Geographic variation in adaptation at the molecular level: a case study of plant immunity genes

Natural selection imposed by interacting species frequently varies among geographic locations and can lead to local adaptation, where alternative phenotypes are found in different populations. Little is known, however, about whether geographically variable selection acting on traits that mediate species interactions is consistent or strong enough to influence patterns of nucleotide variation at individual loci. To investigate this question, we examined patterns of nucleotide diversity and population structure at 16 plant innate immunity genes, with putative functions in defending plants against pathogens or herbivores, from six populations of teosinte (Zea mays ssp. parviglumis). Specifically, we tested whether patterns of population structure and within-population diversity at immunity genes differed from patterns found at nonimmunity (reference) loci and from neutral expectations derived from coalescent simulations of structured populations. For the majority of genes, we detected no strong evidence of geographically variable selection. However, in the wound-induced serine protease inhibitor (wip1), which inhibits the hydrolysis of dietary proteins in insect herbivores, one population showed unusually high levels of genetic differentiation, very low levels of nucleotide polymorphism, and was fixed for a novel replacement substitution in the active site of the protein. Taken together, these data suggest that wip1 experienced a recent selective sweep in one geographic region; this pattern may reflect local adaptation or an ongoing species-wide sweep. Overall, our results indicate that a signature of local adaptation at the molecular level may be uncommon-particularly for traits that are under complex genetic control.