Environmental Engineering Reference
In-Depth Information
Chapter 6
Phylogeographic Analysis of Tick-borne Pathogen
Agustín Estrada-Peña, Victoria Naranjo, Karina Acevedo-Whitehouse,
Atilio J. Mangold, Katherine M. Kocan, and José de la Fuente
INTRODUCTION
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 ge-
netic diversity in A. marginale geographic strains using major surface protein (MSP)
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 vec-
tor-pathogen and host-pathogen interactions and to have evolved under positive selec-
tion 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.
In this study we showed that the phylogeography of A. marginale MSP1a sequenc-
es is associated with world ecological regions (ecoregions) resulting in different evo-
lutionary pressures and thence MSP1a sequences. The results demonstrated that the
MSP1a fi rst (R1) and last (RL) repeats and microsatellite sequences were associated
with world ecoregion clusters with specifi c 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.
The results reported herein provided the fi rst 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.
The genus Anaplasma (Rickettsiales: Anaplasmataceae) contains obligate intracellu-
lar organisms found exclusively within membrane-bound inclusions or parasitophorous
 
 
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