Microbiome of Amblyomma americanum reflects known westward expansion

Amblyomma americanum, the vector of several tick-borne pathogens including those that cause ehrlichiosis, has undergone recent range expansion. We examined the microbiota of 187 A. americanum ticks from 4 regions spanning their historical and current geographic range. Using massively parallel sequencing targeting the 16S rRNA V4 region, we analyzed the bacterial community composition of each individual tick. Consistently, the most prominent members of the community for each sample were either Coxiella, Rickettsia or Rickettsiella. Alpha diversity was significantly different between males and females at all regions tested; male ticks possess more diverse communities than females. Random forest analysis revealed that sex is a significant predictor of microbiome diversity, and indicator species analysis pinpointed 24 taxa that are most associated with males and 1 taxon most associated with females. Regarding beta diversity, both ordination and PERMANOVA analyses revealed significant differences in microbial community composition driven by sex and geographical origin. In addition, a mantel correlogram showed that the dissimilarities were structured by the distance between collection sites. Null model and phylogenetic analyses showed that for females and males, respectively, drift (88.11%; 85.90%), dispersal limitation (11.28%; 12.97%), homogeneous selection (0.61%; 0.10%), and homogeneous dispersal (0.00%; 1.03) were ecological drivers of microbial community dynamics. Microbiota dissimilarities increasing with geographic distances and predominantly driven by non-selective ecological processes are hallmarks of bacterial community isolation. Therefore, our results suggest the microbiome may offer an additional layer of genomic information to study the range expansion of this relevant vector.