Nijhout H F., Kremen C
Department of Zoology, Duke University, Durham, USA
J Insect Physiol. 1998 Mar;44(3-4):287-296. doi: 10.1016/s0022-1910(97)00121-2.
When final (5th) instar larvae of Precis coenia were treated with the juvenile hormone analog (JHA) methoprene, they underwent a supernumerary larval molt, except for certain regions of their imaginal disks, which deposited a normal pupal cuticle. Evidently those regions had already become irreversibly committed to pupal development at the time JHA was applied. By applying JHA at successively later times in the instar, the progression of pupal commitment could be studied. Pupal commitment in the proboscis, antenna, eye, leg and wing imaginal disks occurred in disk-specific patterns. In each imaginal disk there were distinct initiation sites where pupal commitment began during the first few hours of the final larval instar, and from which commitment spread across the remainder of the disk over a 2- to 3-day period. The initiation sites were not always located in homologous regions of the various disks. As a rule, pupal commitment also spread from imaginal disk tissue to surrounding epidermal tissue. The regions of pupal commitment in all disks except those of the wings, coincided with the regions of growth of the disk. Only portions of the disk that had undergone cell division and growth underwent pupal commitment. Shortening the growth period did not prevent pupal commitment in the wing imaginal disk, indicating that, in this disk at least, a normal number of cell divisions was not crucial in reprogramming of disk cells for pupal cuticle synthesis. The apparent growth spurt of imaginal disks that occurs during the last part of the final larval instar is merely the final stage of normal and constant exponential growth. Juvenile hormone (JH) and ecdysteroids appeared to play little role in the regulation of normal imaginal disk growth. Instead, growth of the disks may be under intrinsic control. Interestingly, even though endogenous fluctuation in JH titers do not affect imaginal disk growth, exogenous JHA proved able to inhibit both pupal commitment, cell movement, and growth of the disks during the last larval instar. This function of JH could be important under certain adverse conditions, such as when metamorphosis is delayed in favor of a supernumerary larval molt.
用保幼激素类似物（JHA）烯虫酯处理玫彩艳凤蝶末龄（第5龄）幼虫时，它们会进行额外的幼虫蜕皮，但成虫盘的某些区域除外，这些区域会形成正常的蛹表皮。显然，在施用JHA时，这些区域已经不可逆转地进入了蛹发育阶段。通过在龄期内依次延迟施用JHA，可以研究蛹发育的进程。喙、触角、眼、腿和翅成虫盘中的蛹发育遵循特定于盘的模式。在每个成虫盘中，都有不同的起始位点，在末龄幼虫的最初几个小时内蛹发育开始于此，并在2至3天的时间内从这些位点扩展到盘的其余部分。起始位点并不总是位于不同盘的同源区域。通常，蛹发育也从成虫盘组织扩散到周围的表皮组织。除翅成虫盘外，所有成虫盘中蛹发育的区域与盘的生长区域一致。只有经历了细胞分裂和生长的盘的部分区域才会进行蛹发育。缩短生长期并不能阻止翅成虫盘中的蛹发育，这表明，至少在这个盘中，正常数量的细胞分裂对于将盘细胞重新编程以合成蛹表皮并不关键。在末龄幼虫最后阶段出现的成虫盘明显的生长突增仅仅是正常且持续的指数生长的最后阶段。保幼激素（JH）和蜕皮甾体似乎在正常成虫盘生长的调节中作用不大。相反，盘的生长可能受内在控制。有趣的是，尽管JH滴度的内源性波动不影响成虫盘生长，但外源性JHA在末龄幼虫期能够抑制蛹发育、细胞移动和盘的生长。在某些不利条件下，例如当变态延迟以利于额外的幼虫蜕皮时，JH的这种功能可能很重要。
