The Formation and Maintenance of a Hybrid Zone of Aesculus L. (Sapindaceae) in the Southeastern United States

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Title: The Formation and Maintenance of a Hybrid Zone of Aesculus L. (Sapindaceae) in the Southeastern United States
Author: Modliszewski, Jennifer Louise
Advisors: Thomas R. Wentworth, Committee Member
Michael D. Purugganan, Committee Co-Chair
Philip Awadalla, Committee Member
Qiu-Yun (Jenny) Xiang, Committee Co-Chair
Abstract: Hybrid zones have long been touted by plant evolutionary biologists as an unrivaled phenomenon through which the mechanisms of evolution may be studied as they are occurring. Speciation, introgression, and adaptation may all occur as a result of their formation and thus their study may provide new insight into how these processes occur. The purpose of this study is to determine the manner in which a broad hybrid zone of Aesculus was formed, through the use of chloroplast DNA analysis. This hybrid zone encompasses parts of central and northern Georgia and includes hybrids among three species of Aesculus sect. Pavia: Aesculus flava, A. pavia, and A. sylvatica. These species currently have distinct geographic distributions, with both A. pavia and A. flava absent from the hybrid zone. Previous hypotheses have purported that the Aesculus hybrid zone was formed through either 1) secondary contact of previously isolated species or 2) recurrent long-distance pollen dispersal via the ruby-throated hummingbird, Archilochus colubris. It is also possible that the zone may have originated through a combination of both of these forces. This study consists of two parts, the first involving the verification of maternal inheritance of chloroplasts in Aesculus, and the second involving the assessment of the hypothesis of historical secondary contact, based on patterns of cpDNA variation in hybrid and parental populations. Verification of the inheritance of chloroplasts was accomplished through the sequencing of the matK gene from the parents and progeny of 17 crosses among various Aesculus species. The relative contribution of historical secondary contact to the formation of the hybrid zone was accomplished through PCR-RFLP analysis of three loci in the chloroplast genome: matK, trnD-trnT, and trnH-trnK, from 29 natural populations of Aesculus located within, adjacent to, and broadly separate from the hybrid zone. Haplotypes identified from RFLP analysis of the three loci were sequenced and subjected to phylogenetic and population genetic analyses. Results from the sequencing of the matK gene from controlled crosses verified that chloroplasts are inherited maternally in Aesculus, as in most angiosperms. Twenty-one unique haplotypes were identified via analysis of RFLPs, indicating that the chloroplast genome of Aesculus is highly polymorphic. Phylogenetic and population genetic analyses of sequence and restriction site data revealed that cpDNA haplotypes do not correlate with either spatial or taxonomic boundaries. Haplotypes of A. pavia, a species that is presently physically absent from the hybrid zone, were detected in hybrid populations. Additionally, most populations were fixed for a single haplotype that was unique to that population. These results suggest that while secondary contact has played a historical role in the formation of the hybrid zone, current gene flow via seeds is highly restricted among populations. Furthermore, multiple types of cpDNA, originating from the last common ancestor of the group, are maintained in hybridizing species, suggesting that the common ancestor of species of sect. Pavia was polymorphic. Comparison of these results to those of previous analysis of allozyme markers suggests that intermittent long-distance pollen dispersal has helped to maintain the hybrid zone while localized gene flow due to secondary contact of divergent species was responsible for the initial formation of the hybrid zone. Future studies of the Aesculus hybrid zone might focus on the fitness of hybrid individuals as a determining factor in the continued maintenance of the hybrid zone, as well as genotype-by-environment interactions that may be influencing the physical shape and geographic location of the Aesculus hybrid zone. This study could lead to the development of a new hybrid zone model, which would be particularly well-suited for plants capable of long-distance dispersal.
Date: 2006-02-28
Degree: MS
Discipline: Botany

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