Advisor: Dr. Jenny Xiang

Hybridization between plant species plays an important role in plant evolutionary via creation of new species and novel hybrids adaptive to new environments. Hybrid zones offer a means of studying hybridization and evolution in a naturally occurring system where many generations of hybrid individuals exist, providing ideal systems for studying evolution in progress. In particularly, information on genetic structure and patterns of gene flow in hybrid zones is essential to understanding the formation of a hybrid zone and to predicting the potential evolutionary consequences of hybridization. The present study is to examine the patterns of gene flow in a broad hybrid zone in the genus Aesculus (the buckeyes), which are widely cultivated plants for their ornamental values. The hybrid zone of Aesculus is located in northern Georgia and adjacent areas. Hybridization in the zone involves three of the four species of Sect. Pavia (A. flava, A. pavia, and A. sylvatica) that occur in the SE United States. Previous analyses of morphology and allozymes indicated an asymmetrical structure of the zone in distribution of hybrids and gene introgression (Hardin 1957, dePamphilis and Wyatt 1990). These studies found that hybrids between A. pavia and A. sylvatica occur only within the range of A. sylvatica (in the piedmont), but outside the range of A. pavia (in the coastal areas), that hybrids between A. flava and A. sylvatica occur outside of the range of A. flava (Appalachian Mountains and north), but within the range of A. sylvatica. The presence of genes of A. pavia in the hybrid zone was proposed to occur via long distance pollen dispersal from the coastal areas into the piedmont by hummingbirds, whereas the occurrence of genes of A. flava in the hybrid zone was considered to be a result of long distance seed dispersal along streams flowing down from the mountains (dePamphilis and Wyatt, 1989,1990). The present study will seek evidence from both chloroplast DNA and nuclear DNA markers to test these hypotheses. The maternally inherited chloroplast DNA (confirmed via our genetic analysis of controlled crosses) will provide evidence of seed dispersal (or gene flow via seed dispersal) whereas nuclear DNA markers will document total gene flow by both pollen and seeds. Specifically, highly variable chloroplast intergenic spacer regions and microsatellite and AFLP from nuclear genome will be surveyed in the study to document gene flow in a detailed manner. This work will contribute to a better understanding of evolutionary processes in natural hybridization and their genetic and evolutionary consequences in plants.

Objectives of Research

1. Conduct phylogeographic study determining relative contribution of historical demographic factors such as population range expansions and contraction and secondary contact versus current gene flow on the formation and mainetance of a broad hybrid zone in Aesculus. (IN PROCESS – ALMOST COMPLETED)

2. Examine rates and pattern of evolution in two candidate genes for flowering time (Frigida and Flowering Locus C – this is open to discussion) in natural populations of two closely related species of Aesculus, A. pavia and A. parviflora, and to determine if selection acting on these loci contributed to the origin or reinforcement of a reproductive barrier between the species.

3. Characterize the promoter regions of these two loci in terms of: number and type of binding sites, organization, and sequence variation both within and between binding sites, and in terms of similarities between the two loci, as they are coordinately regulated.

4. Determine functional consequences of variation in promoter regions through analysis of gene expression profiles

5. Reconstruct the phylogeny of the genus Aesculus based on data from candidate genes, and to re-test the biogeographic hypothesis of an Asian origin of the genus, as the origin has recently been disputed.

References:

dePamphilis, C.W., and R. Wyatt. 1989. Hybridization and Introgression in Buckeyes (Aesculus: Hippocastanaceae): A review of the evidence and a hypothesis to explain long distance gene flow. Systematic Botany. 14:593-611.

Hardin, J.W. 1957. Studies in the Hippocastanaceae, III: A hybrid swarm in the buckeyes. Rhodora. 59: 45-51.