Our data indicate that introgression occurs across a large portion of the native species range. Although hybridization was highest in fragmented semi-rural landscapes (sites CT and NC, USA), we also found small numbers of probable hybrids in four other locations, including three national forests. Extensive introgression could alter the gene pool and evolutionary potential of the native species, with potentially cascading ecosystem consequences. Our data suggest that caution should be taken in choosing individuals for restoration efforts.
Most (90.9%) of the probable hybrids contained the J. ailantifolia
chloroplast. As the chloroplast is maternally inherited in Juglans
(Zhang et al. 2003
), this result indicates that most maternal parents of hybrids are J. ailantifolia.
One possible explanation is the higher number of J. cinerea
and thus higher amount of J. cinerea
pollen in the pollen pool, so most paternal parents will be J. cinerea,
similar to observations in Populus
(Lexer et al. 2005
). Alternatively, the explanation may be partial one-way intrinsic incompatibility, which is common in angiosperms (Tiffin et al. 2001
Interestingly, all eight probable backcross individuals are to J. cinerea. This also may be due to high representation of J. cinerea pollen in the pollen pool. While eight is an exceedingly small sample size, we can speculate that a long-term consequence of repeated backcrossing to J. cinerea could be chloroplast capture, in which the J. ailantifolia chloroplast is retained mostly in a J. cinerea nuclear background. However, interaction between hybrids and butternut canker may preserve large blocks of the J. ailantifolia genome, if J. ailantifolia tolerance has a genetic basis. The extent of J. ailantifolia introgression over time may depend on the strength of selection against J. cinerea and asymmetry in survival of backcross individuals.
We note that our Bayesian probabilities are based on a small number of moderately to highly differentiated markers, so assignments are rough estimates. The four unclassifiable individuals and the 13 individuals with less than or equal to 0.80 probability in a single hybrid class reflect the limits of our ability to distinguish genotypic classes with the current marker sets. Genotyping with additional well-differentiated markers will improve the resolution of this analysis and the number of F1 versus advanced generation hybrids may change. Nevertheless, the presence of the J. ailantifolia chloroplast and many highly probable hybrids in multiple populations allows us to strongly suggest that hybrids persist and interbreed with the native species in natural settings, in contrast to expectations for disjunct species.