RESUMEN
Hybridization is a potential tool for incorporating stress tolerance in plants, particularly to pests and diseases, in support of restoration and conservation efforts. Butternut (Juglans cinerea) is a species for which hybridization has only recently begun being explored. This North American hardwood tree is threatened due to Ophiognomonia clavigignenti-juglandacearum (Ocj), the causal fungus of butternut canker disease (BCD), first observed in 1967. Observational evidence in some wild J. cinerea populations indicates that naturalized hybrids of J. cinerea with Japanese walnut (Juglans ailantifolia) may be more tolerant to BCD than non-admixed J. cinerea, but this has not been formally tested in a controlled trial. We aimed to examine potential BCD tolerance within and between J. cinerea and J. cinerea × J. ailantifolia hybrids and to determine if there is a difference in canker growth between BCD fungal isolates. Five-year-old J. cinerea and hybrid trees were inoculated with two Ocj fungal isolates collected from natural infections found in two different sites in Indiana, United States, and a blank control (agar only). Measurements of both artificially induced and naturally occurring cankers were taken at 8-, 12-, 20-, 24-, and 32-month post-inoculation. Differences in canker presence/absence and size were observed by fungal isolate, which could help explain some of the differences in BCD severity seen between J. cinerea populations. Smaller and fewer cankers and greater genetic gains were seen in hybrid families, demonstrating that hybrids warrant further evaluation as a possible breeding tool for developing BCD-resistant J. cinerea trees.
RESUMEN
Climate change may have unpredictable effects on the cold hardiness of woody species planted outside of their range of origin. Extreme undulations in temperatures may exacerbate susceptibility to cold stress, thereby interfering with productivity and ecosystem functioning. Juglans L. and their naturally occurring interspecific F1 hybrids, are distributed natively across many temperate regions, and J. regia has been extensively introduced. Cold hardiness, an environmental and genetic factor yet to be evaluated in many native and introduced Juglans species, may be a limiting factor under future climate change and following species introductions. We evaluated cold hardiness of native North American and Eastern Asian Juglans along with J. regia genotypes using field data from the Midwestern United States (Indiana), controlled freezing tests, and genome sequencing with close assessment of Juglans cold hardy genes. Many Juglans species previously screened for cold-hardiness were genotypes derived from the Midwest, California, and Europe. In 2014, despite general climate adaptation, Midwestern winter temperatures of -30°C killed J. regia originating from California; however, naturalized Midwestern J. regia survived and displayed low damage. Hybridization of J. regia with black walnut (J. nigra) and butternut (J. cinerea) produced F1s displaying greater cold tolerance than pure J. regia. Cold hardiness and growth are variable in Midwestern J. regia compared to native Juglans, East Asian Juglans, and F1 hybrids. Phylogeny analyses revealed that J. cinerea sorted with East Asian species using the nuclear genome but with North American species using the organellar genome. Investigation of selected cold hardy genes revealed that J. regia was distinct from other species and exhibited less genetic diversity than native Juglans species Average whole genome heterozygosity and Tajima's D for cold hardy genes was low within J. regia samples and significantly higher for hybrid as well as J. nigra. We confirmed that molecular and morpho-physiological data were highly correlated and thus can be used effectively to characterize cold hardiness in Juglans species. We conclude that the genetic diversity within local J. regia populations is low and additional germplasm is needed for development of more regionally adapted J. regia varieties.
RESUMEN
Artificial pollination of black walnut (Juglans nigra L.) is not practical and timber breeders have historically utilized only open-pollinated half-sib families. An alternate approach called "breeding without breeding," consists of genotyping open-pollinated progeny using DNA markers to identify paternal parents and then constructing full-sib families. In 2014, we used 12 SSR markers to genotype 884 open-pollinated half-sib progeny harvested from two clonal orchards containing 206 trees, comprised of 52 elite timber selections. Seed was harvested in 2011 from each of two ramets of 23 clones, one upwind and one downwind, based on prevailing wind direction from the west-southwest. One orchard was isolated from wild black walnut and composed of forward selections while the other orchard was adjacent to a natural forest containing mature black walnut composed of backward selections. Isolation significantly increased within-orchard pollination (85%) of the progeny from the isolated orchard compared to 42% from the non-isolated orchard. Neither prevailing wind direction nor seed tree position in the orchard affected paternity patterns or wild pollen contamination. Genetic diversity indices revealed that progeny from both orchards were in Hardy-Weinberg equilibrium with very little inbreeding and no selfing. A significant level of inbreeding was present among the forward selected parents, but not the first generation (backward selected) parents. Some orchard clones failed to sire any progeny while other clones pollinated upwards of 20% of progeny.
