Multi-scale genetic analyses of two Trillium species (Trillium reliquum and Trillium cuneatum ) in space and time
Gonzales, Eva Bercikova
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This dissertation was motivated by an interest to integrate spatial-temporal considerationsinto understanding the evolutionary history of two forest herbaceous species, Trillium reliquumand T. cuneatum, and those mechanisms governing their population genetic processes.The investigation of T. reliquum addressed a hypothesis that this endangered species,surviving today in relict, disjunct populations, was previously widespread, and that it becamerare due to European settlements and subsequent habitat fragmentation. Comparisons of thedistribution of genetic diversity among populations of T. reliquum and T. cuneatum, its morecommon. albeit also fragmented, congener, revealed strong genetic structure among populationsof both species. However, the disjunct T. reliquum populations are much more divergent thanthose of T. cuneatum, in spite of their shared recent history, suggesting that rarity in T. reliquumis more ancient, possibly predating the last glacial episode, rather than a consequence of post-European colonization.Examination of hypotheses emerging from biogeographical and fossil records regardingglacial refugia of T. cuneatum in the southeastern US revealed multiple refugia. Surprisingly, theLower Mississippi Valley refugium, considered by paleoecologists as the main refuge fordeciduous forest species, did not participate in postglacial expansion. Rather, scattered refugialpopulations in Alabama, Georgia and the southern Appalachian Mountains contributed to thecurrent geographic distribution. Even more unexpected is the conclusion that T. cuneatum musthave survived at more northern latitudes than the fossil record indicates. Furthermore, this studyidentified the Ridge and Valley as a corridor for species migration in their response to post-glacial climatic changes.Investigation of within population mechanisms revealed slow acting consequences ofanthropogenic activities on genetic diversity. Although we found evidence for highly localizedgene dispersal in all populations, comparisons among continuous mountain populations withfragmented and environmentally stressed Piedmont populations revealed that fragmentationappears to promote pollen-mediated gene movement and to spatially homogenize geneticvariation. I attributed these differences largely to edge effects and concluded that humandisturbance has increased environmental stress in the Piedmont populations, but has caused onlysubtle genetic and demographic shifts to date.