Research interests

I seek to deepen our understanding of speciation and adaptation by gathering critical genetic data from a variety of species in field and laboratory settings, by developing and applying novel statistical analyses to genetic data, and through the development of theoretical models.

The primary goal of my research is to understand the genetic architecture of boundaries between species. Geographic contact between previously isolated lineages can lead to their coalescence through introgression, or to the maintenance of derived characteristics in divergent lineages. Part of my work involves identifying the genetic conditions that favor either outcome.

A secondary and related goal of my research is to contribute to our growing understanding of the genetics of adaptation. My contribution is primarily in the area of statistical genetics. I am interested in the use of natural hybrid zones for mapping genes underlying adaptive phenotypes. I am also involved in a collaborative project to identify genes that contributed to phenotypic shifts in the domestication of sunflowers.

Genetic architecture of species boundaries

Barriers to gene flow allow discrete groups of organisms, such as species, to persist. In many cases reproductive barriers between taxa are incomplete and hybridization occurs in zones of geographic contact. A central focus of my research is to understand how different genetic architectures influence the fate of hybridizing taxa, and includes studies of the origin of species through hybridization.

Examples of research projects include:

1. One study involved a comparison of introgression in multiple hybrid zones in California and Nebraska. The pattern of genomic isolation between two sunflower species exhibited remarkably little variation among hybrid zones (Buerkle and Rieseberg 2001). This project utilized mapped molecular markers covering 15 of 17 linkage groups, and suggests that intrinsic, genetic factors predominate in the reproductive isolation of this pair of hybridizing species.

   Ongoing work includes estimating variability in reproductive isolation between sunflower species in hybrid zones. I have also contributed to methods development for the analysis of hybrid zones (Lexer et al. 2006).

2. I have modeled the ecological and genetic conditions that affect the origin of species through hybridization, and specifically hybrid species that have arisen without an increase in ploidy (Buerkle et al. 2000).

   In a related study, we used genetic maps from three hybrid species of sunflowers and junction theory to infer the rate of genome stabilization that followed diploid hybrid speciation (Buerkle and Rieseberg 2008).

   I have used related simulation models to study the risk of extinction through hybridization (Buerkle et al. 2003).

Genetics of adaptation

The increasing ease with which diagnostic molecular markers can be developed for divergent lineages (strains, ecotypes, species) is changing the way in which the genetic basis of quantitative phenotypic traits can be studied. In particular, molecular markers make possible the study of specific genomic regions contributing to the expression of quantitative traits and allow an analysis of the genetic basis of adaptation and speciation at multiple levels of resolution (e.g., quantitative trait loci, candidate genes). My initial involvement in this area was as a collaborator on a study of the genetics of divergent skeletal and trophic morphologies in two species of three-spine sticklebacks (Peichel et al. 2001).

Examples of research projects include:

1. Common methods for mapping of quantitative traits require a known pedigree. This requirement rules out many organisms that are long-lived or difficult to rear experimentally, and consequently limits the widespread application of these methods. Hybrid zones contain naturally recombinant individuals that may be used for genetic mapping and Loren Rieseberg and I have described some of the challenges associated with this approach (Rieseberg and Buerkle 2002).

   I continue to seek and develop methods for mapping quantitative traits and identifying regions of the genome under selection in natural hybrid zones. In a collaboration with Christian Lexer and other colleagues who work with Populus, we have made some more progess in developing tools for genome regions under divergent selection in hybridizing species (Lexer et al. 2006).

2. A requirement for many studies of hybrids is a method to quantify the genetic composition of hybrids with an index (Buerkle 2005). I have written and continue to develop software for this purpose.

3. In a collaboration with John Burke at the University of Georgia, we are using coalescent theory and modeling to identify genes that were subject to selection in the domestication of sunflowers.

4. Mike Raboin (M.S. student in the lab) is working with me to investigate different statistical and modeling approaches to genome scans of adaptive variation.

Selected publications

Buerkle, C. A., and L. H. Rieseberg. 2008. The rate of genome stabilization in homoploid hybrid species. Evolution 62: 266-275. (article)

Lexer, C., C. A. Buerkle, J. A. Joseph, B. Heinze, and M. F. Fay. 2007. Admixture in European Populus hybrid zones makes feasible the mapping of loci that contribute to reproductive isolation and trait differences. Heredity 98: 74-84. (abstract)

Buerkle, C. A. 2005. Maximum-likelihood estimation of a hybrid index based on molecular markers. Molecular Ecology Notes 5: 684-687. (abstract)

Buerkle, C. A. 2003. Speciation-a rebirth. New Phytologist 160:14-17. (article)

Phillips, A. T. and C. A. Buerkle. 2003. A computational science case study: classification of hybrids using genetic markers and maximum-likelihood estimates. Inroads-SIGCSE Bulletin 35:94-98. (abstract)

Buerkle, C. A., D. E. Wolf, and L. H. Rieseberg. 2003. The origin and extinction of species through hybridization. in Viability in Plants: Conservation, Management, and Modeling of Rare Plants, pp. 117-141. Springer Verlag.

Rieseberg, L. H., and C. A. Buerkle. 2002. Genetic mapping in hybrid zones. American Naturalist 159: S36-S50. (abstract)

Peichel, C. L., K. Nereng, K. A. Ohgi, B. L. E. Cole, P. F. Colosimo, C. A. Buerkle, D. Schluter, and D. M. Kingsley. 2001. The genetic architecture of divergence between threespine stickleback species. Nature 414: 901-905. (abstract)

Buerkle, C. A., and L. H. Rieseberg. 2001. Low intraspecific variation for genomic isolation between hybridizing sunflower species. Evolution 55: 684-691. (abstract)

Gardner, K., C. A. Buerkle, J. Whitton, and L. H. Rieseberg. 2000. Epistasis in wild sunflower hybrid zones. in Wolf, J., E. D. Brodie III, and M. J. Wade, eds., Epistasis and the Evolutionary Process, pp. 264-279. Oxford University Press.

Buerkle, C. A., R. J. Morris, M. A. Asmussen, and L. H. Rieseberg. 2000. The likelihood of homoploid hybrid speciation. Heredity 84: 441-451. (abstract)