Abstract:
Rhipicephalus microplus is a tick with a one-host life cycle which
takes place on cattle. It has
three life stages during the host phase. Tropical and sub-tropical
regions around the world
make for an ideal climate for both R. microplus and R. decoloratus
ticks. Both tick species are
well documented as vectors for various tick-borne diseases. Several
target site resistance
markers are known for acaricide resistance in R. microplus and the
presence of these markers
in R. decoloratus is a focus of this study. The dieldrin resistance
marker was not observed in
R. decoloratus nor was the pyrethroid resistance marker in the
carboxylesterase gene. The
octopamine/tyramine receptor showed the presence of 49 different SNPs
as compared to with
the NCBI R. microplus entry (AJ010743.1). Maximum parsimony tree
analysis of this gene
segment which included locally sequenced R. decoloratus and R.
microplus tick samples
showed a mix grouping of both species. This would suggest that there
is not a significant
difference between R. decoloratus and R. microplus for the
octopamine/tyramine receptor to
group them as separate. Analysis of the voltage-gated sodium channel
revealed the presence of the resistance marker (L64I) in both R. microplus and R.
decoloratus ticks of South Africa.
Rhipicephalus microplus showed allele frequencies of 58.8% homozygous
resistant and a
32.4% homozygous susceptible, while R. decoloratus had a 72%
heterozygous frequency. A
model for the R. microplus carboxylesterase (RmCaE) was constructed
using 4B0O as a
template (32.2% identity). The model was well within the expected
quality range for
carboxylesterase enzymes. Docking of cypermethrin showed that no
direct interactions were
possible between the resistance SNP site and cypermethrin, but the
loss of a stabilising
interaction in the secondary structure could be the main cause behind
resistance. A partial
model for the voltage-gated sodium channel (RmvNaCh) was constructed
using 4DXW as a
template (30% identity). Docking of the different stereoisomers for
cypermethrin identified two
key atoms (C4 and Cl-) of cypermethrin as interacting partners with
the SNP site. Cyp5 was
also identified as the one stereoisomer which didn’t show a difference
between WT and
mutant docked poses and was still capable of interacting with the SNP
site. This may show cyp5 [(R)-cyano (1S, 3S)] as an important stereoisomer for use on
pyrethroid resistance R.
microplus ticks. Experimental validation of in silico results must be
done in future.
