Although the mechanisms that control growth are now well understood, the mechanism by which animals assess their body size remains one of the great puzzles in biology. The final larval instar of holometabolous insects, after which growth stops and metamorphosis begins, is specified by a threshold size. We investigated the mechanism of threshold size assessment in the tobacco hornworm, Manduca sexta. The threshold size was found to change depending on the amount of exposure to poor nutrient conditions whereas hypoxia treatment consistently led to a lower threshold size. Under these various conditions, the mass of the muscles plus integuments was correlated with the threshold size. Furthermore, the expression of myoglianin (myo) increased at the threshold size in both M. sexta and Tribolium castaneum. Knockdown of myo in T. castaneum led to larvae that underwent supernumerary larval molts and stayed in the larval stage permanently even after passing the threshold size. We propose that increasing levels of Myo produced by the growing tissues allow larvae to assess their body size and trigger metamorphosis at the threshold size.

尽管现在已经很好地了解了控制生长的机制，但是动物评估其体形的机制仍然是生物学中的一大难题。整体代谢昆虫的最后幼虫龄期，此后停止生长并开始变态，由阈值大小指定。我们调查了烟草天蛾曼杜卡六倍体中阈值大小评估的机制。发现阈值大小会根据暴露于不良营养条件下的数量而变化，而低氧治疗始终导致阈值大小降低。在这些各种条件下，肌肉的质量加上外皮的大小与阈值大小相关。此外，myoglianin（myo）在M. sexta和Tribolium castaneum中都在阈值大小时增加。敲除肌在赤拟谷盗导致幼虫后行编外幼虫蜕皮和永久甚至通过阈值大小后留在幼虫阶段。我们建议，由生长中的组织产生的增高的Myo水平可使幼虫评估其体型并在阈值处触发变态。