Geographically extensive hybridization between the forest trees American butternut and Japanese walnut

Abstract

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 Carriè re). Using nuclear and chloroplast DNA markers, we provide evidence for 29 F 1 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.

DNA markers identify hybrids between butternut (Juglans cinerea L.) and Japanese walnut (Juglans ailantifolia Carr.)

“…Cytoplasmic markers alone cannot identify hybridization events where butternut was the female parent. As hybrids become more common in the landscape relative to butternut, this type of hybridization is more likely (Hoban et al 2009). Nuclear markers can also make morphological identification more secure and genetic studies more robust.…”

“…Bixby reasoned that these non-heartnut-producing offspring were in fact (buart) hybrids. Buartnuts were subsequently propagated and cultivated as orchard or farmyard nut trees because of their vigor and fertility; most were likely the offspring of planted J. ailantifolia or their hybrid progeny, so they contained J. ailantifolia chloroplasts (Hoban et al 2009). Because there were many more J. cinerea trees than J. ailantifolia over the past 150 years, pollination of a J. cinerea by J. ailantifolia has been relatively rare, and when it occurred, the hybrid offspring typically had to compete in a forest environment and were less likely than hybrids (growing in yards and orchards) to reproduce.…”

“…Japanese walnut and butternut are extremely similar vegetatively, but the nut of most hybrids resembles the (distinctively shaped) butternut, which is why hybrids are commonly confused with butternuts Woeste and Pijut 2009). Biologists have expressed concern over the possibility of genetic invasion of butternut by Japanese walnut because, unlike most Juglans hybrids, buartnuts are highly fruitful and vigorous (Ostry and Woeste 2004;Hoban et al 2009). Because buartnut hybrids apparently are able to cross with other buartnut hybrids, both parental species, and may even self-pollinate, their progeny can present confusing combinations of traits (Ross-Davis and Woeste 2008).…”

“…Because buartnut hybrids apparently are able to cross with other buartnut hybrids, both parental species, and may even self-pollinate, their progeny can present confusing combinations of traits (Ross-Davis and Woeste 2008). Hoban et al (2009) identified hybrids using a probabilistic approach based on differences in microsatellite allele frequencies in butternut versus Japanese walnut, but additional tools are needed in cases where greater taxonomic certainty is required and for breeding (Michler et al 2005).…”

Characterisation and inheritance of nuclear microsatellite loci for use in population studies of the allotetraploid Salix alba–Salix fragilis complex

“…However, NewHybrids has recently also been used to successfully study natural hybridisation, using similar numbers of microsatellite loci, in black poplar, Populus nigra L. and Populus x canadensis Moench. , Mediterranean evergreen oaks, Quercus suber and Quercus ilex (Burgarella et al 2009) and North American butternut and Japanese walnut, Juglans cinerea L. and Juglans ailantifolia Carrière (Hoban et al 2009). The markers described here may also be useful for studying clonality within the S. alba-S. fragilis complex.…”