Genotypic Differences and Life-History Trade-Offs in the Freshwater Zooplankton, Daphnia Pulicaria, Under Natural Levels of Food Limitation

Abstract

The schedule of growth, reproduction and survivorship of an individual are the key components of life history, which reflect fitness performance of a genotype. Examining the variation in life history performance among genotypes provides an avenue for which genotypic fitness differences can be compared. The parthenogenetic freshwater zooplankton, Daphnia pulicaria, is a model organism to investigate and disentangle the genotypic from the phenotypic influences of life history variation. While Daphnia spp. life history has been extensively studied, few have examined the effect of low food concentrations on life history traits. Since Daphnia spp. are frequently subjected to periods food limitation caused by natural phytoplankton cycles, it is necessary to understand how individuals respond in low food environments in order to obtain an accurate representation of life history responses among genotypes. In this study, I conducted a set of highly controlled laboratory experiments using multiple genotypes of Daphnia pulicaria under a range of food-limited conditions in order to gain insight into the environmental and genotypic responses of life history traits. I measured a suite of life history traits, growth, reproduction and survivorship, as discrete elements for each individual and synthesized these traits into a representation of the life history schedule. This provided an accurate method to compare genotypes and allowed for identification of trade-offs between life history traits. My results indicate a significant effect of low food concentration on life history traits, causing a decrease in performance of all traits. Additionally, genotypic differences occurred in most traits, however these differences only manifested within the intermediate food levels. There were no genotypic differences in daily reproductive rate, indicating that genotypes only differ in the number of individuals reproducing within an environment. Allocation-based trade-offs among these life history traits were examined within a genotype, however the only trade-off identified was between reproduction and survivorship and this only manifested at low food levels. Even though these genotypes do not co-exist in the same lakes, similar patterns occurred in trade-offs among genotypes. This potentially indicates that trade-offs among genotypes occur due to the same physiological mechanisms witnessed at the individual level.