The repetitive genome of the Ixodes ricinus tick reveals transposable elements have driven genome evolution in ticks

Background: Ticks are obligate blood-feeding parasites associated with a huge diversity of diseases globally. The hard tick Ixodes ricinus is the key vector of Lyme borreliosis and tick-borne encephalitis in Western Eurasia. Ixodes ticks have large and repetitive genomes that are not yet well characterized. Results: Here we generate two high-quality I. ricinus genome assemblies, with haploid genome assembly sizes of approximately 2.15 Gbp. We find transposable elements comprise at least 69% of the two I. ricinus genome assemblies, amongst the highest proportions found in animals. The transposable elements in ticks are highly diverse and novel, so we constructed a repeat library for ticks using our I. ricinus genome assemblies and the high-quality genome assembly of I. scapularis, another major tick vector of Lyme borreliosis. To understand the impact of transposable elements on tick genomes we compared their accumulation in the two Ixodes sister species. We find transposable elements in these two species to have distinctive post-speciation patterns, suggesting transposable elements are drivers of genome evolution in ticks. Conclusions: The I. ricinus genome assemblies and our tick repeat library will be valuable resources for biological insights into these important ectoparasites. Our findings highlight that further research into the impact of transposable elements on the genomes of blood-feeding parasites is required.