Functional genomics of intraspecific variation in carbon and phosphorus kinetics in Daphnia

Understanding how the genome interacts with the environment to produce a diversity ofphenotypes is a central challenge in biology. However, we know little about how traits involved innutrient processing interact with key ecological parameters, such as the supply of mineral nutrients,particularly in animals. The framework of ecological stoichiometry uses information on the contentof key elements such as carbon (C) and phosphorus (P) in individuals to predict the success ofspecies. Nevertheless, intraspecific variation in content and the underlying mechanisms thatgenerate such variation has been poorly explored. We studied two genotypes (G1 and G2) ofDaphnia pulex that exhibit striking genotype environment (G E) interaction in response toshifts in dietary stoichiometry (C:P). G1 had higher fitness under C:P 100 diet, while G2 performedbetter in C:P 800. Dual 14C/33P radiotracer assays show that G1 was more efficient in Cprocessing, while G2 was more efficient in P use. Microarrays revealed that after 3 days ofincubation, the genotypes differentially expressed 25% (7,224) of the total genes on the arrayunder C:P 100 diet, and 30% (8,880) of genes under C:P 800. These results indicate largedifferences in C and P use between two coexisting genotypes. Importantly, such physiologicaldifferences can arise via differential expression of the genome due to alterations in dietarystoichiometry. Basic frameworks such as ecological stoichiometry enable integration ofphysiological and transcriptomic data, and represent initial steps toward understanding theinterplay between fundamental ecological parameters such as nutrient supply and importantevolutionary processes such as G E interactions