In this study, the major sensory genes (i.e., CSPs, GRs, IRs, OBPs, ORs and SNMPs), which perhaps regulate Q. mendeli to locate its host L. invasa, are first reported, providing valuable information for exploring how parasitoids use sensory genes to locate gall-making pests.
CSPs are widespread in the antenna and other chemical sensory organs of insects38 and are involved in the chemical perception and related behavior of insects39. Compared to the total number of insects in the world, CSPs have been identified in only a few species of insects to different degrees, such as Coleoptera40, Hemiptera41, Hymenoptera12, Lepidoptera42 and Orthoptera43, whose numbers show interspecific diversity. For example, the number of CSPs varies from 4 CSPs in Drosophila melanogaster to 22 in B. mori44. In this study, 3 candidate CSPs were identified, which are less than those in the parasitoids Ch. cunea (11), Sclerodermus sp. (10), Me. pulchricornis (8) and Tric. dendrolimi (7)2,4,12,16 and are greater than those in the parasitoids Cot. chilonis (2) and Ap. ervi (2)6,11. Previous research confirmed that generalists seem to be specifically suited for the processing of odorant mixtures, and they respond in a similar manner to plant volatiles45. For example, Ch. cunea is a generalist and has multiple hosts (e.g., Stilpnotia salicis L., Ivela ochropoda Eversmann, Clostera anachoreta Fabricius, Semiothisa cinerearia Bremer & Gray and Clania variegeta Snellen), while Cot. chilonis mainly parasitizes larvae of the genus Chilo Zincken46,47. It can be deduced that the number of CSPs relates to their host range. For the specialist Q. mendeli, the use of CSPs is expected to cope with the variability in host availability44. Previous studies also revealed that CSPs could be involved in the solubilization of hydrocarbons in the stratum corneum to recognize its companion48. Therefore, CSPs in Q. mendeli should be associated with its obligate parasitic characteristics, indicating that QmenCSPs may function in the chemical sensing of L. invasa and its shelter host eucalyptus trees. A similar story has been confirmed for M. mediator, which can accurately find and then parasitize its hidden host Agrotis segetum Denis and Schiffermüller49. Interestingly, MmedCSP1 had a strong reaction with methyl salicylate, pentane, ocimene, β-ionone, 3,4-dimethylbenzaldehyde, 2-hexanone and cis-3-hexe-1-ol, suggesting that MmedCSP1 can function in chemical sensing of the plant volatiles of M. mediator’s host A. segetum49. Our results showed that QM_comp26540 is in the same clade as MmedCSP1, meaning that QM_comp26540 has the closest relationship with MmedCSP1. Encouragingly, significant GC-EAD responses of Q. mendeli antenna to eucalyptus volatile α-phellandrene and 1,8-cineole were observed50. Thus, QM_comp26540 may function as a chemosensor, which is involved in the process of recognizing plant volatiles from eucalyptus when Q. mendeli searches its host, L. invasa.
GRs are widespread in gustatory organs of insects that respond to various taste-related soluble compounds, and cuticular hydrocarbons and odorants, such as sugars, amino acids, salts, bitter compounds, CO2 and pheromones, can be recognized and combined by GRs51,52. To date, GRs in some model insects with genome reports have been identified, such as A. gambiae (76), B. mori (69), D. melanogaster (68) and N. vitripennis (58)53,54. In this study, 10 candidate GRs were identified, which was similar to other parasitoids, such as An. japonicus (8)13, Sclerodermus sp. (6)2 and M. mediator (6)8. This could be attributed to the sequencing depth and species-functional specificity of GRs51. DmelGR5 and DmelGR64 in D. melanogaster are receptor proteins for sweet taste and are used to detect glucose, sucrose, maltose, maltitol and cottonseed sugar55. For Q. mendeli, females that were fed a honey solution or honey solution + young eucalyptus leaves lived for a longer time than those who underwent other treatments, including flowers, gall leaves, water, galled leaves + honey solution, no food and young leaves23. Therefore, GRs in Q. mendeli should play a key role in recognizing sugar and fresh eucalyptus leaves via various soluble compounds56. Interestingly, QM_comp11847 was in the same clade as the sugar receptor NvitGR1, which was used to recognize the only source of nutrients from host Lucilia caesar L. for the offspring of N. vitripennis57,58. Thus, QM_comp11847 may be involved in recognizing host organisms and sugars, which helps Q. mendeli to quickly access energy from these molecules.
IRs, which evolve from the ionotropic glutamate receptor (iGluR), are a new class of sensory proteins mainly in taste organs/sensilla that respond to food components, such as sugars, salts, water and bitter compounds, and detect small temperature differences59–61. In this study, 21 candidate IRs were identified, which was more than in the parasitoids Me. pulchricornis (19)12, Ch. cunea (10)4, M. mediator (6)8, Sclerodermus sp. (3)2, Mi. cingulum (3)9 and An. japonicus (3)13. Physiological recordings from taste sensilla in D. Melanogaster and other insects have revealed responses of taste neurons to salts, sugars, water, bitter compounds and a large diversity of other tastants59,60. Taste sensilla are widely distributed on the antennae of Q. mendeli28, suggesting that QmenIRs may function as taste receptors. The phylogenetic tree showed that QmenIRs were spread across the tree branches and clustered with homologous IRs from other species, which suggested that QmenIRs may be functionally conserved. QM_comp21031 was located in the same clade as BmorIR21a of B. mori, DmelIR21a of D. melanogaster and TcasIR21a of Trib. castaneum, indicating that QM_comp21031 has the closest relationship with insect IR21a, which can mediate cool sensing in Drosophila62. Thus, QM_comp21031 may perceive changes in temperature since insect IR21a can achieve both heat avoidance and heating63,64. Taking the oviposition features into consideration, we deduced that female Q. mendeli may be capable of sensing surface heat on galls related to L. invasa damage, which requires further exploration.
OBPs are crucial in insect olfactory perception and are the first step in the recognition of chemical stimuli from the outside environment3. In some model insects, OBPs have been identified to different degrees, such as B. mori (57)65, D. melanogaster (51)66, Trib. castaneum (46)67 and A. gambiae (44)68. In this study, 56 candidate OBPs were identified, which was more than in the parasitoids Ae. bambawalei (54)14, Ch. cunea (25)4, Tric. dendrolimi (24)16, M. mediator (20)8, Me. pulchricornis (16)12, T. japonicum (15)3, Ap. ervi (15)11, Sclerodermus sp. (10)2, Cop. floridanum (8)15, Cot. chilonis (8)6 and As. hispinarum (8)17. The number of Q. mendeli OBPs identified was less than that in the parasitoid Cot. vestalis (74)5. Previous studies revealed that OBPs in parasitoids play a key role in binding and transporting hydrophobic odorants from the environment to sensory receptors69. In Q. mendeli, a significant behavioral response to the gall volatiles D-limonene and decanal was observed (unpublished data). Relevant QmenOBPs function in chemical sensing of these volatiles characterizing L. invasa and its shelter host eucalyptus trees, which bioinformatics analysis could help to target. Our results showed that QM_comp21238 is the same clade as MmedOBP10 of M. mediator, which is involved in the process of recognizing β-ionone and Nonanal when they find the location of their hidden host A. segetu49,70. Thus, QM_comp21238 may also be involved in the process of recognizing similar odorants or ligands when Q. mendeli locates its shelter host, L. invasa.
ORs are thought to play critical roles in the perception of chemosensory stimuli by insects52. The number of ORs in parasitoids vary greatly1,4. In this study, 30 candidate ORs were identified, which was more than in the parasitoid Tric. dendrolimi (9)16, Mi. cingulum (9)9 and Sclerodermus sp. (8)2. Previous studies revealed that the OR of M. mediator play an important role in recognizing plant volatiles, such as nonanal and farnesene, which provided a key start to manipulate and develop ORs in wasps to find hosts and use them as biological tools for pest control71. The RNAi investigation of the role of MmedOrco, the M. mediator ortholog of Drosophila Or83b, supported the assumption that this highly conserved gene plays a similar role in insects71,72. QmenORs may function as chemoreceptors to recognize plant volatiles from eucalyptus. Our results showed that QM_comp20892 is in the same clade as DmelOR10a of D. melanogaster, which plays a role in responding to odorants such as methylsalicylate and acetophenone73,74. For Q. mendeli, QM_comp20892 may be involved in the process of recognizing similar odorants or ligands.
SNMPs are involved in cellular signal transduction and play a role in odor detection5. Two SNMPs are normally broadly identified in different insects, e.g., parasitoids Ch. cunea and Sclerodermus sp.2,4. It has been reported that SNMP1 and SNMP2 are both expressed in antennae sensilla and have different expression patterns4,75. In Ch. cunea, CcunSNMP1 is a morphine receptor of neurons, and CcunSNMP2 is mainly expressed in supporting cells and the lymph of antennal sensilla4, while the location and expression patterns of SNMPs in Q. mendeli should be further studied since this information should be associated with their functions. SNMP1 of D. melanogaster is involved in pheromone detection and enhances the Ca2+ responses served in signal transduction76. SNMP1 of M. mediator was determined to participate in both pheromone and general odor detection77. In contrast, the general functional mechanism of SNMP2 in parasitoids is still poorly understood. In addition, Q. mendeli is a uniparental parasitoid that is not required in the male search for mating22. Thus, the function of QmenSNMPs may include an oviposition pheromone receptor rather than a sex receptor and a membrane protein with unknown functions, which needs to be further explored.
Chemical detection involves a series of complicated processes that require participation and interactions by multiple cascades of sensory proteins. Insect sensory proteins are capable of functional cooperation and division. First, OBPs and CSPs are both chemically binding proteins to various odorants and can also respond to the same chemicals, e.g., MmedOBP10 and MmedCSP1 are involved in the process of recognizing β-ionone and nonanal when they find the location of their hidden host A. segetu49,70. Second, ORs and IRs are both chemoreceptors, while there are differences in the process of recognizing odor substances78. For example, IRs are better at detecting long-lasting odor pulses, and they are less sensitive, suggesting that they are better at close-range odor detection. In contrast, ORs are more sensitive and better at resolving brief (low molecular flux) pulsed stimuli78,79. Moreover, features of functional organization have emerged between behavioral response profiles of OBPs and electrophysiological response profiles of ORs73. Therefore, the sensory genes in Q. mendeli should systematically act on the process of locating their gall-making host, and the biological functions of these genes and their products are still poorly known. Overall the overall sensory genes of the wasp reported here provide valuable insight into the molecular mechanisms of olfaction, which help pave the way for the host location of Q. mendeli in gall-making pests.