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Biol Lett. Jun 23, 2009; 5(3): 324–327.
Published online Mar 18, 2009. doi:  10.1098/rsbl.2009.0031
PMCID: PMC2679929
Geographically extensive hybridization between the forest trees American butternut and Japanese walnut
Sean M. Hoban,1 Tim S. McCleary,1 Scott E. Schlarbaum,2 and Jeanne Romero-Severson1*
1Department of Biological Sciences, University of Notre Dame, 327 Galvin Life Science Building, Notre Dame, IN 46556, USA
2Department of Forestry, Wildlife and Fisheries, University of Tennessee, Knoxville, TN 37996, USA
*Author for correspondence (jromeros/at/
Received January 16, 2009; Revised February 20, 2009; Accepted February 24, 2009.
We investigate the question of naturally occurring interspecific hybrids between two forest trees: the native North American butternut (Juglans cinerea L.) and the introduced Japanese walnut (Juglans ailantifolia Carrière). Using nuclear and chloroplast DNA markers, we provide evidence for 29 F1 and 22 advanced generation hybrids in seven locations across the eastern and southern range of the native species. Two locations show extensive admixture (95% J. ailantifolia and hybrids) while other locations show limited admixture. Hybridization appears to be asymmetrical with 90.9 per cent of hybrids having J. ailantifolia as the maternal parent. This is, to our knowledge, the first genetic data supporting natural hybridization between these species. The long-term outcome of introgression could include loss of native diversity, but could also include transfer of useful traits from the introduced species.
Keywords: conservation, interspecific hybridization, introgression, Juglans cinerea
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