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(American Journal of Botany. 2006;93:377-388.)
© 2006 Botanical Society of America, Inc.

Population Biology

Ancestral chloroplast polymorphism and historical secondary contact in a broad hybrid zone of Aesculus (Sapindaceae)1

Jennifer L Modliszewski2, David T Thomas2, Chuanzhu Fan3, Daniel J Crawford4, Claude W dePamphilis5 and Qiu-Yun (Jenny) Xiang2,6

2Department of Botany, North Carolina State University, Raleigh, North Carolina 27695-7612 USA 3Department of Ecology and Evolution, University of Chicago, Chicago, Illinois 60637 USA 4Department of Ecology and Evolutionary Biology and the Natural History Museum and Biodiversity Research Center, University of Kansas, Lawrence, Kansas 66045-2106 USA; 5Department of Biology and Huck Institutes of Life Sciences, Pennsylvania State University, University Park, Pennsylvania 16802 USA

ABSTRACT

Knowledge regarding the origin and maintenance of hybrid zones is critical for understanding the evolutionary outcomes of natural hybridization. To evaluate the contribution of historical contact vs. long-distance gene flow in the formation of a broad hybrid zone in central and northern Georgia that involves Aesculus pavia, A. sylvatica, and A. flava, three cpDNA regions (matK, trnD-trnT, and trnH-trnK) were analyzed. The maternal inheritance of cpDNA in Aesculus was confirmed via sequencing of matK from progeny of controlled crosses. Restriction site analyses identified 21 unique haplotypes among 248 individuals representing 29 populations from parental species and hybrids. Haplotypes were sequenced for all cpDNA regions. Restriction site and sequence data were subjected to phylogeographic and population genetic analyses. Considerable cpDNA variation was detected in the hybrid zone, as well as ancestral cpDNA polymorphism; furthermore, the distribution of haplotypes indicates limited interpopulation gene flow via seeds. The genealogy and structure of genetic variation further support the historical presence of A. pavia in the Piedmont, although they are at present locally extinct. In conjunction with previous allozyme studies, the cpDNA data suggest that the hybrid zone originated through historical local gene flow, yet is maintained by periodic long-distance pollen dispersal.

Key Words: Aesculus • cpDNA inheritance • hybrid zone • phylogeography • Pleistocene • Sapindaceae • secondary contact • southeastern United States






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