Trade-offs in growth on rich and poor quality resources: Intraspecific variation in sensitivity to resource quality in the freshwater zooplankton Daphnia pulicaria
Populations maintain a high degree of genetic diversity in many ecologically important traits. Many animal populations maintain variation in acquisition and allocation of resources. Highly sensitive individuals have a high growth rate on rich quality resources but suffer a fitness decline on poor quality resources. This strategy is contrasted with low sensitivity to resource quality. Low sensitivity individuals have little to no difference in fitness on rich and poor quality resources. We collected Daphnia pulicaria individuals from 6 populations during 2 consecutive spring and summers, genotyped them for genetically distinct maternal lines, and assayed their clonal offspring’s growth rate – a fitness proxy – on rich and poor quality resources. We predict that seasonal differences in resource quality will lead to a high sensitivity strategy being favored in the spring when resource conditions are rich. However, when resource quality declines in the summer, few highly sensitive genotypes would remain as their fitness is reduced and low sensitivity genotypes would dominate the population.
Our assays demonstrate that mean sensitivity to resource quality is higher in the spring when resource conditions are of rich quality. Population mean sensitivity declines in the summer when resource conditions are comparatively poor. Initial assessment of population mean sensitivity indicates that there is no difference in sensitivity to resource quality between populations. However, there is a high degree of variation within each of the six populations, which may reflect our sampling design for these assays in which was specifically chose unique genotypes. Ongoing molecular analyses will quantify the relative frequency of each of our assayed genotype in their populations. Once complete, this will reveal the proportion of the population that are composed of high sensitivity or low sensitivity genotypes.