Clarifying the Conservation Status of Northern California Black Walnut (Juglans hindsii) Using Microsatellite Markers

Abstract

The conservation status of the northern California black walnut (Juglans hindsii (Jeps.) Jeps. ex R. E. Sm.) has been a source of considerable confusion and controversy. Although not currently legally protected by either Federal or State Endangered Species Acts, this species is given conservation status by the California Native Plant Society and the California Department of Fish and Wildlife, and some California counties require mitigation for removal of individuals of this species, especially older trees. Despite the current widespread distribution in northern California and southern Oregon of trees that match J. hindsii morphologically, there are only three or four sites where the species is known to have occurred prior to extensive settlement of California by Europeans in the mid-19th century. This has led to the suspicion that trees found in other places may not be genetically pure J. hindsii but may instead be descendants of lineages that experienced past gene flow from one or more other species. In addition, despite its more distant relationship, the cultivated walnut (J. regia L.) readily hybridizes (as the male parent) with J. hindsii, producing morphologically identifiable ‘Paradox' hybrids, which occur spontaneously and are widely planted as rootstocks and street trees. Finally, recent collections of J. hindsii from southern California have raised questions about the respective geographic distributions of J. hindsii and southern California black walnut (J. californica S. Watson). We analyzed genotypes at 10 microsatellite loci for 158 mostly wild J. hindsii trees, as well as some orchard waifs, from 10 counties in northern and southern California and one county in southern Oregon, including representatives of putative original native populations. We also sampled several Paradox hybrids, 10-20 standards for each of the five North American black walnut species, and six standards for J. regia. Bayesian cluster analyses with the program STRUCTURE revealed that at least 71.5% of the putatively wild J. hindsii represent genetically pure members of that species, while the remaining trees show evidence of past hybridizations with one or more of the other North American black walnut species. We found no evidence of introgression of J. regia into J. hindsii. The results suggest that individual J. hindsii trees should not have conservation status.