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 <i>Aesculus</i> 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 <i>Aesculus</i> sect. <i>Pavia</i>: <i>Aesculus flava</i>, <i>A. pavia</i>, and <i>A. sylvatica</i>. These species currently have distinct geographic distributions, with both <i>A. pavia</i> and <i>A. flava</i> absent from the hybrid zone. Previous hypotheses have purported that the <i>Aesculus</i> 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, <i>Archilochus colubris</i>. 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 <i>Aesculus</i>, 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 <i>matK</i> gene from the parents and progeny of 17 crosses among various <i>Aesculus</i> 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: <i>matK</i>, <i>trnD</i>-<i>trnT</i>, and <i>trnH</i>-<i>trnK</i>, from 29 natural populations of <i>Aesculus</i> 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 <i>matK</i> gene from controlled crosses verified that chloroplasts are inherited maternally in <i>Aesculus</i>, as in most angiosperms. Twenty-one unique haplotypes were identified via analysis of RFLPs, indicating that the chloroplast genome of <i>Aesculus</i> 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 <i>A. pavia</i>, 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. <i>Pavia</i> 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 <i>Aesculus</i> 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 <i>Aesculus</i> 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.
| Identifer | oai:union.ndltd.org:NCSU/oai:NCSU:etd-10172005-221504 |
| Date | 30 November 2005 |
| Creators | Modliszewski, Jennifer Louise |
| Contributors | Thomas R. Wentworth, Michael D. Purugganan, Philip Awadalla, Qiu-Yun (Jenny) Xiang |
| Publisher | NCSU |
| Source Sets | North Carolina State University |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://www.lib.ncsu.edu/theses/available/etd-10172005-221504/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to NC State University or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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