ORIGINAL RESEARCH PAPER

A novel point mutation of acetylcholinesterase in a trichlorfon-resistant strain of the oriental fruit fly Bactrocera dorsalis(Diptera: Tephritidae)

Jian-jun Jiang • Kang Zhou • Guang-wen Liang •

Ling Zeng • Shuo-yang Wen

Received: 8 May 2013 / Accepted: 6 November 2013 / Published online: 18 December 2013

� The Japanese Society of Applied Entomology and Zoology 2013

Abstract Acetylcholinesterase (AChE) is the target

enzyme of organophosphorus and carbamate insecticides.

We applied trichlorfon to select resistant strains of Bac-

trocera dorsalis Hendel in the laboratory. Two trichlorfon-

resistant strains, the Tri-R1 strain with 18.23-fold resistance

and the Tri-R2 strain with 69.5-fold resistance, were

obtained. Three known mutations, I159V, G433S and

Q588R were identified in AChE of two resistant strains,

and a novel mutation, G365A, was identified in the more

resistant Tri-R2 strain. The modeled 3-D-structure of AChE

showed that G365A and G433S are closely adjacent in the

gorge above the catalytic site S235. Mutations of G365A

and G433S resulted in a steric hindrance by stronger Van

der Waals force between two sites. Such a minor structural

change might block insecticides from squeezing through

the gorge to reach the active site, but not the natural sub-

strate. Compared with the susceptible strain, the AChE

activity of the Tri-R1 strain and the Tri-R2 strain was 0.87-

and 0.67-fold, the Km value of the Tri-R1 strain and the Tri-

R2 strain was 0.11- and 0.10-fold, the Vmax value of two

resistant strains was 0.26- and 0.15-fold, whereas, the I50 to

trichlorfon significantly increased by 9.07- and 13.19-fold.

These results suggested that the novel point mutation

G365A of AChE might be involved in increasing resistance

to trichlorfon in the resistant strain of oriental fruit fly.

Keywords Ace gene � Mutation �Acetylcholinesterase � Trichlorfon � Bactrocera

dorsalis � Insecticide resistance

Introduction

More and more intractable pests have emerged which are less

susceptible to insecticides in the field. Such increased

insecticide resistance is mainly caused by increasing num-

bers of alleles which are resistant to the target molecule. It is

known that the resistant alleles originate from gene ampli-

fication (Field et al. 1988; Mouches et al. 1986), over-tran-

scription (Field et al. 1988; Fournier et al. 1992a), or point

mutations (Ffrench-Constant et al. 1993; Fournier et al.

1992b; Mutero et al. 1994). Acetylcholinesterase (AChE, EC

3.1.1.7), acts as a synaptic terminator of nerve impulses

through hydrolysis of the neurotransmitter acetylcholine,

and is a target enzyme of organophosphorus and carbamate

insecticides (Casida and Quistad 2004; Mutero et al. 1994).

A couple of resistance-associated amino acid substitutions in

insecticide-insensitive AChE have been identified from

some insects (Menozzi et al. 2004; Mutero et al. 1994;

Vaughan et al. 1997; Walsh et al. 2001). There are five amino

acid substitutions, F115S, I199V, I199T, G303A, and

F368Y, in Drosophila melanogaster Meigen, these residues

are located near the active site of AChE and are involved in

organophosphorus and carbamate resistance (Mutero et al.

1994). In Aedes aegypti Linnaeus, three amino acid substi-

tutions, F105S, G285A, and F350Y, have been identified

from resistant strains; the IC50 values increased to different

levels according to different combinations of substitutions

J. Jiang � G. Liang � L. Zeng (&) � S. Wen (&)

Department of Entomology, College of Natural Resources

and Environment, South China Agricultural University,

Guangzhou 510642, China

e-mail: zengling@scau.edu.cn

S. Wen

e-mail: shywen@scau.edu.cn

K. Zhou

Hefei National Laboratory for Physical Sciences at the

Microscale, School of Life Sciences, University of Science

and Technology of China, Hefei 230026, Anhui, China

123

Appl Entomol Zool (2014) 49:129–137

DOI 10.1007/s13355-013-0232-0

(Vaughan et al. 1997). Five substitutions, V180L, G262V,

G262A, F327Y and G365A, were in Musca domestica Lin-

naeus. Among them, a novel G262V mutation resulted in up

to 100-fold resistance to certain compounds (Walsh et al.

2001). Two amino acid substitutions, A302S of AChE1 and

F139L of AChE2, were found in Aphis gossypii Glover (Li and

Han 2004). S291G was identified from Leptinotarsa decem-

lineata Say (Zhu et al. 1996). I214V and G488S were also

found in AChE of Bactrocera oleae Gmelin (Vontas et al.

2002). Other than above mentioned mutations, the active sites

remained highly conservative in insects. In D. melanogaster,

S238 of an acyl-ester intermediate, and E405 and H518 of a

charge relay system are three very conservative active sites

(Harel et al. 2000). Organophosphates and carbamates are

irreversible inhibitors of AChE. Some resistance-associated

mutations which are located very close to the active site of

AChE change the structure of the gorge. Therefore, the

organophosphates and the carbamates are blocked and cannot

get through to the active sites (Vontas et al. 2002).

The oriental fruit fly, Bactrocera dorsalis Hendel, is a seri-

ous pest causing significant financial loss in southern China.

The main strategy to control this pest is chemical control. The

field monitoring has indicated that this pest has shown very high

resistance to trichlorfon, fenitrothion, spinosad, alphamethrin

and abamectin (Hsu 2000; Vontas et al. 2011; Zhang 2007). So

far, three substitutions, I159V, G433S, and Q588R, have been

identified in AChE as associated with pesticide-resistance in B.

dorsalis (Hsu et al. 2006). In order to provide experimental

evidence of possible resistance arising under strong insecticide

pressure, we applied trichlorfon to select resistant strains in the

laboratory and obtained two highly resistant strains. The cDNA

of the ace gene was also cloned and sequenced from all studied

strains. The possible structural change of AChE in resistant

strains and the potential mechanism underlying resistance is

discussed in this study.

Materials and methods

Susceptible strain

The susceptible strain was collected from Qingxin County,

Guangdong Province, China. No specific permits were

required for collecting fruit flies from this area. Flies were

maintained in cages on an artificial diet (Jin et al. 2011) at

26 ± 2 �C and 70 ± 5 % relative humidity (RH) under a

14:10 h light cycle.

Insecticides

Technical grade trichlorfon powder (90 %) was provided

by Nantong Jiangsu Agrochemical Co., Ltd (Nantong,

Jiangsu, China).

Selection of insecticide-resistant strains and bioassays

Trichlorfon-resistant strains were established from 3- to

5-day-old adult flies of the susceptible strain (F0) by using

the residual film method (WHO 1970) with the trichlorfon

in the laboratory. The first three generations, F1–F3, were

selected with the concentration of insecticide based on the

lethal concentration of 50 % (LC50) in the previous

selection experiments (Zhang 2007). After the third gen-

eration, the LC50 was determined for each of the strains and

the concentration based on the LC50 was then used for the

next treatment. Briefly, the pesticide was diluted with

acetone into working concentrations and then smeared on

the surface of the 250-ml Erlenmeyer flask. Fifty 3- to

5-day-old adult flies were trapped in the flask, which was

sealed with gauze. Then, the Erlenmeyer flask was stood

upside down for 24 h at 26 ± 2 �C, 70 ± 5 % RH under a

14:10 h light cycle. In total, about 2,000 flies were treated.

The survivors were placed in a new cage for the treatment

of the next generation. The resistance level was determined

by the ratios of LC50 of resistant strains to the LC50 of the

susceptible strain. The susceptible strain treated with ace-

tone was used as a negative control. The bioassay was done

with four replicates, 20 flies were used for each replicate.

Data analysis

The data was analyzed according to Jin et al. (2011). The

mortality data were corrected by using Abbott’s formula

(Abbott 1925). The toxicity regression and the LC50 of

insecticides were calculated using Microsoft Office Excel

(Zhang et al. 2002).The statistical analysis was performed

by using SPSS (Jia 2006).

RNA extraction and synthesis of cDNA

Total RNA was extracted individually from 3- to 5-day-old

adult flies with Trizol (Invitrogen, USA) according to the

user manual. Five individuals were sampled from the SS

and Tri-R1 strains, and 10 individuals were sampled from

the Tri-R2 strain. The first-strand cDNA was synthesized

by using the PrimeScript II 1st Strand cDNA Synthesis Kit

(Takara, Dalian) according to the user manual.

PCR amplification and sequencing

From the full-length cDNA sequence of ace from B. dor-

salis (AY155500), two pairs of primers were designed for

amplifying the ace cDNA from investigated strains. The

aceF1 (50-CGAGCAGTGTTAATAAGGTG-30) and the

aceR1 (50-AAATGCCCGAGTAGGAGT-30) were for a

1,258-bp fragment, the aceF2 (50-AATGCGTCGCTGTTG

T-30) and the aceR2 (50-CATTTATTGCCTGTCTGT-30)

130 Appl Entomol Zool (2014) 49:129–137

123

were for a 1,047-bp fragment. Two fragments partly over-

lapped into the ace full-length cDNA. The 50-ll PCR

reaction mixture included 5 ll first-strand cDNA, 0.8 ll

each of the primers, 0.4 mM dNTPs, 1.25 U Taq DNA

polymerase and 5 ll 109 PCR buffer (plus Mg2?). The

PCR reactions were performed under the following condi-

tions: pre-heat at 94 �C for 2 min, followed 35 cycles of

94 �C for 30 s, 52 �C for 30 s, 72 �C for 1.5 min, finally

extended at 72 �C for 5 min. On a 1.5 % agarose/EtBr gel,

5 ll of PCR product was detected. Then, the PCR ampli-

fication was purified with a DNA gel extraction kit (Omega

Bio-Tek, China), and the purified products were cloned into

the pGEM-T easy vector (Promega, USA) and sequenced

by the Invitrogen company (Guangzhou, China). Sequences

were analyzed with Mega 5.0 (Tamura et al. 2011).

Modeling of the 3-D structure of AChE

The potential 3-D structure of resistant strains of B. dor-

salis were modeled based on the Drosophila template 1qo9

(Harel et al. 2000) under the automated mode of modeling

online (http://swissmodel.expasy.org/) (Arnold et al. 2006;

Guex and Peitsch 1997; Schwede et al. 2003). The gorge

position was determined according to the structure of

Drosophila AChE (Harel et al. 2000). A part of the gorge

among substitutions of S433 and A365 and the active site

S235 was calculated by the program CAVER (http://www.

caver.cz/) (Chovancova et al. 2012). The program PyMOL

(http://www.pymol.org/) (DeLano 2002) was used for

drawing the spatial figure. The Van der Waals (VDW)

surfaces and the atom type of substitution amino acid

residues were computed by the Swiss-Pdb Viewer 4.1.0

(http://spdbv.vital-it.ch/download_prerelease.html).

Assays of AChE activity

The enzyme activity for the susceptible strain and two

resistant strains was surveyed. Six energetic 3- to 5-day-old

adult flies were homogenized in 4 ml of 0.02 M phosphate

buffer solution (PBS) (pH 7.6) containing 0.1 %Triton

X-100 (V/V) using a glass homogenizer in an ice bath. Three

replicates were made for each strain. The homogenates were

centrifuged at 12,000 rpm for 15 min at 4 �C, and the

supernatant was carefully collected for further use. The

Coomassie Brilliant Blue G-250-based colorimetric assay

(Bradford 1976) with the standard protein bovine serum

albumin (BSA) was used to determine the protein content of

the supernatant. A modified Ellman procedure (Gorun et al.

1978) was used to assay the enzyme activity. Briefly, the

supernatant was diluted to a concentration of 0.8–1.6 lg/ll

protein as the crude enzyme solution for determining AChE

activity. A 200-ll reaction mixture contained 20 mM PBS

(pH 7.6), 0.8 mM substrate acetylthiocholine iodide

(ATchI), and 50 ll crude enzyme solution. This was incu-

bated at 25 �C for 25 min, 1.8 ml dithiobis nitrobenzoic acid

(DTNB) was added, then the absorbance was checked at

412 nm under the spectrophotometer (emM = 13.6). An

inactivated crude enzyme solution which had been heated at

100 �C for 5 min was employed instead of the crude enzyme

as the control solution. In addition, the Km, Vmax and I50

values were also determined for each strain. The serial

concentrations of substrate (ATchI) were set as 0.2, 0.6, 1.0,

1.4 and 1.8 mM. A Lineweaver–Burk plot (Li 2003) was

used to calculate Km and Vmax values. To determine the I50

value, the 10 g/l trichlorfon acetone solution was further

diluted with 0.02 M PBS buffer (pH 7.6) into different

concentrations for assay. The acetone diluted with 0.02 M

PBS buffer (pH 7.6) was used as a parallel control instead of

the trichlorfon acetone solution. The 100-ll reaction mixture

contained 50 ll diluted solution and 50 ll enzyme solution.

This was incubated at 25 �C for 2 min, then 100 ll 0.8 mM

ATchI was added to the reaction mixture to assay the surplus

enzyme activity. The I50 was determined based on log-con-

centration vs log% inhibition regression analysis.

Results

Resistance of resistant strains

Two resistant strains were obtained after 24 generations of

trichlorfon selection. The LC50 and resistance ratios of two

resistant strains and the susceptible strain are shown in Table 1.

The resistant strain Tri-R1 showed an 18.23-fold resistance and

the resistant strain Tri-R2 showed a 69.5-fold resistance.

Resistance-associated mutations

We cloned and sequenced cDNA of ace using two pairs of

primers based on the ace cDNA sequence of the wild type

Table 1 Resistance ratio to trichlorfon based on the lethal concen-

trations of 50 % (LC50)

Strain LC50 (95 % CI)

(mg/l)

Linear regression

equation

Slope RR

SS 2.655

(2.212–3.430)

y = -3.506 ? 3.396x 3.396 1.00

Tri-R1 48.32

(44.62–51.68)

y = -4.519 ? 5.650x 5.650 18.23

Tri-R2 184.2

(169.1–200.6)

y = -3.721 ? 3.850x 3.850 69.50

The assay was done with four replicates, 20 flies were used for each

replicate. The resistance ratio (RR) was calculated from the LC50 of

the resistant strain divided by the LC50 of the susceptible strain

SS susceptible strain, Tri-R1 trichlorfon-resistant strain 1, Tri-R2 tri-

chlorfon-resistant strain 2

Appl Entomol Zool (2014) 49:129–137 131

123

B. dorsalis (Hsu et al. 2006). After alignment of two

fragments of the ace gene, a 2,022-bp length of ace cDNA

sequence was obtained from the susceptible and resistant

strains. The sequences from the susceptible strain were

100 % identical to the known wild type ace cDNA

sequence. The sequence encodes 55 amino acid residues of

a signal peptide and 618 amino acid residues of a mature

peptide (Fig. 1). Different numbers of mutations were

detected from the two resistant strains. Mutations from

each strain are shown in Table 2. As the results show, three

known mutations, I159V, G433S and Q588R, were found

in the two resistant strains. It was very interesting to find a

novel mutation at the 365 site in the Tri-R2 strain (Gen-

Bank accession number: KC536628) (Table 2; Fig. 1).

Besides those missense mutations, some silent mutations

were also identified in the resistant strains (Table 2). In

order to compare the mutations of resistant strains with the

AChE of D. melanogaster, we analyzed the alignment of

amino acid sequences as shown in Fig. 1. The 55-residue

signal peptide of B. dorsalis was longer than the 38-residue

signal peptide of D. melanogaster AChE. Three residues at

the active sites S235, E364 and H477, remained conser-

vative in B. dorsalis (Fig. 1).

Potential 3-D structure changes of AChE in resistant

strains

We submitted the amino acid sequences of mature AChE

of the resistant Tri-R2 strain of B. dorsalis online to

model the AChE 3-D structure. The B. dorsalis AChE

was automatically modeled with the template 1qo9 of

D. melanogaster. The identity of the amino acid

sequence of B. dorsalis AChE was 84 % with the

sequence of 1qo9. The modeling sequence didn’t include

the known mutation Q588R, because 1qo9 only ran from

D3 to A574: equivalent to D3-Q571 of B. dorsalis

AChE. As Fig. 2 shows, B. dorsalis AChE has a gorge

similar to that of Drosophila AChE, which allows the

substrate to get through to the active site (Fig. 2a, Harel

et al. 2000). G433S and the novel mutation G365A were

adjacent in the gorge above the active site S235 (Fig. 2).

Figure 3 indicates the side-chains and Van der Waals

volumes of three amino acid residues at sites 432–434

and 364–366 of B. dorsalis AChE. Glycin is the simplest

amino acid, with a small Van der Waals (VdW) volume.

Substitution at site 433 changes the amino acid from G

to S, serine changes the side-chain with a –CH3 and the

Van der Waals volume increases to 73 A3. Substitution

at site 365 changes the amino acid from G to A, alanine

changes the side-chain with a –OH and the Van der

Waals volume increases to 67 A3. We further computed

the Van der Waals surfaces of two mutations and their

neighbor residues (Fig. 3). When mutation only happened

at site 433 in resistant strains, the VdW surfaces between

S433 and G365 might overlap a little more (Fig. 3b) than

those between G433 and G365 (Fig. 3a). When mutations

happened at both site 433 and site 365 in highly resistant

strains, the VdW surface overlapped even further

between S433 and A365 (Fig. 3c). Such slight changes of

structure might increase steric hindrance, resulting in a

narrower gorge than that of the native gorge. If the

insecticides are bigger than the native AChE, the smaller

space might block the bigger molecules from the gorge,

stopping the bigger molecule from binding to the active

site S235.

Assays of AChE activity and kinetic parameters

Parameters of AChE, such as enzyme activity, Km and Vmax

are shown in Table 3. The AChE activity of the two

resistant strains was significantly different from the sus-

ceptible strain. Compared with the susceptible strain, the

AChE activity of Tri-R1 strain and Tri-R2 strain was 0.87-

and 0.67-fold respectively. The Km and Vmax values of the

two resistant strains were also significantly decreased

compared with the susceptible strain. The Km value of the

Tri-R1 strain and the Tri-R2 strain was 0.11- and 0.10-fold

and the Vmax value of the two resistant strains was 0.26-

and 0.15-fold, whereas, the I50 to trichlorfon significantly

increased by 9.07- and 13.19-fold for Tri-R1 strain and Tri-

R2 strain respectively.

Discussion

We have shown here the selection of trichlorfon-resistant

strains with a novel mutation. The novel mutation along

with other known mutations resulted in potential 3-D

structural changes of AChE associated with high insec-

ticide resistance. Organophosphorus compounds were

designed to serve as competitive inhibitors of the natural

substrate of AChE, phosphorylating the active-site serine,

leading to irreversible inhibition of the enzyme, resulting

in accumulation of acetylcholine in the synapses which

disables the termination of nerve impulses. In resistance-

associated mutations located within the active-site gorge

of AChE, the altered amino acids are larger than those

residues in the wild-type AChE. They have a steric effect

on the 3-D structure, or alter the orientation of the

active-site residues. Different mutations reduce the AChE

catalytic efficiency to different levels according to the

combination of mutations. In B. oleae, G433S resulted in

a 35–40 % reduction in AChE catalytic efficiency. The

combination of G433S and I159V produced up to a

16-fold decrease in insecticide sensitivity (Vontas et al.

2002).

132 Appl Entomol Zool (2014) 49:129–137

123

Fig. 1 Alignment of AChE of D. melanogaster (Dm), susceptible

(Bd-SS) and resistant strains (Bd-Tri-R1 and Bd-Tri-R2) of B.

dorsalis. The signal peptide is 38aa in D. melanogaster and 55aa in B.

dorsalis. An asterisk indicates the active site of the acyl-ester

intermediate. The number signs indicate the active sites of the charge

relay system. Three known mutations in B. dorsalis are marked with

solid circles. The novel mutation in the resistant strains is indicated

with an open circle

Appl Entomol Zool (2014) 49:129–137 133

123

The phenomenon that multiple mutations in B. dorsalis

AChE work together to raise the level of resistance, is

consistent with earlier studies in other insects (Ffrench-

Constant et al. 1998; Menozzi et al. 2004; Mutero et al.

1994; Shi et al. 2004). The novel mutation G365A and the

known mutation G433S are located in different helices.

Interestingly, they were found to be adjacent and located

above the active site S235 in the gorge (Fig. 2). The Van

der Waals volume of substituted amino acids serine and

alanine are slightly larger than the original glycin. The

surface force between these two residues became a little bit

stronger, and might have resulted in changing the gorge

space (Fig. 2). The gorge space of wild-type AChE allows

the natural substrates to get through to reach the active site,

as well as the elaborately designed organophosphorus

compounds. However, the substituted G365A and G433S

slightly enlarged the overlying area of Van der Waals

surface. The change is very minor, but might be sensitive

Fig. 2 The potential 3-D structure of the resistant strains of B.

dorsalis. The 3-D structure was modeled based on the Drosophila

template 1qo9 (Harel et al. 2000) online (http://swissmodel.expasy.

org/) under the automated mode of modeling. The modeled pdb file

was introduced into the PyMOL molecular graphics system to draw

the 3-D structure (a) and the substrate entrance tunnel (b). The gorge

position in a was determined according to the structure of Drosophila

AChE (Harel et al. 2000). The red frame in a was enlarged into b to

show the spatial relationships between the gorge, substitutions of

S433 and A365, and the active site S235. We used the program

CAVER to calculate the substrate entrance tunnel (brown mesh); it is

a part of the gorge. The active site of S235 is colored in pink. The

known substitutions of V159 and S433 and the novel substitution of

A365 are colored in yellow. They are shown as sticks with the main

chain hidden (color figure online)

Table 2 Mutations in resistant strains

Strain Missense Silent

I159V G365A G433S Q588R N97 V171 Q244 R362 Y371 F373

SS ATA GGC GGC CAA AAT GTC CAG AGA TAC TTT

Tri-R1 GTA GGC AGC CGA AAT GTC CAG CGA TAC TTC

Tri-R2 GTA GCC AGC CGA AAC GTT CAA AGA TAT TTC

Nucleotide substitution is underlined. Numbers indicate sites of mutation in the mature AChE sequence, excluding the signal peptide sequence

SS susceptible strain, GenBank accession number: KF257933; Tri-R1 trichlorfon-resistant strain 1, GenBank accession number: KC556803; Tri-

R2 trichlorfon-resistant strain 2, GenBank accession number: KC536628

134 Appl Entomol Zool (2014) 49:129–137

123

enough to increase steric hindrance and change the gorge

space, therefore, it might block the trichlorfon from

entering the gorge, but not the natural substrate of

acetylcholine. To confirm this mechanism, we need further

studies on the dynamic interaction between AChE, target

insecticides and natural substrates.

In addition, research shows that many insects have two

ace genes (ace1 and ace2), for example, in Aanopheles

gambiae Giles, Culex pipiens Linnaeus, A. gossypii and so

on. In these species, the ace1 gene is related to pesticide

resistance (Baek et al. 2005; Bourguet et al. 1996; Javed

et al. 2003; Li and Han 2002; Malcolm et al. 1998; Ni et al.

2003; Weill et al. 2002, 2003). However, there is only one

ace gene in D. melanogaster, M. domestica, and Bemisia

tabaci Gennadius: this gene is an ortholog of the ace2 gene

of other insect species and it is related to pesticide resis-

tance (Fournier et al. 1989; Hall and Spierer 1986; Hsu

et al. 2006; Javed et al. 2003; Kozaki et al. 2001; Walsh

et al. 2001). In B. dorsalis, only one ace gene has been

reported (Hsu et al. 2006). The ace gene we cloned in this

study appears to be an ortholog of the ace2 gene of mos-

quitoes and B. oleae (Hawkes et al. 2005; Hsu et al. 2006;

Vontas et al. 2002). So far, it is still unclear whether there

is one ace gene in B. dorsalis, or two, as in B. oleae. The

number of ace gene in B. dorsalis will be confirmed by

Southern hybridization in future sutdies.

The Km value is a crucial indicator of the affinity of the

enzyme to its substrate. When the Km value of altered

AChE decreased, the affinity of the enzyme to its substrate

increased, whereas the affinity to other substrates

decreased. This decreased Km value has been observed in

the resistant populations of house flies (Bull 1992; Dev-

onshire and Moores 1984). The level of AChE activity, and

the Km and Vmax values of resistant strains, were signifi-

cantly lower compared with that of the susceptible strain,

while the I50 value was increased (Table 3). There was also

a significant difference between the two resistant strains in

AChE activity, the Vmax value and the I50 value. The AChE

activity and the Vmax value of the Tri-R strain are signifi-

cantly lower than that of the Tri-R1 strain. The I50 value of

the Tri-R strain is significantly lower than that of the Tri-R1

strain. These results suggest that the novel mutation might

be the inducement for such change of the AChE activity

and the resistance. The alteration of the AChE enzyme

activity is a major contributor to the resistance of B. dor-

salis to trichlorfon. The novel mutation occurring together

Fig. 3 The Van der Waals surfaces of amino acid residues at

substituted sites of Bactrocera AChE. Amino acid residues at the

substituted sites and their neighbor residues were loaded to compute

the surfaces of target amino acids using Swiss-Pdb Viewer 4.1.0

(http://spdbv.vital-it.ch/download_prerelease.html). Substitution at

site 433 changed G to S, the side-chain changed to –CH3 and the Van

der Waals volume increased to 73 A3 for S. Substitution at site 365

changed the amino acid from G to A, the side-chain changed to –OH

and the Van der Waals volume increased to 67 A3 for A. a Suscep-

tible strain with G365 and G433. b The Tri-R1 strain with 18.23-fold

resistance has one substitution at S433. c The Tri-R2 strain with 69.5-

fold resistance has both substitutions at A365 and S433. The relative

position of A365 and S433 were shown in Fig. 2b

Table 3 AChE activity and kinetic parameters (mean ± SE) of the crude AChE extract from susceptible and resistant strains of oriental fruit fly

Strain AChE activity (nmol/min/mg protein) Km (lmol/l) Vmax (nmol/min/mg protein) I50 (trichlorfon) (lmol/l)

SS 39.50 ± 3.689a 12.92 ± 1.121a 416. 7 ± 23.33a 1.028 ± 0.167c

Tri-R1 34.51 ± 1.374b 1.430 ± 0.194b 107.9 ± 3.954b 9.335 ± 0.893b

Tri-R2 26.47 ± 0.318c 1.360 ± 0.179b 61.67 ± 1.466c 13.59 ± 0.705a

ATchI (acetylthiocholine iodide) was used as the substrate in this study. There were three replicates for each sample. The same letters following

the means are not significantly different at the 0.05 level (Turkey’s, p = 0.05)

SS susceptible strain, Tri-R1 trichlorfon-resistant strain 1, Tri-R2 trichlorfon-resistant strain 2

Appl Entomol Zool (2014) 49:129–137 135

123

with other mutations increased the resistance ratio in

resistant strains.

In some insects, the increased expression level of AChE

caused insecticide resistance (Berrada and Fournier 1997;

Charpentier and Fournier 2001; Gao and Zhu 2002).

However, it was found that the level of expression of the ace

gene was roughly equivalent in individuals from both the

susceptible and resistant strains in B. dorsalis (Hsu et al.

2008). In this study, mutations of the ace gene have been

found to be associated with pesticide resistance in B. dor-

salis. In addition to the over-expression and mutations of

the ace gene associated with resistance, there might be other

factors involved in pesticide resistance yet to be discovered.

Early detection of mutations and resistance is very

important for monitoring resistant strains in the field and

developing effective control strategies. We have provided

here evidence of a resistance increasing tendency, a novel

mutation and the potential 3-D structure change of AChE

regarding this resistance. These results will shed light on

the mechanism of resistance, and supply basic data for pest

monitoring and the development of control strategies.

Acknowledgments This work was supported by the Research

Project of National Public Service in Agriculture (200903047). We

appreciate two anonymous reviewers’ crucial suggestions and

detailed comments for improving the quality of the manuscript.

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