Out with the old, in with the new? Younger Daphnia clones are competitively superior over centuries-old ancestors

Abstract

A laboratory microcosm experiment was conducted to examine the roles of food quantity and food quality on the competitive abilities of clones of the keystone aquatic zooplankter, Daphnia pulicaria. Using methods of resurrection ecology, clones were established by hatching dormant eggs from sediment layers of a lake (South Center, MN, U.S.A) that were separated by centuries of environmental change (nutrient enrichment). Two sets of paired clones (“modern” vs. “ancient”) were competed against each other in a 2 × 2 factorial design of high/low food quantity and high/low food quality. Experimental carbon:phosphorus (C:P) ratios were designed to mimic food quantity and food quality parameters respective to each of the two time periods from which the clones were extracted to test whether clones are competitively superior under conditions representative of the time period in which they existed. Contrary to predictions, we found that “modern” clones were able to outcompete “ancient” clones under all food treatments. We discuss potential mechanisms (i.e., mutational input, viability of long-dormant eggs, differences in phenotypic plasticity) that may have differentially impacted the performance of experimental clones. Our results should aid in deciphering microevolutionary dynamics observed in resurrection ecology studies, a powerful tool that can illuminate adaptive dynamics of organisms to environmental changes on long-term (centuries-long) temporal scales.