Distinguished Professor

Ph.D. Washington State University, 1987

Program Affiliation: Evolution, Ecology and Behavior | Plant Biology

Research Groups Affiliation: Evo-Devo | Evolution | Genetics | Genomics & Bioinformatics | Plant Biology

Office: (812) 855-7614
Lab: (812) 855-9018
Fax: (812) 855-6705
Email Loren

Rieseberg Lab website

The Rieseberg lab integrates high-throughput genomic methods, bioinformatics, ecological experiments, and evolutionary theory to study the origin and evolution of species, domesticated plants, and weeds. Some of the problems we are currently working on are described below:

Speciation -- Our primary research interest concerns how new plant species arise (Science 317:910-914) – one of the most fundamental questions in biology.  Much of this work focuses on members of the sunflower genus Helianthus, but we also analyze patterns of variation in other plant and animal groups to make more general conclusions about speciation.  Problems that have attracted our attention recently include the nature of species (Nature 440:524-527), the importance of advantageous alleles in holding species together, the evolutionary forces underlying phenotypic diversification (PNAS 99:12242-12245), the frequency of polyploid speciation, the role of hybridization in evolution (ARES 28:359-389), and the contribution of chromosomal rearrangements to speciation (TREE 16:351-358).

In Helianthus, our goals are to identify and order the genetic changes responsible for the origin of species in this group and to understand how the new species survive, evolve, and interact after they are formed. Specific phenomena that appear to be critical for speciation in this group include hybridization, ecological divergence, and chromosomal rearrangements.

Hybridization has played a major role in the evolution of wild sunflowers, contributing both to adaptation within species and to the origin of entirely new species. We have employed phylogenetic reconstruction, quantitative trait locus (QTL) analyses, field experiments, computer simulations, and comparative genomic approaches to identify hybrid taxa, determine how they have become reproductively isolated (Science 272:741-745), estimate their ages (PNAS 95:11757-11762), and assess the contribution of hybrid gene combinations to adaptation (Science 301:1211-1216).  Current work focuses on the identification of genetic mutations underlying ecological and phenotypic differences in the hybrid taxa and to estimate the scale of recombination in the hybrid species’ genomes.

Speciation in Helianthus appears to have been driven by habitat differentiation. Even hybrid species are strongly divergent ecologically from their parental species. We are employing a combination of ecophysiological studies, QTL approaches, field experiments, candidate gene analyses, and hitchhiking mapping to identify the traits and genes responsible for habitat divergence in this group (Molecular Ecology 12:1225-1235; New Phytologist 161:225-233). We are particularly interested in the molecular and ecological genetic basis of salt tolerance and flowering time.

Chromosomal sterility barriers partially isolate most wild sunflower species. We are using comparative genomic approaches to map the rearrangements (Nature 375:313-316 and to assess their affects on hybrid fitness, interspecific gene flow, and the accumulation of species differences (Genetics 152:713-727; Genetics 171:291-303).

Domestication --

The dramatic, human-mediated transformations associated with plant domestication provide a model for studying phenotypic evolution. My lab has exploited this situation by studying the domestication of sunflower. Our genetic studies showed that sunflower was easily domesticated; there were few major QTLs and many wild QTL alleles had effects in the direction of the cultivar (Genetics 161:1257-1267). More recently, we effectively settled a high-profile controversy by establishing that sunflower was domesticated in the eastern United States and not in southern Mexico. This finding is of some general importance because it further establishes the eastern U.S. as one of five regions in the world in which agriculture arose independently (Nature 430:201-205).

Current work includes a large-scale search for associations between sequence variation at candidate genes and domestication traits in sunflowers. Another project focuses on the domestication and improvement of Noug, an oilseed crop indigenous to Ethiopia.

Invasiveness – Invasive plants represent a major threat to the economy and environment, with annual economic costs to North America of $35-40 billion. We are using a combination of common garden, hitchhiking mapping, microarray analyses, and association studies to identify specific genetic changes associated with invasiveness and to determine whether any of these changes evolve repeatedly in different groups of plants. We are targeting Compositae weeds: including spotted knapweed, starthistle, Canada thistle, ragweed, and common sunflower.  As part of the Compositae Genome Project (see below), we have developed gene catalogs for several of these weeds and have conducted genome scans and expression profiling for invasiveness in one of them, the common sunflower.  Also, a large-scale genome scan of ancestral and invasive starthistle populations is underway.

Compositae genomics –In collaboration with labs at UC Davis, U. Georgia, U. Massachusetts, and Cal Poly Pomona, we are developing genomic resources and tools for the Compositae, the largest and most ecologically diverse family of flowering plants.  These resources include ~700,000 ESTs for crops and weeds in the family, with another 150,000 underway (http://cgpdb.ucdavis.edu/).  We also have developed detailed genetic linkage maps, QTL populations, functional maps, and cDNA and Affymetrix microarrays for sunflower.  These resources underlie most of the other projects described here.

Conservation - Our work in the area of conservation genetics has focused on the effects of hybridization between rare and common species and between crops and their wild relatives. It is easy to see how hybridization could have a negative impact on rare species. If hybrids are sterile or inviable, then rare populations may have reduced fitness because gametes are wasted in the formation of unfit hybrid individuals. On the other hand, if hybrids are fertile and vigorous, hybridization can lead to the genetic assimilation of the rare taxon by a numerically larger one. My lab group has documented several examples where rare species are in danger of genetic swamping, such as North America's rarest tree, the Catalina Island Mahogany. We also have generated computer simulation models to assess the kinds of ecological and genetic conditions under which extinction through hybridization is most likely (Conservation Biology 15:1039-1053.

The impact of hybridization between crops and their wild relatives is less clear, but this issue has taken on a very high profile because of concerns about the escape of genetically engineered genes (transgenes) into the natural environment. We have documented very high levels of gene flow between cultivated and wild sunflower, indicating that transgene escape is likely. We also have collaborated with several other groups to assess the fitness consequences of individual transgenes in a wild type background. The spread of a transgene will depend almost entirely on its fitness effects. Transgenes that are negatively selected will not spread beyond the crop margin, whereas highly advantageous transgenes will rapidly sweep across a population system. Along with collaborators, we have shown that a Bt transgene that kills some species of insects is highly advantageous and likely to escape (Ecological Applications 13:279-286), but that a transgene affecting a fungal pathogen is unlikely to do so (Science 300:1250).  Our recent work tests the strength of selection against domestication QTLs and asks whether linkage with transgenes could limit the spread of the latter.

Rieseberg, L.H., and J.H. Willis. Plant speciation. 2007. Science 317:910-914.

Rieseberg, L.H., T.E. Wood, and E. Baack.  2006.  The nature of plant species. Nature 440:524-527.

Harter, A.V., K.A. Gardner, D. Falush, D.L. Lentz, R. Bye, L.H. Rieseberg.  2004.  Origin of extant domesticated sunflowers in eastern North America. Nature 430:201-205.

Burke, J.M., and L.H. Rieseberg. 2003. The fitness effects of transgenic disease resistance in wild sunflowers. Science 300:1250.

Rieseberg, L.H., O. Raymond, D.M. Rosenthal, Z. Lai, K. Livingstone, T. Nakazato, J.L. Durphy, A.E. Schwarzbach, L.A. Donovan, and C. Lexer. 2003. Major ecological transitions in annual sunflowers facilitated by hybridization. Science 301:1211-1216.

Rieseberg, L. H., A. Widmer, M. A. Arntz, and J. M. Burke. 2002. Directional selection is the primary cause of phenotypic diversification. Proceedings of the National Academy of Sciences USA 99:12242-12245.