| Research Interests
We combine classical evolutionary and genetic approaches with state-of-the-art massively parallel sequencing to answer questions about the evolution of traits and organisms. Presently, our lab projects are focused in two areas. First, we are trying to advance our understanding of the causes and consequences of immense sexual variation in plants, where some species and lineages are hermaphrodites or have various modes of fluid gender di- or tri-morphism, some of the former self-fertilize while others are obligate outcrossers, and nearly all are somewhat flexible. We use powerful genomic tools to further characterize genes involved in determining sexual systems, particularly at self-incompatibility loci, which are close analogs of human immune defenses (MHC-HLA loci). Second, we are attempting to quantify the effects of ecological specialization on the propensity of lineages to diversify or become extinct. This project is focused on groups of flowering plants as well as passerine birds.
Breeding and Mating System Evolution in Flowering Plants
Motivated by Stebbins' (1957) hypothesis that self-fertilization is an evolutionary dead-end, and the growing opposition to this line of thinking, 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; Igic and Busch 2013).
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.
Ecological Specialization and Generalization
A recent set of projects from the lab aims to uncover patterns of evolution of ecological specialization, as well as their causes and consequences. This work departs from our standard fare in that it is partly focused on bird lineages.
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).
Literature Cited (see Publications for in-house citations)
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.