Genetic Formation of Paradox Hybrids (Juglans L.) Revealed by nrDNA IGS8-ETS1 Region

Genus Juglans (walnuts) is one of the most economically important tree crops for healthy food and wood production. Paradox is a famous rootstock in the walnut industry for a number of important features, including fast growth and resistance to some diseases. Paradox commonly refers to black walnut-Persian walnut hybrid. Nuclear rDNA IGS region, typically bi-parentally inherited, with rapid evolution and broad existence in all eukaryotic genomes, was found to be of importance in revealing genetic background of the walnut hybrids. Both parental genetic components (around 87.50% to 88.89%) and novel genetic components (around 11.11% to 12.50%) were detected in the nrDNA IGS8-ETS1 region of the Paradox genome. The inheritance was commonly one-parent-dominated in each hybridization event. Our results indicated that genetic formation of Paradox hybrids involved in J. regia (sect. Juglans) and the following 6 black walnut species (sect. Rhysocaryon), i.e., J. hindsii, J. californica, J. major, J. nigra, J. microcarpa and J. hirsuta. The nrDNA IGS8-ETS1 region is helpful in understanding the genetic basis of hybrids.