Comment on Nature's new cover paper


The cover paper of this week's nature (Body plan innovation in treehoppers through the evolution of an extra wing-like appendage) raised a big question to developmental biology: although the body plans of insect are pretty stable, there are exceptions.
For most insects, the first thoracic segment does not grow any appendage. However, treehopper does. It develops a wing-like structure on this segment.
By checking the expression patterns of several wing-specific genes, the authors found the appearance of wing-like structure on the first thoracic segment in treehopper is due to a loss of function for the repression of appendage genes by a regulatory gene. In other words, the repression is removed and thus the potential of growing appendage is released. The authors further conclude that this serves as an evidence that the stability of dorsal appendage formation pattern in insects is not due to developmental constraint but to selection.
I argue that their last conclusion is likely but not solid. There are two reasons. 1) Just finding the regulation pattern of several genes correlating with appendage formation does not mean cause-and-effect relationship. Thus, there might be other important changes also contributing to the peculiarity of treehopper. With a set of mutations in a system-level that determine the appearance of the wing-like structure in the first thoracic segment, we cannot say there is no developmental constraint. 2) In order to prove that the peculiarity is due to selection, an ecological and/or behavioral study is needed to find what is special about treehopper among other insects that makes the wing-like structure in the first thoracic segment adaptive.
In conclusion, I am not convinced by their argument about selection over development constraint as the reason for stable dorsal appendage formation pattern in insects.

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