Mutation status and homology modelling of acaricide-binding proteins in Rhipicephalus ticks

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.