Phylogeographic Analysis of Tick-borne Pathogen - Recent Advances and Issues in Environmental Science - page 88

with specifi c ecoregion clusters and were not arranged according to geographical
features. The different evolutionary pressures operating over different MSP1a repeats
were demonstrated previously [6], but the possibility of using the MSP1a R1 repeat as
a biogeographical marker has only been suggested [5]. In contrast, MSP1a RL repeat
sequences, while still linked to a similar set of ecoregion clusters, did not evolve under
positive selection. Consequently, RL repeat sequences were not good genetic markers
for the characterization of A. marginale biogeography and evolution.
Analysis of MSP1a microsatellite sequences demonstrated that A. marginale
strains are associated with specifi c ecoregion clusters, thus corroborating the results
obtained with repeat sequences. These results may have a functional signifi cance. It
has been shown that the SD-ATG distance between the ribosome binding site (Shine-
Dalgarno sequence) and the translation initiation codon affects gene expression in
prokaryotes [41]. Little is known about the regulation of gene expression in A.
marginale [6]. However, as shown here in E. coli , the length of the MSP1a microsat-
ellite could affect the expression of MSP1a, which varies during A. marginale multi-
plication in both tick cells and bovine erythrocytes, thus affecting pathogen infection
and transmission [42]. Since, MSP1a repeats and microsatellites are unambiguously
associated to ecoregion clusters, these results suggested a new factor that may affect
the effi ciency by which different A. marginale strains are transmitted under different
environmental conditions.
A. marginale is an obligate intracellular parasite, which alternates between the
tick vector and the vertebrate host. Our hypothesis was that the link between pathogen
strains and defi nite portions of the environmental envelope could refl ect the effects of
climate on the tick vector. Temperature and rainfall, which are indirectly captured by
the specifi c signatures of NDVI, are the main factors affecting the ecology and popu-
lation dynamics of tick species [43] and these operate at critical levels of selection
of tick populations, selecting also specifi c strains of the pathogen. This framework is
further obscured by the “noise” produced by invasive events of the pathogen (by cattle
movement or other factors), or by selection of strains transmitted mechanically in ar-
eas where ticks are eradicated by acaricide application, contributing to the absence of
total consistence between ecoregion clusters and strains.
Adequate reports exist about distribution, seasonal dynamics and abundance of
R. microplus populations in the study area, allowing a direct comparison with results
presented herein. Ecoregion cluster 1 contained the R1 repeats with the lowest per-
centage of conserved amino acids and the highest positive selection pressure, in areas
with high temperature and medium rainfall. R. microplus ticks are common in these
areas and a strict seasonality in tick population dynamics has been reported, allowing
for a high selection of tick populations due to winter mortality [44]. Ecoregion cluster
2 contained sites with constant high temperature and rainfall. In these sites, R. microplus
ticks are abundant throughout the year without marked seasonality and climate is not
a limiting factor in tick mortality [44, 45]. In ecoregion cluster 3, tick populations
suffer drastic limitations in effectiveness because of low and inadequate rainfall [46],
and this high selection pressure on tick populations might be adverse for pathogen
transmission and selection. The R1 repeat sequences in ecoregion clusters 2 and 3 had