De Novo Genome and Transcriptome Assembly of the Canadian Beaver (Castor canadensis)

Si Lok, Tara A Paton, Zhuozhi Wang, Gaganjot Kaur, Susan Walker, Ryan K C Yuen, Wilson W L Sung, Joseph Whitney, Janet A Buchanan, Brett Trost, Naina Singh, Beverly Apresto, Nan Chen, Matthew Coole, Travis J Dawson, Karen Ho, Zhizhou Hu, Sanjeev Pullenayegum, Kozue Samler, Arun Shipstone, Fiona Tsoi, Ting Wang, Sergio L Pereira, Pirooz Rostami, Carol Ann Ryan, Amy Hin Yan Tong, Karen Ng, Yogi Sundaravadanam, Jared T Simpson, Burton K Lim, Mark D Engstrom, Christopher J Dutton, Kevin C R Kerr, Maria Franke, William Rapley, Richard F Wintle, Stephen W Scherer

The Canadian beaver (Castor canadensis) is the largest indigenous rodent in North America. We report a draft annotated assembly of the beaver genome, the first for a large rodent and the first mammalian genome assembled directly from uncorrected and moderate coverage (< 30 ×) long-reads generated by single-molecule sequencing. The genome size is 2.7 Gb estimated by k-mer analysis. We assembled the beaver genome using the new Canu assembler optimized for noisy reads. The resulting assembly was refined using Pilon supported by shortreads (80 ×) and checked for accuracy by congruency against an independent short-read assembly. We scaffolded the assembly using the exon-gene models derived from 9805 full-length open reading frames (FL-ORFs) constructed from the beaver leukocyte and muscle transcriptomes. The final assembly comprised 22,515 contigs with an N50 of 278,680 bp and an N50-scaffold of 317,558 bp. Maximum contig and scaffold lengths were 3.3 and 4.2 Mb, respectively, with a combined scaffold length representing 92% of the estimated genome size. The completeness and accuracy of the scaffold assembly was demonstrated by the complete and precise exon placement for 91.1% of the 9,805 assembled FL-ORFs and 83.1% of the BUSCO (Benchmarking Universal Single-Copy Orthologs) gene set used to assess the quality of genome assemblies. Well-represented were genes involved in dentition and enamel deposition, defining characteristics of rodents with which the beaver is well-endowed. The study provides insights for genome assembly and an important genomics resource for Castoridae and rodent evolutionary biology.