The role of genetic relatedness and trait similarity in the outcome of pairwise interactions between genotypes of eelgrass (Zostera marina)

Background/Question/Methods

Genetic diversity within key species has been increasingly tied to the performance of entire communities. In parallel with species diversity experiments, most current genetic diversity studies use only the number of genotypes (genotypic richness) as a metric of biodiversity. However, because functional similarity among genotypes may be influenced by common ancestry, assemblages of more distantly related genotypes might have higher performance because they encompass a wider niche breath (complementarity). Here we explicitly test the influence of genetic relatedness and trait distance on the performance of pairs of genotypes of the clonal seagrass species Zostera marina (eelgrass). Specifically, we ask: 1) Are relatedness and functional trait distance correlated? 2) How do the trait similarity of genotypes and their genetic relatedness influence aggregate performance? To test these questions we conducted a field experiment using eelgrass genotypes with known pairwise relatedness and trait measurements. We used all 15 possible combinations of 6 genotypes to create pairs that covered a range of relatedness and planted 5 replicates of each pair in Bodega Harbor, CA. The experiment ran for approximately one year and we assessed performance of pairs using measures of shoot density (recorded at monthly intervals) and final plant biomass.

Results/Conclusions

Surprisingly, we found that the multivariate trait distance between genotype pairs and pair relatedness were not significantly correlated. Our field experiment showed that increasing trait distance was correlated with higher pair performance, but that genetic relatedness was not significantly correlated with performance. Genotypic identity did influence performance, with some genotypes being particularity strong or week performers, however identity did not explain as much variation in productivity as did trait distance. On average genotype pair performance did not differ significantly from that expected based on genotype performance in monoculture. However, there was a trend where pairs with lower trait distance performed worse than expected based on monocultures and pairs with higher trait distance performed better than expected. Our results support the hypothesis that functional trait distance, a measure of niche breadth, can influence assemblage performance, but that genetic relatedness is a relatively poor predictor of these ecological distances. Understanding which metrics of biodiversity best predict assemblage performance can give insight into the mechanisms linking genetic diversity and ecosystem function. Evaluating biodiversity metrics can also help set conservation priorities and aid in restoration efforts, which is especially important for a foundation species like Z. marina, which provides critical habitat and ecosystem services.