Reference genome of California walnut, Juglans californica, and resemblance with other genomes in the order Fagales.

Juglans californica, California walnut, is a vulnerable small tree that is locally abundant but restricted to woodland and chaparral habitats of Southern California threatened by urbanization and land use change. This species is the dominant species in a unique woodland ecosystem in California. It is one of 2 endemic California walnut species (family Juglandaceae). The other species, Northern California black walnut (J. hindsii), has been suggested controversially to be a variety of J. californica. Here, we report a new, chromosome-level assembly of J. californica as part of the California Conservation Genomics Project (CCGP). Consistent with the CCGP common methodology across ~150 genomes, we used Pacific Biosciences HiFi long reads and Omni-C chromatin-proximity sequencing technology to produce a de novo assembled genome. The assembly comprises 137 scaffolds spanning 551,065,703 bp, has a contig N50 of 30 Mb, a scaffold N50 of 37 Mb, and BUSCO complete score of 98.9%. Additionally, the mitochondrial genome has 701,569 bp. In addition, we compare this genome with other existing high-quality Juglans and Quercus genomes, which are in the same order (Fagales) and show relatively high synteny within the Juglans genomes. Future work will utilize the J. californica genome to determine its relationship with the Northern California walnut and assess the extent to which these 2 endemic trees might be at risk from fragmentation and/or climate warming.