Elevated CO2 alters genetic dominance hierarchies in common ragweed, an allergenic plant

Background/Question/Methods

Because of ongoing changes to the earth’s environment, plant genotypes that dominate within mixed populations today may differ markedly from those that are favored in the future.  In this study, we tested whether and how the dominance of different genotypes may change in response to projected atmospheric CO₂ conditions in the noxious plant species, common ragweed.  We grew twelve maternal lines under competitive conditions at either ambient or twice-ambient CO₂.  Using a mixed model framework, we combined classical quantitative genetics approaches with a priori predictions about intraspecific competition to test the relative performance of dominant and subordinate genotypes.  Applying our data to this framework, we tested among four specific hypotheses for the impact of CO₂  on the genetic hierarchy: a) there is no interaction between CO₂ and genotype and hence no change in genotypic dominance hierarchies; (b) competition among genotypes is exacerbated because dominant genotypes at ambient CO₂ benefit proportionately more under elevated CO₂; (c) subordinate genotypes benefit proportionately more than dominants and therefore the hierarchy is less pronounced under elevated CO_2; (d) subordinate genotypes ‘catch-up’ to dominants and there is no genotypic hierarchy under elevated CO₂; or (e) ‘dominance reversal’ where dominants become suppressed and subordinate genotypes become dominant under elevated CO₂.  We then examined the evolutionary implications using a contextual analysis to measure the strength and direction of selection on size in the two treatments.

 

Results/Conclusions

Our results demonstrate reduced competition among genotypes, as well as a surprising and complete reversal in the size hierarchy.  Genotypes that are competitively suppressed at ambient levels become dominant under experimental doubling of CO₂.  Subordinate plants, in turn, boost their reproductive allocation to that of dominants, shrinking the fitness gap among all genotypes in high CO₂.  Extending our data to a contextual analysis framework, we further show that natural selection on size is dampened at elevated CO₂, because an individual’s position within the size hierarchy becomes less important for reproduction than it is in ambient conditions.  This work highlights useful analytical tools for understanding competition in an evolutionary context, and points to potential future ecological and evolutionary changes in this widespread and weedy allergenic plant species.