1 INTRODUCTION
Olfaction plays incredibly important roles for insects to search for food, mate, spawning locations and avoid predators by detecting molecule clues in their environment such as host plant volatiles, sex pheromones and alarm pheromones (Vogt, 2005; Pelosi et al., 2006; Dweck et al., 2013; Li and Liberles, 2015). During the reception of odor molecules, many key proteins are involved, including odorant binding proteins (OBPs), chemosensory proteins (CSPs), odorant receptor (ORs), odorant degrading enzymes (ODEs), ionotropic receptors (IRs), and sensory neuron membrane proteins (SNMPs) (Rützler and Zwiebel, 2005; Leal, 2013). Among these proteins, ODEs act as a rapid odorant inactivator to maintain a low stimulus level within antennal sensilla in the process of odorant signal reception (Vogt et al., 1985). Once olfactory receptor neurons (ORNs) were stimulated by odor molecules, they must be rapidly removed by ODEs to prevent the accumulation of stimulus and restore the sensitivity of the sensory neuron (Leal, 2013). There are a few insect ODEs that have been functional characterized to date, such as carboxylesterases (CXEs), cytochrome P450s, glutathione S-transferases (GSTs), UDP-glycosyltransferases (UGTs), aldehyde oxidases, epoxide hydrolases, and alcohol dehydrogenases (Vogt, 2005; Leal, 2013).
Insect carboxylesterases belong to the carboxyl/cholinesterase family, a branch of the α/β –hydrolase fold superfamily. The enzymes in this superfamily hydrolyze ester bonds, which are present in many insect and plant odors, insect pheromones and hormones as well as pesticides (Vogt, 2005; Durand et al., 2010a; He et al., 2015). The first insect ODE was an antennae-specific esterase (ApolSE) identified from the sensillar fluid of male silkworm Antheraea Polyphemus antennae (Vogt and Riddiford, 1981). It was characterized as a pheromone degrading enzyme (PDE) that can rapidly degrade sex pheromone [(6E ,11Z )-hexadecadienyl acetate, Z11-16: Ac] in vivo andin vitro (Vogt and Riddiford, 1981; Vogt et al. 1985; Ishida and Leal, 2005). The other antennae-specific pheromone-degrading esterase, PjapPDE, was identified from antennae of the Japanese beetlePopillia japonica (Ishida and Leal 2008). Kinetic studies showed that PjapPDE is involved in rapid inactivation of sex pheromone (R)-japonilure. In Drosophila , one male antennal specific expressed esterase, Esterase-6 has been proved to be an odorant-degrading enzyme involving in modulating sex pheromone response (Chertemps et al. 2012). Additionally, many antennal CXEs have been identified from the moths (Maïbèche-Coisne et al., 2004; Merlin et al. 2007; Jordan et al., 2008; Durand et al., 2010b; He et al. 2014a, b, c), honey bee (Kamikouchi et al., 2004), and fruit fly (Younus et al, 2014). Recent years, with more advanced sequencing technologies, moreCXEs were identified in olfactory organs by antennal transcriptome analysis (Merlin et al., 2007; Liu et al., 2015; Zhang et al., 2016; Sun et al., 2017; Zhang et al., 2017b). However, the molecular mechanisms of the CXEs involved in pests foraging, mating and selecting oviposition sites are not fully investigated.
The oriental fruit moth or peach moth, Grapholita molesta(Busck), is a notorious fruit pest throughout the world (Cardé et al., 1979; Zhang et al., 2018). The larvae prefer eating shoot and pulp inside of fruits, which causes extensive fruit damage and leads to serious economic losses if not controlled (Myers et al., 2007). Many odorant molecules that G. molesta uses for host plant location, oviposition and mating are ester compounds, such as (Z )-8-dodecenyl acetate (Z8-12:Ac) and (E )-8-dodecenyl acetate (E8-12:Ac) belonging to critical sex pheromone components released by female moths of G. molesta (Allred, 1995), and butyl acetate, hexyl acetate, hexyl hexanoate and butyl hexanoate that are volatiles of host plants (pear, apple and peach) (Zhao et al., 2013; Tian et al., 2018). Therefore, we hypothesize that G. molestaantennal CXEs may play important roles in plant odor or insect pheromone degradation and are potentially involved in modulating the foraging and mating behavior of G. molesta .
In the present study, putative GmolCXEs were identified and analyzed from the transcriptomic analysis of G. molesta larva heads. Then the expression patterns of GmolCXE s of adults and larvae were examined under normal feeding conditions and after exposure with odors from female moths or ripe fuji apples and crown pears. FiveGmolCXEs were selected for further functional studies by combining RNA interference (RNAi), behavior, electrophysiological and chromatographic studies. The proposed research provides a new insight into functions of odorant degrading esterases in modulating adult and larval behavior of G. molesta that potentially can be good targets to control this notorious agricultural pest.