Cusson M, Laforge M, Miller D, Cloutier C, Stoltz D
Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, Sainte-Foy, Québec, Canada.
Gen Comp Endocrinol. 2000 Mar;117(3):343-54. doi: 10.1006/gcen.1999.7406.
The parasitic wasp Tranosema rostrale transmits a polydnavirus (PDV) to its host, Choristoneura fumiferana, during oviposition. Last-instar C. fumiferana larvae parasitized by T. rostrale early in the stadium fail to undergo metamorphosis, and injection of the wasp's calyx fluid (CxF; contains PDV) into healthy caterpillars induces a dose-dependent delay in initiation of metamorphosis (D. Doucet and M. Cusson, 1996, Entomol. Exp. Appl. 81, 21-30). In the present work, parasitization and injection of CxF (0.5 female equivalent) on the first day of the last stadium both prevented the rise in hemolymph 20-hydroxyecdysone (20HE) titer observed between day 4 and day 7 in control and saline-injected larvae. Similarly, juvenile hormone esterase (JHE) activity was depressed following parasitization or CxF injection, whereas control larvae displayed a peak on day 4. However, neither parasitism nor injection of CxF on day 1 prevented the JH-producing glands from turning off during the first half of the last stadium. Likewise, low but clearly detectable JH titers were observed in the first hours following the molt but very low titers, at or near the detection limit of our radioimmunoassay, were seen in both control and parasitized larvae on day 4. Prothoracic glands showed no apparent sign of degeneration 4 days after injection of CxF but had significantly smaller cells than saline-injected larvae 7 days postinjection. It is not clear whether this was a direct effect of T. rostrale PDV. Thus, disruption of spruce budworm metamorphosis by T. rostrale CxF involves depression of 20HE titers but is not associated with a measurable increase in the level of JH, as shown for some other host-parasitoid systems. In view of the latter observation, we put forward three hypotheses regarding the functional significance of the observed suppression of JHE activity in developmentally arrested C. fumiferana larvae.
寄生黄蜂Tranosema rostrale在产卵时会将一种多DNA病毒（PDV）传播给其寄主——烟芽夜蛾（Choristoneura fumiferana）。在龄期早期被Tranosema rostrale寄生的末龄烟芽夜蛾幼虫无法完成变态，将黄蜂的萼液（CxF；含有PDV）注射到健康毛虫体内会诱导变态起始出现剂量依赖性延迟（D. 杜塞和M. 库森，1996年，《昆虫学实验与应用》81卷，21 - 30页）。在本研究中，在末龄期第一天进行寄生和注射CxF（相当于0.5只雌蜂的量）均阻止了在对照幼虫和注射生理盐水的幼虫中于第4天至第7天观察到的血淋巴20 - 羟基蜕皮酮（20HE）滴度的升高。同样，寄生或注射CxF后，幼虫的保幼激素酯酶（JHE）活性受到抑制，而对照幼虫在第4天出现活性峰值。然而，在末龄期的前半段，无论是寄生还是在第1天注射CxF都未能阻止产生保幼激素的腺体停止分泌。同样，在蜕皮后的最初几个小时观察到保幼激素滴度较低但明显可检测到，不过在第4天，对照幼虫和被寄生幼虫中的保幼激素滴度都非常低，处于或接近我们放射免疫测定的检测极限。注射CxF 4天后，前胸腺没有明显的退化迹象，但在注射7天后，其细胞明显小于注射生理盐水的幼虫。尚不清楚这是否是Tranosema rostrale PDV的直接作用。因此，Tranosema rostrale的CxF对云杉芽虫变态的干扰涉及20HE滴度的降低，但与保幼激素水平的可测量增加无关，这与其他一些寄主 - 寄生蜂系统的情况不同。鉴于后一观察结果，我们针对发育停滞的烟芽夜蛾幼虫中观察到的JHE活性抑制的功能意义提出了三个假说。
