Uncovering the population genetic structure of allopatric and sympatric duckweed populations

Background/Question/Methods

The dynamic interplay between competition and evolution has been a focus of both theoretical and empirical research for many decades. Our lab studies how competition drives rapid evolution and tests how this evolution impacts species coexistence using duckweed; a small rapidly asexually reproducing aquatic plant. One major goal of this research is to test whether such evolutionary-coexistence dynamics depend on the history of the populations under study. We here present the first phase of this work asking whether naturally occurring sympatric and allopatric populations differ in their genetic structure across independent pairs of populations. One hypothesis being tested is that if competitors reduce available niche space than genotypic diversity should be reduced in sympatry. To address this question, we sampled nearby allopatric and sympatric populations of both Lemna minor and Spirodela polyrhiza from four different regions across three states. In each population, we genotyped at least 66 individuals using between 8 and 12 microsatellite markers. We then conducted a series of population genetic analyses. In addition, we conducted a large field experiment competing the most common genotypes with the other species and quantified growth and competitive ability and will correlate these traits with the frequency and distribution of genotypes.

Results/Conclusions

Preliminary population genetic results suggest that across regions, genotypic richness of S. polyrhiza was five times lower than that of L. minor. Across the four independent pairs of allopatric and sympatric sites we did not see a clear impact of the competitive environment on genotypic richness. Phylogeographic analyses revealed that genotypes from similar geographic regions were generally more closely related and only a few genotypes were found across regions suggesting local diversification and limited dispersal. Genotypes also tended to differ among nearby sympatric and allopatric populations. Allelic diversity did seem to be higher in sympatric populations. Analyses of genetic distance similarly revealed stronger impacts of geographic distance (region) and weaker impact of the competitive environment. We are still processing the data for growth rate and competitive ability but will be including in the presentation. Our preliminary conclusions are that genotypic composition is most strongly structured by region, can differ quite substantially among nearby bodies of water, and only somewhat impacted by current competitive environment. How these factors correlate with variation in traits remains to be determined.