Walnuts (Juglans spp.) are economically important nut and timber species with a worldwide distribution. Using the published Persian walnut genome as a reference for the assembly of short reads from six Juglans species and several interspecific hybrids, we identified simple sequence repeats in 12 Juglans nuclear and organellar genomes. The genome-wide distribution and polymorphisms of nuclear and organellar microsatellites (SSRs) for most Juglans genomes have not been previously studied. We compared the frequency of nuclear SSR motifs and their lengths across Juglans, and identified section-specific chloroplast SSR motifs. Primer pairs were designed for more than 60,000 SSR-containing sequences based on alignment against assembled scaffold sequences. Of the >60,000 loci, 39,000 were validated by e-PCR using unique primer pairs. We identified primers containing 100% sequence identity in multiple species. Across species, sequence identity in the SSR-flanking regions was generally low. Although SSRs are common and highly dispersed in the genome, their flanking sequences are conserved at about 90 to 95% identity within Juglans and within species. In a few rare cases, flanking sequences are identical across species of Juglans. This comprehensive report of nuclear and organellar SSRs in Juglans and the generation of validated SSR primers will be a useful resource for future genetic analyses, walnut breeding programs, high-level taxonomic evaluations, and genomic studies in Juglandaceae.

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.

Pistachio is one of the most economically important nut crops worldwide. However, there are no reports describing the chloroplast genome of this important fruit tree. In this investigation, we assembled and characterized the complete pistachio chloroplast sequence. The Pistacia vera chloroplast genome was 160,598 bp in size, similar to other members of Anacardiaceae (149,011–172,199 bp) and exhibited the typical four section structure, including a large single copy region (88,174 bp), a small single copy region (19,330 bp), and a pair of inverted repeats regions (26,547 bp). The genome contains 121 genes comprised of 87 protein-coding genes, 30 tRNA genes and 4 rRNA genes. Thirteen intron-containing genes were identified in the genome wherein two genes had more than two introns. The genomic patterns of GC content resembled those for other Anacardiaceae. P. vera displayed the highest number of simple sequence repeats (SSRs) among the genera studied, which may be useful for molecular marker development and future population studies. Amino acid analysis revealed that Leucine is the most frequent (10.69%) amino acid in the chloroplast genome followed by Isoleucine (8.53%) and Serine (7.77%). Cysteine (1.30%) and Tryptophan (1.74%) were the least frequent amino acids. Phylogenetic analysis revealed P. vera is most like its taxonomically close relative P. weinmaniifolia, followed by Rhus chinensis; all placed taxonomically in the tribe Rhoeae. Members of Anacardiaceae were most closely related to Rhoeae, followed by members of Spondieae. The reports of this chloroplast genome will be useful for future conservation studies, genetic evaluation and breeding of P. vera, and more comprehensive phylogenetic analysis of the Pistacia species and its closely-related genera.

Chestnut blight, a disease that has spread rampantly among American (Castanea dentata (Marsh.) Borkh.) and European chestnut (C. sativa Mill.) trees, results from infection by the fungal pathogen Cryphonectria parasitica (Murrill) M.E. Barr (C. parasitica). This fungus was introduced in the early 1900s and has almost functionally eliminated chestnut trees from the North American landscape. In 2017, we collected chestnut blight samples from two sites (Site B, (Fulton Co., IN) and Site C (Marshall Co., IN)). At the Fulton County planting, Site B, cankers had formed, healed over, and the trees were healthy. However, at the second site in Marshall County, (Site C), cankers continued to propagate until all of the chestnut trees had died back to the ground. Research evidence worldwide has indicated that these visual clues likely result from the presence of a hypovirus. Upon closer inspection and the subsequent isolation and reproduction of spores, no hypovirus has been identified from either site. Here, we present a curious coincidence where one site has completely succumbed to the disease, while the other has been able to spring back to health.