Pronounced local adaptation and genetic divergence have been found between closely adjacent pea aphid populations on alfalfa and clover. However, little is known about the balance between selection and gene flow that currently maintains the observed genetic differentiation of aphids on the two hosts. On both hosts, the performance of transplanted individuals is much lower than that of residents, indicating strong disruptive selection. Pea aphids are capable of flying between fields of the different crops and specialist clones can interbreed and produce viable offspring, so migration and gene flow are physically possible. How much gene flow actually takes place? The major questions addressed in this proposal are (1) is the host-associated divergence maintained by selection in the face of free gene flow between crops? and (2) if, as we expect, gene flow is restricted, by what ecological and genetic mechanisms does this occur? First, the extent of inter- host gene flow will be determined using both direct and indirect methods, with measures of gene flow between fields of the same crop as a reference point. Preliminary results suggest that populations on the two crops are neither completely isolated nor panmictic. Various mechanisms that could act to limit gene flow between these divergent populations will be then evaluated, including habitat choice by alate (winged) forms, selection against migrants, phenological separations between mating population on the two crops, and reduced fitness of hybrids due to developmental failure and/or lack of adaptation to the parental environments. The results of these experiments will provide a detailed view of the mechanisms by which closely adjacent populations in different habitats remain genetically differentiated despite the potential to interbreed. This study represents a unique opportunity to study mechanisms of local adaptation and population divergence. These experiments will also provide a case study of gene flow restriction that has implications for the study of speciation, and the process by which insect pests adapt to specific crop plants.
