Thesis

Phenotypic plasticity and morphological variation in a native submerged aquatic plant

Morphological variations in widely distributed plants may be driven by phenotypic plasticity or by underlying genetic differences. In the brackish open water region of the San Francisco Estuary (SFE), a population of sago pondweed, (Stuckenia pectinata) shows at least two distinct growth forms, which are so different that they were previously thought to be two species. This study confirmed the S. pectinata species identity for all morphologies through sequencing of the nuclear ribosomal internal transcribed spacer (ITS) region. I investigated phenotypic plasticity in response to flow variations in a common garden, and found that morphological traits are plastic but distinct morphotypes did not converge. I then used four microsatellite loci to investigate differentiation between the two morphotypes at three sites within Suisun Bay and the western Sacramento-San Joaquin Delta, and two populations from other central California sites for comparison. I found very few multilocus genotypes (unique combinations of alleles across two or more loci), which could be a reflection of low levels of clonal diversity, or a byproduct of low resolution in my methods. Lastly, I investigated the influence of plant morphology on the ecologically important invertebrate epibiont community. I found a positive relationship of plant surface area, leaf count, and leaf density with invertebrate abundance only at the more saline of the two sites sampled, which had a different invertebrate assemblage than the fresher site. Results of this study suggest that the two growth forms may provide different ecosystem functions and services, which could influence management and restoration decisions in the region.

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