Host Location Mechanism Of Sclerodermus Sp., A Parasitoid Of Monochamus Alternatus Hope

Parasitic wasps form a unique mechanism of searching, finding and attacking hosts in the long-term coevolution. Infochemicals play an important role in that process. The parasitoid wasp, Sclerodermus sp.(Hymenoptera: Bethylidae), which was recently identified in Yunnan Province in China, is an obligatory ectoparasite that preys on the larvae of the Japanese pine sawyer beetle, Monochamus alternatus Hope(Coleoptera: Cerambycidae), a major vector of the pine wood nematode, Bursaphelenchus xylophilus(Steiner and Buhrer) Nickle and gets strong control capability. It will provide a basis for better using Sclerodermus sp. to control M. alternatus if we can understand the mechanism of host location of Sclerodermus sp. However, how to locate their hosts and the mechanism of olfactory reception of Sclerodermus sp. remains unclear. Therefore, we firstly used scanning electron microscopy to examine the external morphology of the antennal sensilla and identified the infochemicals in host location of Sclerodermus sp. Then, we constructed the antennal transcriptome of Sclerodermus sp. and determined the function of SspOBPs in the olfactory reception. The main results are as follows: 1. Ultrastructural observation of antennal sensilla of Sclerodermus sp.Antennae of females and males Sclerodermus sp. comprised the scape, pedicel and 11 flagellomere segments. Based on the morphology of the sensilla in each sex, seven types of sensillum were identified: sensilla trichodea(Tr.1, Tr.2 and Tr.3), sensilla basiconica(Ba.1, Ba.2, and Ba.3), sensilla styloconica(St.1 and St.2), sensilla placodea(Pl), sensilla coeloconica(Co), sensilla squamiforma(Sq), and Bohm’s bristles(B.b). Tr.2, Ba.1, and St.1 were only found in females, whereas Ba.2, Ba.3, and St.2 were only observed in males. The other sensilla were identified in both males and females. 2. Infochemicals in host location of Sclerodermus sp.The GC-EAD tests showed that nine compounds from volatiles of masson pine infested by the larvae of M. alternatus elicited strong EAG responses to Sclerodermus sp., including 2-bornene or 2,7,7-trimethyl bicyclo[2.2.1]hept-2-ene, α-pinene, β-pinene, γ-terpinene, tetramethyl-benzene, pinocamphone, α-longipinene, D-germacrene, and an unknown compound. The H-tube olfactometer tests showed that both α-longipinene and γ-terpinene had significant attraction effect for Sclerodermus sp. 3. Antennal transcriptome and odorant binding protein expression profiles of Sclerodermus sp.The analysis of the transcriptome yielded 51,830,552 clean reads. A total of 46,269 unigenes were assembled, among which 29,582 unigenes exhibited significant similarity(E-values ≤ 10-5) to sequences in the NCBI nonredundant protein database. Gene ontology(GO) and cluster of orthologous groups(COG) analyses were used for the functional classification of these unigenes. We identified ten odorant binding proteins(OBPs), ten chemosensory proteins(CSPs), eight olfactory receptors(ORs), three ionotropic receptors(IRs), six gustatory receptors(GRs), and two sensory neuron membrane proteins(SNMPs).A q-PCR analysis was conducted of the expression of the SspOBPs from Sclerodermus sp. in different tissues and development stages. The highest levels of SspOBP5, SspOBP6, SspOBP7, and SspOBP9 expression were observed in the antennae of adult females. The highest levels of SspOBP1, SspOBP2, and SspOBP4 expression were observed in the abdomen of winged females. The highest levels of SspOBP3 and SspOBP10 expression were observed in larvae and pupae, respectively, whereas SspOBP8 transcripts was expressed at high levels in both larvae and pupae. 4. The binding property of SspOBP7 of Sclerodermus sp.The binding property of SspOBP7 with 18 kinds of host plant volatiles was measured at neutral(pH=7.4) and acidic(pH=5.0) environment using competitive fluorescence binding assay. At acidic environment, although SspOBP7 showed weak binding affinities with most compounds, SspOBP7 showed high binding affinities with butylated hydroxytoluene only. At neutral environment, SspOBP7 showed high binding affinities with six compounds, including S-(-)-limonene, terpinolene,(+)-β-pinene, myrcene,(-)-caryophyllene oxide, and butylated hydroxytoluene. In comparison, SspOBP7 showed much higher binding affinities with five compounds at neutral than at acidic environment, including S-(-)-limonene, terpinolene,(+)-longifolene,(-)-isolongifolene, 2-methoxy-4-vinylphenol.kosher.