco Estuary, there are at least two distinct growth forms of a native pondweed in the genus Stuckenia. We determined that all plants collected from throughout the region are S. pectinata by sequencing the nuclear ribosomal internal transcribed spacer (ITS) region. This led us to question if different morphologies result from intraspecific genetic differentiation or differences in water flow, which we explored using microsatellites and a common garden experiment, respectively. As only four of nine microsatellite loci developed in Europe were polymorphic for our samples, there was low resolution to detect differences; however, some multilocus genotypes found only in narrow-leaf samples hinted of within-species variation. Further, while morphological traits were plastic in response to flow manipulation, distinct morphotypes did not converge under the same conditions, implicating genetic differences. Regardless of mechanisms for contrasting growth forms, leaf size and quantity may affect available habitat for ecologicallyimportant invertebrates. We found narrow/dense leaves supported greater invertebrate densities at both a low-salinity, and especially, a brackish site. The latter was dominated by an amphipod that we determined strongly prefers to consume narrower leaves. We conclude that both environmental conditions and genetic differentiation play a role in S. pectinata morphology. That these differences propagate to higher trophic levels calls for further investigation into the genetic basis for, as well as spatial extent of, the two morphotypes.