Phenotypic plasticity, the expression of alternate physiological states, morphological forms and/or distinct behaviors in response to specific environments, is a universal phenomenon in animals. Although the evolution of plastic responses is well understood theoretically, little is known about the genetic and molecular mechanisms of this response and the importance of plasticity in disease. The pea aphid Acyrthosiphon pisum, an emerging model system, exhibits an adaptive plasticity in which environmental modifications to meiosis result in parthenogenetic clonal reproduction. How aphids have evolved plasticity in this fundamental process that promotes recombination and reassortment of alleles is unknown. The goal of the proposed research is to understand meiotic plasticity at the molecular level. I propose to examine the discrete molecular differences that produce sexual and asexual meiosis in aphids using cell biological and genomic approaches. Candidate genes and pathways identified by these experiments will be functionally tested using reverse genetics and small molecule inhibitors. These experiments represent the first step toward a molecular characterization of a dramatic example of adaptive plasticity in an ancient, highly conserved process.