Wache S, Jones D
Graduate Center for Toxicology, Chandler Medical Center, University of Kentucky, Lexington, USA
J Insect Physiol. 1998 Sep;44(9):755-765. doi: 10.1016/s0022-1910(98)00008-0.
In normally regulated larval metamorphosis of Trichoplusia ni, a 4th, 5th or other numbered instar is a 'preultimate' instar, and will normally continue larval molting, if the larva has not yet surpassed the critical (minimal) size threshold corresponding to attainment of the 'ultimate' (metamorphic) instar. Natural injection of T. ni embryos with venom/calyx fluid of female Chelonus sp. near curvimaculatus caused 'penultimate' 4th or 5th instar larvae that would normally molt at least once more, to a 5th/6th instar, to instead precociously metamorphose without another larval molt. These effects were observed in naturally-injected insects that never contained either a parasite larva, a viable parasite embryo, or a parasite egg. These data demonstrate that this effect of venom/calyx fluid of this wasp to induce precocious metamorphosis, at an instar earlier than would otherwise have typically occurred under normal growth conditions, does not require the presence of a parasite larva. Other data did indicate the parasite larva contributes an additional effect that causes a 4th instar host (that from its size would normally require not just one, but at least two more larval molts to reach the metamorphic instar) to not grow to the size metamorphic threshold, but to instead, precociously metamorphose at an even smaller size than occurs with the venom/calyx fluid alone. Additionally, arylphorin was precociously highly expressed in parasitized hosts in a manner independent of a decline in the host JH titer. Therefore, the main target of the venom/calyx fluid activity to induce precocious metamorphosis appears to be an event upstream of the decline in JH production by the corpora allata. Pseudoparasitized hosts become developmentally arrested as precocious prepupae and express a 2.7kb polydnavirus transcript. The larger (but still subthreshold size) larvae showed less suppressed prepupal ecdysteroid titers, less developmental suppression, and a much weaker expression of that transcript. A general model for mechanisms of action of chelonine venom/calyx fluid, and larvae, to cause precocious host metamorphosis and suppressed prepupal development is presented that is based on the current 'size threshold' model of normal lepidopteran development, rather than the older, displaced 'instar count' model. By basing the model for chelonine regulation of host development on the current 'size threshold' model for normal development, the proposed model for chelonine action both accounts for observations reported on various species of that subfamily and makes useful, testable predictions.
在粉纹夜蛾正常调控的幼虫变态过程中，第4龄、第5龄或其他编号龄期是“次终龄”，如果幼虫尚未超过与达到“终龄”（变态龄期）相对应的临界（最小）大小阈值，通常会继续进行幼虫蜕皮。用近弯斑 Chelonus 属雌性的毒液/萼液自然注射粉纹夜蛾胚胎，会使通常至少再蜕皮一次至第5/6龄的“倒数第二”第4或第5龄幼虫，反而早熟变态，不再进行另一次幼虫蜕皮。在从未含有寄生虫幼虫、活寄生虫胚胎或寄生虫卵的自然注射昆虫中观察到了这些效应。这些数据表明，这种黄蜂的毒液/萼液诱导早熟变态的作用，即在比正常生长条件下通常发生的龄期更早的龄期诱导早熟变态，并不需要寄生虫幼虫的存在。其他数据确实表明，寄生虫幼虫会产生额外的效应，导致第4龄宿主（从其大小来看，正常情况下不仅需要一次，而且至少需要两次以上幼虫蜕皮才能达到变态龄期）无法生长到变态大小阈值，而是在比仅用毒液/萼液时更小的大小就早熟变态。此外，芳基脂蛋白在被寄生宿主中早熟且高度表达，其方式与宿主保幼激素滴度的下降无关。因此，毒液/萼液诱导早熟变态活性的主要靶点似乎是咽侧体产生保幼激素下降上游的一个事件。假寄生宿主会发育停滞成为早熟预蛹，并表达一种2.7kb的多角体病毒转录本。较大（但仍低于阈值大小）的幼虫显示出预蛹蜕皮甾体滴度受抑制程度较低、发育抑制较轻，且该转录本的表达也弱得多。基于当前鳞翅目正常发育的“大小阈值”模型，而不是较旧的、已被取代的“龄期计数”模型，提出了一个关于 Chelonus 毒液/萼液和幼虫导致宿主早熟变态及预蛹发育受抑制作用机制的通用模型。通过将 Chelonus 对宿主发育调控的模型基于当前正常发育的“大小阈值”模型，所提出的 Chelonus 作用模型既解释了该亚科各种物种报道的观察结果，又做出了有用的、可检验的预测。
