A pseudomolecule assembly of the Rocky Mountain elk genome reveals putative immune system gene loss near chromosomal fissions

Abstract

Rocky Mountain elk (Cervus canadensis) is a major reservoir for Brucella abortus in the Greater Yellowstone area, which has significant economic implications to the cattle industry. Vaccination attempts against intracellular bacterial diseases in elk populations have not been successful due to a negligible adaptive cellular immune response. A lack of genomic resources has impeded attempts to better understand why vaccination does not induce protective immunity. To overcome this limitation, PacBio, Illumina, and HiC sequencing with a total of 686-fold coverage was used to assemble the elk genome into 35 pseudomolecules. A robust gene annotation was generated resulting in 18,013 gene models and 33,422 mRNAs. The accuracy of the assembly was assessed using synteny to the red deer and cattle genomes identifying several chromosomal rearrangements, fusions and fissions. Because this genome assembly and annotation provide a foundation for genome-enabled exploration of Cervus species, we demonstrate its utility by exploring the conservation of immune system-related genes. We conclude by comparing cattle immune system-related genes to the elk genome, revealing nine putative gene losses in elk. Author SummaryBrucellosis, also known as contagious abortion, is a bacterial disease that commonly affects livestock and remains prevalent in Rocky Mountain elk (Cervus canadensis). Since the 1920s the USDA has led a program to eradicate Brucellosis from cattle, yet wild Rocky Mountain elk continue to be a source of transmission. Attempts to vaccinate wild elk herds have been unsuccessful, due to a poor and short-lived immune response. To investigate the genetic basis for this inherent difference, we created the first genome and annotation for the Rocky Mountain elk. This genome assembly is of the highest quality and contains single linear sequences for all 35 chromosomes. In order to generate gene models, an array of RNA-Seq data and proteins from many different organ tissues and cells were used in gene prediction software. Specifically, we compare cattle immune system genes with the Rocky Mountain elk, revealing the putative loss of nine immune-system related genes in elk.