The lab seeks to advance our understanding of mating system evolution. Motivated by Stebbins' (1957) hypothesis that self-fertilization is an evolutionary dead-end, our work aims to uncover the causes and consequences of mating system transitions. We are also broadly interested in the link between genotype, phenotype, and environment. Until recently, we pursued this interest principally in the context of self-incompatibility systems, where the fitness consequences of new phenotypic variants are well understood.

Self-incompatibility (SI) is defined as the ability of plants to recognize and reject their own pollen. It commonly involves linked pollen- and style-expressed genes, which must co-evolve to maintain the ability to recognize each other. Genes associated with self-recognition systems such as SI in plants, mating type loci in fungi, the sex determination locus in hymenopterans, and the MHC in jawed vertebrates experience selection for rare alleles, and thus harbor extreme allelic polymorphism. This polymorphism is typically manifested as a spectacular number of alleles maintained in natural populations (up to 200; Lawrence 2000), and high molecular divergence among alleles (Ioerger et al. 1990), implying long times to coalescence of allelic polymorphism. Because of these unique characteristics, the study of SI can provide information concerning historical population genetic changes (e.g. bottlenecks; Richman et al. 1996, Igic et al. 2004) and the order of mating system transitions (Igic et al. 2004, 2006, 2007; Igic and Kohn 2006).

We have extended this research program to uncover the genetic architecture of variation in mating systems. We merge quantitative trait locus (QTL) mapping approaches, which elucidate the magnitude and direction of such differences, and field trials to examine the adaptive consequences of alternative mating systems in independent pairs of sister species. The principal question is whether adaptive trajectories use parallel or novel genetic mechanisms in response to parallel selection.

In addition, the lab is entering a number of exciting collaborations with young faculty and postdocs around the world. These are aimed at improving the methods for reconstruction of ancestral states, detection of differential diversification rates, and finding co-evolved regions/residues of interacting molecules under positive selection (in the absence of experimental evidence). Finally, we are beginning an exciting period of speciation research on sister taxa in the nighshade family that are thought to be experiencing selection for reproductive isolation through reinforcement. Recently acquired collaborators include Emma Goldberg, Dan Merl, and Mario Vallejo-Marin.

Literature Cited

Igic, B., L. Bohs, and J.R. Kohn. 2004. Historical inferences from the self-incompatibility locus. New Phytologist 161:97-105

Igic, B., L. Bohs, and J.R. Kohn. 2006. Ancient polymorphism reveals unidirectional breeding system shifts. Proceedings of the National Academy of Sciences 103:1359-1363.

Igic, B. and J.R. Kohn. 2006. Bias in the studies of outcrossing rate distributions. Evolution 60:1098-1103.

Ioerger, T. R., A. G. Clark, and T.-h. Kao. 1990. Polymorphism at the self-incompatibility locus in Solanaceae predates speciation. Proceedings of the National Academy of Sciences 87:9732-9735.

Lawrence, M.J. 2000. Population genetics of homomorphic self-incompatibility polymorphisms in flowering plants. Annals of Botany 85:221-226.

Richman, A. D., M. K. Uyenoyama, and J. R. Kohn. 1996. Allelic diversity and gene genealogy at the self-incompatibility locus in the Solanaceae. Science 273:1212-1216.

Stebbins, G. L., 1957. Self fertilization and population variability in the higher plants. American Naturalist 91:337-354.