Insect Control: Biological and Synthetic Agents - Index of
Insect Control: Biological and Synthetic Agents - Index of
Insect Control: Biological and Synthetic Agents - Index of
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A1.4. Resistance<br />
The increasing number <strong>of</strong> reports <strong>of</strong> pyrethroid<br />
resistance in the field has fuelled further studies<br />
to determine the molecular basis <strong>of</strong> resistance, <strong>and</strong><br />
recent progress in this area is summarized in<br />
Table A1. Several cytochrome P450s that are overexpressed<br />
in insecticide-resistant strains <strong>and</strong> are able<br />
to metabolize pyrethroids have been identified. For<br />
example, the Anopheles gambiae enzyme, CYP6P3,<br />
is upregulated in pyrethroid-resistant populations<br />
from Ghana <strong>and</strong> Benin <strong>and</strong> it metabolizes both<br />
a-cyano (type 2) <strong>and</strong> non-a-cyano (type 1) pyrethroids<br />
(Muller et al., 2008). Another CYP6 enzyme<br />
from the mosquito An. minimus has also been demonstrated<br />
to have activity against deltamethrin, supporting<br />
its likely role in resistance (Boonsuepsakul<br />
et al., 2008). A number <strong>of</strong> P450s from Helicoverpa<br />
armigera have been implicated in pyrethroid resistance<br />
by metabolism studies (Table A1), although<br />
genetic mapping has suggested that the in vitro ability<br />
<strong>of</strong> certain P450s (such as the CYP6B7 <strong>and</strong> CYP6B6)<br />
to metabolize pyrethroids may not be sufficient evidence<br />
to prove their role in resistance (Grubor <strong>and</strong><br />
Heckel, 2007). The application <strong>of</strong> RNA silencing techniques<br />
has demonstrated the importance <strong>of</strong> CYP6BG1<br />
from the diamondback moth, Plutella xylostella,<br />
in conferring permethrin resistance (Bautista et al.,<br />
2009). Recently, the gene duplication events in<br />
An. funestus as a possible contributing mechanism<br />
fortheincreasedP450activityinpyrethroidresistance<br />
populations were identified in this enzyme family in<br />
insects (Wondji et al., 2009).<br />
The number <strong>of</strong> insect <strong>and</strong> mite species documented<br />
as containing mutations in the pyrethroid target<br />
site, the voltage-gated sodium channel <strong>of</strong> nerve<br />
membranes, continues to grow (see Table A1).<br />
Many <strong>of</strong> these are clustered within localized ‘‘hotspots,’’<br />
including the domain IIS4-S5 linker, IIS5 <strong>and</strong><br />
IIS6 helices <strong>and</strong> the corresponding regions <strong>of</strong><br />
domains I <strong>and</strong> III (reviewed in Davies et al., 2007),<br />
<strong>and</strong> have been instrumental to identify a clearly<br />
defined binding site for these compounds at the<br />
sodium channel pore (see Section 1.5). The functionality<br />
<strong>of</strong> many <strong>of</strong> these mutations has now been<br />
confirmed by analysis <strong>of</strong> their properties in Xenopus<br />
oocyte-expressed insect channels, <strong>and</strong> some <strong>of</strong> the<br />
more commonly identified mutations at residues,<br />
such as M918, T929, L1014 <strong>and</strong> F1538, confer<br />
insensitivity across a range <strong>of</strong> pyrethroid structures.<br />
The resistance mechanism is corroborated when the<br />
same, or similar, mutations are found to occur in a<br />
new species; for example, the recent identification <strong>of</strong><br />
F1538I in the spider mite Tetranychus urticae<br />
(Tsagkarakou et al., 2009). Other mutation sites<br />
A1: Addendum 31<br />
have been identified that are clearly linked to the<br />
resistance phenotype but have not been functionally<br />
characterized by in vitro assay; for example, V1016I<br />
in Aedes aegypti (Saavedra-Rodriguez et al., 2007).<br />
A note <strong>of</strong> caution, however, is that, not all reported<br />
mutations have been so clearly linked to pyrethroid<br />
resistance <strong>and</strong> some may represent natural polymorphisms<br />
within the sodium channel sequence.<br />
Also, secondary processing events known to generate<br />
developmental <strong>and</strong>/or tissue-specific diversity <strong>of</strong> mature<br />
sodium channel is<strong>of</strong>orms may also affect the level<br />
<strong>of</strong> resistance imparted by a particular mutation; for<br />
example, the expression <strong>of</strong> alternatively spliced<br />
sodium channel transcripts in resistant diamondback<br />
moth populations produced distinct channels with<br />
different sensitivities to pyrethroids (Sonoda et al.,<br />
2008). Finally, in populations with multiple resistance<br />
mechanisms, the relative contribution <strong>of</strong> each mutation<br />
to the resistance phenotype is <strong>of</strong>ten poorly defined.<br />
This has been explored in Culex mosquitoes<br />
<strong>and</strong> revealed a complex interplay between metabolic<br />
<strong>and</strong> target site resistance (Hardstone et al., 2009).<br />
A1.5. Pyrethroid–Sodium Channel<br />
Interactions<br />
One area in which there has been significant progress<br />
over the past 5 years is in the development<br />
<strong>of</strong> a detailed molecular model showing a putative<br />
binding site for pyrethroids at the insect sodium<br />
channel. This was made possible by publication <strong>of</strong> a<br />
series <strong>of</strong> high-resolution crystal structures for closely<br />
related potassium channels, including the eukaryotic<br />
voltage-gated K + channel Kv1.2 (Long et al., 2005),<br />
that provided a structural template on which to model<br />
the insect (housefly) sodium channel sequence<br />
(O’Reilly et al., 2006). Close examination <strong>of</strong> the<br />
pore region <strong>of</strong> this model revealed a long, narrow<br />
cavity between the IIS4-S5 linker, IIS5 helix <strong>and</strong><br />
IIIS6 helix that could accommodate a range <strong>of</strong> pyrethroid<br />
structures as well as DDT (see Figure A1).<br />
Several known features <strong>of</strong> pyrethroid mode <strong>of</strong> action<br />
<strong>and</strong> resistance are supported by this model (O’Reilly<br />
et al., 2006), including (1) a hydrophobic cavity accessible<br />
to lipid-soluble pyrethroids, (2) the observation<br />
that the binding site is formed by movement <strong>of</strong><br />
the IIS4-S5 linker towards the other two helices during<br />
channel activation, which is consistent with<br />
electrophysiological evidence that pyrethroids bind<br />
preferentially to the open state <strong>of</strong> the channel, (3)<br />
that this high-affinity binding would stabilize the<br />
open state to slow channel inactivation, consistent<br />
with the pyrethroid-induced tail currents seen following<br />
membrane repolarization, (4) the proposed binding<br />
site includes many known mutation sites in the