POTTER, DANIEL* AND FANGYOU GAO. Department of Pomology, University of California, Davis, CA 95616. - Black, English, Royal, or Paradox? - developing molecular markers for walnut (Juglans) species, cultivars, and hybrids.
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