The most important rootstock in the California walnut industry is Paradox, which refers to the offspring of a California black walnut pollenized by an English walnut (Juglans regia). Paradox was developed by Luther Burbank, who did not distinguish between Northern (Juglans hindsii) and Southern (J. californica) California black walnut. Although it is generally accepted that Paradox designates hybrids between J. hindsii and J. regia, the name is commonly applied to any black walnut - English walnut hybrid. Moreover, due to gene flow among black walnut species, the genealogy of Paradox hybrids may also include species such as J. major and J. nigra. Since the nuts from which Paradox seedlings are grown are collected from wild trees, their genetic backgrounds are not generally known. In conjunction with a large study aimed at evaluating Paradox hybrids from different industry sources, we have been working on developing molecular markers that can be used to infer the parentage (maternal black walnut species and paternal English walnut cultivar) of individual Paradox seedlings. Chloroplast (trn gene spacers) and nuclear (ITS) DNA sequences have been useful for distinguishing species and PCR-based screens have been developed based on these sequence differences. ISSR markers have been used to distinguish cultivars. Our results indicate that, among industry Paradox sources, there is considerable genetic contribution from species other than J. hindsii. The markers developed in this study also have applications in verifying cultivar identification and genealogy and in investigating the parentage of other hybrids, such as Royal hybrids, designated by Burbank as the hybrid between eastern (J. nigra) and California black walnut.

Key words: cultivar identification, hybrids, ISSRs, Juglans, molecular markers