Phylogeographic Analysis of Tick-borne Pathogen - Recent Advances and Issues in Environmental Science - page 89

higher number of conserved amino acids and lower positive selection pressure when
compared with R1 sequences in ecoregion cluster 1. Finally, the R1 repeat sequences
ascribed to ecoregion cluster 4 had the highest percentage of conserved amino acids
and the lowest positive selection pressure, recorded only in sites where R. microplus
ticks are absent because of the low yearly temperature and thus other tick species act
as vectors of A. marginale [18]. The analysis of MSP1a microsatellite sequences also
supported differences among all ecoregion clusters, except for ecoregion clusters 3
and 4 where R. microplus has low prevalence or is absent.
Some R1 repeat sequences such as A, B, D, and alpha as well as microsatellite
genotypes C-D, G and H were present across several ecoregion clusters. These se-
quences appeared in A. marginale strains collected in zones where R. microplus ticks
are common (ecoregion clusters 1 and 2) and in sites, such as central Argentina and
southern parts of the USA, where R. microplus has been prevalent in the past but has
been eradicated [46]. Additionally, these sequences were also found in sites where
other tick vectors such as Dermacentor spp. are prevalent [18]. These R1 repeats and
microsatellite sequences could have evolved from ancestor pathogen strains transmit-
ted by R. microplus as the main vector, and then evolved under lower selection pres-
sure, due to pathogen transmission by other tick species or mechanically. The pres-
ence of these sequences in sites where R. microplus has been historically absent (that
is, north-western USA) and now adapted to transmission by Dermacentor spp. ticks,
could be interpreted as invasive events. The results reported here showed that lowest
selection pressure exist in sites where Dermacentor spp. ticks are the main biological
vectors or where mechanical transmission is predominant because of eradication of
R. microplus . Therefore, R1 repeats are evolving under high selection pressure only
in sites where R. microplus is the main vector and is subjected to selection because
of climate constraints. This hypothesis did not explain the absence of MSP1a genetic
diversity in Australia. Analysis of four A. marginale strains in Australia revealed the
presence of a single repeat type 8 [11]. We would expect evolution of A. marginale
MSP1a towards different repeat sequences, sharing the consensus sequence found in
ecoregion cluster 1, into which R1 type 8 is ascribed, even in the case of a single in-
vasive event. Reasons accounting for such a lack of diversity are currently unknown,
but the combined pressure exerted by tick population structure [47, 48] the A. centrale
vaccine, acaricide treatments, and cattle movement for pathogen and tick control may
have impacted on A. marginale genetic diversity in Australia [48].
A. marginale exclusively infects cattle and wild ruminants [2]. Such high host
specifi city may results in a relatively low impact of vertebrate host factors on the
evolution of A. marginale strains, thus leaving tick-pathogen interactions as the main
contributing factor affecting its biogeography and evolutionary history. However, as
previously discussed, cattle movement may have contributed to the genetic diversity
of A. marginale strains worldwide [11]. Nevertheless, the results reported herein may
be relevant in studying the evolution of other vector-borne pathogens. Many vector-
borne pathogens, such as some Babesia, Theileria, Rickettsia, Ehrlichia and Plasmodium
species, are also highly host-specifi c [49] and vector-pathogen interactions may play a
crucial role in their evolution and biogeography [35].