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Genomics of the Spotted Owl (Strix occidentalis) and Barred Owl (Strix varia) in Western North America


We report here the assembly of a northern spotted owl (Strix occidentalis caurina) genome. We generated Illumina paired-end sequence data at 90X coverage using nine libraries with insert lengths ranging from approximately 250 - 9,600 nucleotides and read lengths from 100-375 nucleotides. The genome assembly is comprised of 8,108 scaffolds totaling 1.26 × 109 nucleotides in length with an N50 length of 3.98 × 106 nucleotides. We calculated the genome- wide fixation index (FST) of S. o. caurina with the closely related barred owl (S. varia) as 0.819. We examined nineteen genes that encode proteins with light-dependent functions in our genome assembly as well as in that of the barn owl (Tyto alba). We present genomic evidence for loss of three of these in S. o. caurina and four in T. alba. We suggest that most light-associated gene functions have been maintained in owls and their loss has not proceeded to the same extent as in other dim-light-adapted vertebrates.

We describe here the successful assembly of the complete mitochondrial genomes of the northern spotted owl (Strix occidentalis caurina) and the barred owl (S. varia). We utilized sequence data from two sequencing methodologies, Illumina paired-end sequence data with insert lengths ranging from approximately 250 nucleotides (nt) to 9,600 nt and read lengths from 100-375 nt and Sanger-derived sequences. We employed multiple assemblers and alignment methods to generate the final assemblies. The circular genomes of S. o. caurina and S. varia are comprised of 19,948 nt and 18,975 nt, respectively. Both code for two rRNAs, twenty-two tRNAs, and thirteen polypeptides. They both have duplicated control region sequences with complex repeat structures. We were not able to assemble the control regions solely using Illumina paired-end sequence data. By fully spanning the control regions, Sanger-derived sequences enabled accurate and complete assembly of these mitochondrial genomes. These are the first complete mitochondrial genome sequences of owls (Aves: Strigiformes) possessing duplicated control regions. We searched the nuclear genome of S. o. caurina for copies of mitochondrial genes and found at least nine separate stretches of nuclear copies of gene sequences originating in the mitochondrial genome (Numts). The Numts ranged from 226-19,522 nt in length and included copies of all mitochondrial genes except tRNAPro, ND6, and tRNAGlu. Strix occidentalis caurina and S. varia exhibited an average of 10.74% (8.68% uncorrected p- distance) divergence across the non-tRNA mitochondrial genes.

We analyzed low-coverage, whole-genome sequence data from fifty-one barred owl (Strix varia) and spotted owl (S. occidentalis) individuals to investigate recent introgression between these two species in western North America. Although we obtained genomic confirmation that these species can hybridize and backcross, we found no evidence of widespread introgression. Plumage characteristics of western S. varia that suggested admixture with S. occidentalis appeared unrelated to S. occidentalis ancestry and may instead reflect local selection.

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