Institute of Evolutionary Biology (CSIC-Universitat Pompeu Fabra), Passeig Maritim 37, 08003 Barcelona, Spain.
Philos Trans R Soc Lond B Biol Sci. 2019 Oct 14;374(1783):20180415. doi: 10.1098/rstb.2018.0415. Epub 2019 Aug 26.
The three modes of insect postembryonic development are ametaboly, hemimetaboly and holometaboly, the latter being considered the only significant metamorphosis mode. However, the emergence of hemimetaboly, with the genuine innovation of the final moult, represents the origin of insect metamorphosis and a necessary step in the evolution of holometaboly. Hemimetaboly derives from ametaboly and might have appeared as a consequence of wing emergence in Pterygota, in the early Devonian. In extant insects, the final moult is mainly achieved through the degeneration of the prothoracic gland (PG), after the formation of the winged and reproductively competent adult stage. Metamorphosis, including the formation of the mature wings and the degeneration of the PG, is regulated by the MEKRE93 pathway, through which juvenile hormone precludes the adult morphogenesis by repressing the expression of transcription factor E93, which triggers this change. The MEKRE93 pathway appears conserved in extant metamorphosing insects, which suggest that this pathway was operative in the Pterygota last common ancestor. We propose that the final moult, and the consequent hemimetabolan metamorphosis, is a monophyletic innovation and that the role of E93 as a promoter of wing formation and the degeneration of the PG was mechanistically crucial for their emergence. This article is part of the theme issue 'The evolution of complete metamorphosis'.
表变态、不全变态和全变态，后者被认为是唯一重要的变态模式。然而，真正的最后一次蜕皮的出现标志着半变态的起源，也是全变态进化的必要步骤。半变态源于表变态，可能是由于早期泥盆纪的翼龙类昆虫的翅膀出现而产生的。在现存的昆虫中，最后一次蜕皮主要是通过前胸腺（PG）的退化来实现的，在此之后，昆虫会进入具有翅膀且具有生殖能力的成虫阶段。变态过程，包括成熟翅膀的形成和 PG 的退化，是由 MEKRE93 途径调控的，通过该途径，保幼激素通过抑制转录因子 E93 的表达来阻止成虫形态发生，从而触发这种变化。MEKRE93 途径在现存的变态昆虫中似乎是保守的，这表明该途径在翼龙类的最后共同祖先中就已经存在了。我们提出，最后一次蜕皮以及随之而来的半变态变态是一个单系创新，而 E93 作为翅膀形成和 PG 退化的促进者的作用，对于它们的出现具有机械上的重要性。本文是主题为“完全变态的进化”的一部分。
