Deep sequencing of 16Ixodes ricinusticks unveils insights into their interactions with endosymbionts

Abstract

AbstractBackgroundIxodes ricinusticks act as vectors for numerous pathogens that present substantial health threats. Additionally, they harbour vertically transmitted symbionts, some of which have been linked to diseases. The difficulty of isolating and cultivating these symbionts has hampered our understanding of their biological role, their potential to cause disease, and their modes of transmission. To expand our understanding on the tick symbiontMidichloria mitochondriiand onRickettsia helvetica, which has been linked to disease in humans, we utilized deep sequencing on sixteen individual adult female ticks collected from coastal dune and forested areas in the Netherlands.ResultsBy employing a combination of second and third-generation sequencing techniques, we successfully reconstructed the complete genomes ofM. mitochondriifrom eleven individuals,R. helveticafrom eight individuals and the mitochondrial genome from all ticks. Additionally, we visualised the location ofR. helveticain tick organs and constructed genome-scale metabolic models (GEMs) of both symbionts to study their environmental dependencies.Our analysis revealed a strong cophylogeny between M. mitochondrii and mitochondrial genomes, suggesting frequent maternal transmission. In contrast, the absence of cophylogeny between R. helvetica and the mitochondrial genomes, coupled with its presence in the receptaculum seminis of I. ricinus females, raises the possibility of paternal transmission of R. helvetica. Notably, the genetic diversity of R. helvetica was found to be very low, except for the rickA virulence gene, where the presence of up to thirteen insertions of a33nt-long repeat led to significant variability. However, this variation could not account for the differences in infection prevalence observed across eight distinct locations in the Netherlands.ConclusionsBy employing deep sequencing, it becomes feasible to extract complete genomes and genetic data of symbionts directly from their host organisms. This methodology serves as a robust means to gain fresh insights into their interactions. Our observations, which suggest paternal transmission ofR. helvetica, a relatively unexplored mode of transmission in ticks, require validation through experimental investigations. The genetic variations identified in therickA virulence gene ofR. helveticahave the potential to influence the infectivity and transmission dynamics ofR. helvetica