Telomere-to-telomere assembly detects genomic diversity in Canadian strains of Borrelia burgdorferi

Borrelia burgdorferi, the bacteria causing Lyme disease, has a complex genome comprising a linear chromosome and a combination of linear and circular plasmids. The atypical hairpin structure at the telomere of linear replicons and the highly paralogous plasmids make the genome assembly challenging. We developed a genome assembly pipeline using Oxford Nanopore Technologies (ONT) long read and Illumina short read to overcome these challenges. Using ONT reads enabled us to completely assemble the hairpin telomeres of the linear replicons along with a novel lp28 subtype plasmid and the complete circular plasmids of nine B. burgdorferi strains from five geographical regions in Canada. Although these strains are highly similar across the conserved genomic regions, variability was observed predominantly at the right telomeric ends. Comparative analyses revealed that all nine strains carry a ∼2-10 kb right telomeric end identical to the linear plasmid lp28-1, which leads to variability in the telomere length and the gene content. Additionally, we observed diversity at the hairpin telomeric sequences of the linear chromosomes. Further analysis showed that the nine strains belong to seven ospC types and have diverse plasmid profile, highlighting the genomic diversity among the strains from the same geographical locations. Overall, these findings suggest that even the B. burgdorferi strains from close geographical locations can carry substantial genomic variation, especially at the telomeres and with respect to their plasmid content, emphasizing it to be a possible mechanism of rapid evolution within these Canadian strains.