| Diadegma semiclausum (Hell(?)n) (Hymenoptera:Ichneumonidae) is an important naturalenemy of the diamondback moth,Plutella xylostella L.(Lepidoptera:Plutellidae) in manyregions of the world.The physiological effects of parasitism by this endoparasitoid on itshost were studied in this dissertation.The major results are summarized as follows:1 Immune change of P.xylostella larvae parasitized by D.semiclausumNecrosis ofprohemocytes in hematopoietic organ was observed while cell death wasabsent in the mature hemocytes in the parasitized larvae,which was related to the declinedtotal hemocyte count per microliter (THC).THC in the host hemolymph declined sharply by12 h post-parasitization and then remained at a low level.When hemocvtes of the parasitizedlarvae were cultured in vitro,encapsulation ability was suppressed coincidently with theinhibited spreading ability and depressed melanization;but phagocytosis ability of thehemocytes was not affected.After 24 hours,spreading,encapsulation and melanization ofthe hemocytes in the parasitized host larvae could be normally occurred;however.phagocytosis ability was significantly inhibited.In the aspect of humoral immune system inthe parasitized host larvae,activation of the prophenoloxidae was inhibited,and thebacteriostatic ability of hemolymph was decreased.2 Effect of parasitism by D.semiclausum on the activities of detoxification system andprotective enzymes of its host larvaThe detoxification enzymes of host larva were altered by parasitism,resulting in (1) theactivity of glutathione -s- transferase in the host larvae was increased after parasitization;(2)activities of mixed-functionoxidases or carboxylesterase were not significantly affected;(3)activity of acetylcholinesterase was decreased.The protective enzymes of host larva were also altered by parasitism,and the resultswere as follows:(1) activity of superoxide dismutase was decreased to the half level of thecontrol,however,this enzyme was reactivated after 12 hours post parasitism;(2) activity ofcatalase was not affected;and (3) activity of peroxisome was significantly altered.3 Effect of parasitism by D.semiclausum on the host larval nutrient metabolism Hypertrehalosemia was observed in P.xylostella larvae parasitized by either C.vestalisor D.semiclausum.Results of our experiments showed that it was caused by theaccelerated degradation of glycogen in the fat body and probably by lipid mobilization inthe larvae parasitized by C.vestalis.However,increased trehalose in the larvae parasitizedby D.semiclausum accompanied by decreased accumulation of glycogen in the fat bodiesand unchanged lipid concentration.Additionally,it is demonstrated that neither glycogenphosphorylase nor trehalase was responsible for elevated trehalose in the parasitizedlepidopteran larvae.Moreover,protein content in the hemolymph was significantly affected by parasitism,and the balance among the proteinases in the midguts was disrupted.Activities oftrypsinase/BApNAase/TAMEase in the parasitized larvae were drastically changed;andfurthermore,sensitivities of these three enzymes to pH were also altered.Effect of parasitism by D.semiclausum on the host lipid metabolism was mainlyhappened in the late host larval and prepupal stages.The late 4th instar larvae began toaccmulate substance in fat body,and the weight of fat body was gradually increased;however,the fat body weight of the parasitized larvae in the same stages showed adecreasing trend.Change of the amount of total lipid in D.semiclausum was significantlycorrelated with that of host larval haemolymph (P = 0.2976 H + 4.9877,r~2= 0.9824;P:total lipid in D.semiclausum larva,H:total lipid in P.xylostella larval hemolymph) andpre-pupal fat body (P = 476.61-56.705Ln(F),r~2=0.9887;P:total lipid in D.semiclausumlarva,H:total lipid in P.xylostella prepupal fatbody).Profiles of organic compound fromparasitized/non-parasitized larval haemolymph/fat body,and parasitoid larva were similarwith that from parasitzed pre-pupal fat bodies;however,that from non-parasitizedpre-pupal fat bodies was different.4 Parasitism-induced effects on metabolic efficiency and host growth in P.xylostellalarvae parasitized by D.semiclausumParasitization by C.vestalis resulted in significant decrease in the rates of growth,feeding,excretion,assimilation,and respiration,but the final dry rate of respiration at theend of last larval stadium was elevated.The ECI (.percent efficiency of conversion of ingested food to body substance) and ECD (percent efficiency of conversion of digestedfood to body substance) were also reduced as the result of parasitization,but digestibilitywas increased.All these parameters in the larvae parasitized by D.semiclausum at 24 hpost-parasitization were changed much less significantly.In spite of the similarities of theparasitism-induced effects caused by these endoparasitoids,the final metabolic rate,that is,the rate of intake of nutrients required to compensate for metabolism,was much lower inthe larvae parasitized by C.vestalis than that in the larvae parasitized by D.semiclausum.5 Changes in some organ of the host larva parasitized by D.semiclausumAfter parasitized by C.vestalis,structures of some organs in host larvae were altered,forexample,cell death was happened in the cells,which could be developed into secretory cells,of the silk gland before the cellular differentiation was completed.Some ultrastrcuturalchanges in neural cells in the brain were presented,i.e.,tumefied/abnormal karyon,alteredelectronica density,disappeard cellular membrane,cytoplasmic degradation.Futhermore,gap between cells became larger,cellular conjunction disappeared,synaptic vesicles weredisrupted,and accumulation of the neurosecretion was decreased.However,changesmentioned above were not so significant in the P.xylostella larvae parasitized by D.semiclausum.6 Individual development of D.semiclausum:oogenesis with associated polydnavirus,external morphology and development of immature stagesThe egg of D.semiclausum is hymenopteriform,lacking appendages.The larvae appearto be hypermetamorphic with four instars.The pupal period has two distinct phases:(a)phase I:appears larval-like with remarkable groups of urate cells;(b) phase II:exarate pupalappearance with a pigmentation sequence of eyes,ocelli,ma ndibles,thorax,abdomen,andantennae.D.semiclausum female was found to carry a kind of PDVs (DsIV) as its majorparasitoid-associated factors beside venom for the first time.Protein content,most of whichwas ranged from 35 kDa to 220 kDa.in each venom reservior was 1.016μg.DslVs weretypical ichneumonid polydnaviruses with the following characteristics:( 1 ) the virus particles were assembled in the nuclei of calyx cells where they acquire an initial (inner) membrane,then migrate through the cytoplasm and budd out into the lumen of the ovary,and at thistime they acquire a second envelope (outer membrane);(2) the particle,which had adiameter of 130-150 nm with a length of 300-400 nm,has a genome comprising severalDNA segments.Ovary of female D.semiclausum had 12 orarioles,and belonged to the polytrophic type.Ultrastructural observations demonstrated that several cells existed in a gametangium,andone of the central cells of the cluster differentiated into an oocyte while the remainingbecame nurse cells.Germ cells continued to develop in the vitellarium until mature;howeverthe nurse cells were in gradual degradation simultaneously.The calyx region was emptywhen the adult was just emerged;and after 2-3 days,mature eggs began to enter the calyxregion and surrounded by calyx fluid but without DsIVs attached. |
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