POTTER, DANIEL*, FANGYOU GAO, SCOTT BAGGETT, JAMES R. MCKENNA, and GALE H. MCGRANAHAN. Department of Pomology, 1045 Wickson Hall, University of California, One Shields Ave., Davis, CA 95616. - Defining the sources of Paradox: DNA sequence markers for North American walnut (Juglans L.) species and hybrids.
One of the most important rootstocks in the California walnut industry
is Paradox, which refers to the offspring of a California black walnut
pollinized by a Persian walnut (Juglans regia). Paradox was
developed by Luther Burbank, who did not distinguish between northern
(J. 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 - Persian walnut hybrid.
Moreover, due to gene flow among black walnut species, the genealogy
of Paradox hybrids may also include species such as Arizona (J.
major) and Eastern black (J. nigra) walnut. 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 to develop molecular markers
that can be used to infer the parentage of individual Paradox
seedlings. Representatives of the five black walnut species from North
America were screened for variability in the ITS regions of the
nuclear ribosomal DNA and in three noncoding regions from the
chloroplast genome, the trnT-trnL,
trnL-trnF, and trnD-trnT spacers. Unique
sequence markers were identified for each species, and total DNA
extracts from 27 Paradox source trees were tested for those markers.
Chloroplast DNA profiles were used to trace the maternal lineages of
the Paradox source trees, while the ITS data provided evidence as to
whether or not the source trees were themselves hybrids. Our results
indicate that, among industry Paradox sources, there is considerable
genetic contribution from species other than J. hindsii.
Key words: chloroplast DNA, interspecific hybrids, Juglans, nucleotide sequence markers, rootstock, superimposed nucleotide additivity patterns