About Dr. Igic's Research

Our lab seeks to advance our understanding of mating system evolution. Mating system changes affect the magnitude and distribution of genetic variation. 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; 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. One of the goals of this line of research is to properly order mating system transitions and ask how contrasting mating systems influence evolutionary fate of lineages: is the transition to selfing indeed a dead-end?

In addition, the lab is entering a number of exciting collaborations aimed at improving the methods for reconstruction of ancestral states, detection of differential diversification rates between different plant lineages, finding co-evolved regions/residues of interacting molecules under positive selection (in the absence of experimental evidence), and identifying genomic regions and genes responsible for reproductive isolation and ecological divergence between species.

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.

Representative Publications

Goldberg, E. E. and B. Igic. 2008. On phylogenetic tests of irreversible evolution. Evolution 62:2727-2741.

Igic B., R. Lande, and J. R. Kohn. 2008. Loss of self-incompatibility and its evolutionary consequences. International Journal of Plant Sciences 169:93-104.

Igic B. and J.R. Kohn. 2006. Bias against obligate outcrossers in studies of the distribution of outcrossing rates in plants. Evolution 60:1098-1103.

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