Pleiotropic genes can influence the course of evolution by producing correlated responses to selection and potentially hindering adaptive evolution if alternative alleles have antagonistic effects on fitness traits. Little is known about the pleiotropic effects of natural variation in any gene on fitness and the selective agents acting on this variation. In this thesis, I examined the pleiotropic effects of natural variation in the Dopa decarboxylase (Ddc) gene of Drosophila melanogaster. I show that natural Ddc polymorphism associated with life span variation has pleiotropic effects on larval defenses against parasitoid wasps, larval foraging behavior, and larval feeding rates. I measured these traits because Ddc catalyzes the production of two neurotransmitters, dopamine and serotonin, which have known effects on behavior and immunity in invertebrates and vertebrates. Homozygote and heterozygote effects of two Ddc alleles with opposing life spans were measured for each larval trait. Larvae homozygous or heterozygous for the long-lived Ddc allele had a better immune response against the parasitoid, Aphaereta sp., but were parasitized more by Leptopilina boulardi than larvae homozygous for the short-lived Ddc allele. Similar differences in parasitism avoidance were found when larvae with different Ddc genotypes were exposed to natural rates of parasitism by L. boulardi in the field, confirming the ecological relevance of the Ddc polymorphism. Larvae with the long-lived Ddc allele moved shorter distances while foraging and fed slower than larvae homozygous for the short-lived Ddc allele, which supports a known trade off between immune resistance and larval competitive ability. Heterozygous larvae have higher levels of dopamine than larvae homozygous for the short-lived Ddc allele suggesting thatDdc polymorphism has functional consequences for the observed phenotypic variation. I also conducted a multi-year field study (2004-2006) to investigate the interrelationships between Drosophila and its parasitoids in two natural populations in the Baltimore, MD area, which allowed documentation of ecological variables that may select for differences in larval fitness traits influenced by Ddc
