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Background  The tick-borne pathogen  Anaplasma marginale , which is endemic worldwide, is the type species of the genus  Anaplasma  (Rickettsiales: Anaplasmataceae).  Rhipicephalus  ( Boophilus )  microplus  is the most important tick vector of  A. marginale  in tropical and subtropical regions of the world. Despite extensive characterization of the genetic diversity in  A. marginale  geographic strains using major surface protein sequences, little is known about the biogeography and evolution of  A. marginale  and other  Anaplasma  species. For  A. marginale , MSP1a was shown to be involved in vector-pathogen and host-pathogen interactions and to have evolved under positive selection pressure. The MSP1a of  A. marginale  strains differs in molecular weight because of a variable number of tandem 23-31 amino acid repeats and has proven to be a stable marker of strain identity. While phylogenetic studies of MSP1a repeat sequences have shown evidence of  A. marginale -tick co-evolution, these studies have not provided phylogeographic information on a global scale because of the high level of MSP1a genetic diversity among geographic strains.    Results  In this study we showed that the phylogeography of  A. marginale  MSP1a sequences is associated with world ecological regions (ecoregions) resulting in different evolutionary pressures and thence MSP1a sequences. The results demonstrated that the MSP1a first (R1) and last (RL) repeats and microsatellite sequences were associated with world ecoregion clusters with specific and different environmental envelopes. The evolution of R1 repeat sequences was found to be under positive selection. It is hypothesized that the driving environmental factors regulating tick populations could act on the selection of different  A. marginale  MSP1a sequence lineages, associated to each ecoregion.    Conclusion  The results reported herein provided the first evidence that the evolution of  A. marginale  was linked to ecological traits affecting tick vector performance. These results suggested that some  A. marginale  strains have evolved under conditions that support pathogen biological transmission by  R. microplus , under different ecological traits which affect performance of  R. microplus  populations. The evolution of other  A. marginale  strains may be linked to transmission by other tick species or to mechanical transmission in regions where  R. microplus  is currently eradicated. The information derived from this study is fundamental toward understanding the evolution of other vector-borne pathogens.

bmc biology

Phylogeographic analysis reveals association of tick-borne pathogen, Anaplasma marginale, MSP1a sequences with ecological traits affecting tick vector performance