Genotype x environment interactions, stoichiometric food quality effects, and clonal coexistence in Daphnia pulex.

ABSTRACT

The role of stoichiometric food quality in influencing genotype coexistence and competitive interactions between clones of the freshwater microcrustacean, Daphnia pulex, was examined in controlled laboratory microcosm experiments. Two genetically distinct clones of D. pulex, which show variation in their ribosomal rDNA structure, as well as differences in a number of previously characterized growth-rate-related features (i.e., life-history features), were allowed to compete in two different arenas (1) batch cultures differing in algal food quality (i.e., high vs. low carbonphosphorus (CP ratio) in the green alga, Scenedesmus acutus); (2) continuous flow microcosms receiving different light levels (i.e., photosynthetically active radiation) that affected algal CP ratios. In experiment 1, a clear genotype x environment interaction was determined with clone 1 out-competing clone 2 under high nutrient (i.e., low food CP) conditions, while the exact opposite pattern was observed under low nutrient (i.e., high CP) conditions. In experiment 2, clone 1 dominated over clone 2 under high light (higher CP) conditions, but clonal coexistence was observed under low light (low CP) conditions. These results indicate that food (nutrient) quality effects (hitherto an often overlooked factor) may play a role in microevolutionary (genotypic) responses to changing stoichiometric conditions in natural populations.