Genetic and genomic variability of Spiroplasma and Midichloria endosymbionts associated with the tick Ixodes frontalis

Ixodes frontalis, an ornithophilic tick species, is widely distributed all over Europe exhibiting two genetically diverging haplogroups based on differences in the cytochrome c oxidase subunit 1 mitochondrial gene. Despite its broad distribution, little is known about the presence of symbiotic bacteria in I. frontalis, while symbionts are generally widespread in ixodid ticks and responsible for important effects on host fitness. We collected I. frontalis from France and Italy (n = 277) and assessed that the most prevalent haplogroup was A (73%). We then investigated the presence of the symbionts, Midichloria mitochondrii and Spiroplasma ixodetis. They were both found at a high prevalence in adult ticks (66% and 77% respectively), while the number of positive immature ticks was significantly lower (18% for both). The experimental analysis of larvae hatched from egg clutches obtained from four females hints at vertical transmission of both symbionts. We obtained three genomes of Spiroplasma and one of Midichloria, and used them to perform comparative genomic analysis. Average nucleotide identity among available Spiroplasma or Midichloria genomes from I. frontalis are all extremely high, suggesting low genetic variability for both symbionts. Gene presence/absence analysis confirmed the presence of B vitamin synthesis genes in the genome of M. mitochondrii, and also showed the presence of the ETX/MTX2 gene, the RIP family and a partial Spaid-like gene in S. ixodetis. This gene repertoire indicates a nutritional role for Midichloria, while for S. ixodetis we hypothesize a role of this bacterium as a defensive symbiont or a manipulator of the host reproduction.