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Beetle horns evolved from wing serial homologs

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Science  22 Nov 2019:
Vol. 366, Issue 6468, pp. 1004-1007
DOI: 10.1126/science.aaw2980

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Where do horns come from?

One of the most pronounced examples of a sexually selected trait is the prothoracic horns of scarab beetles, which, in the most extreme cases, can be nearly half as long as the length of the beetle. It is fairly easy to understand how selection might have shaped these horns, but understanding how development shaped them from a hornless ancestor is a much more complex proposition. Hu et al. show that these horns are generated from wing homologs and argue that many other insect traits may have followed similar transcriptional paths (see the Perspective by Nijhout).

Science, this issue p. 1004; see also p. 946

Abstract

Understanding how novel complex traits originate is a foundational challenge in evolutionary biology. We investigated the origin of prothoracic horns in scarabaeine beetles, one of the most pronounced examples of secondary sexual traits in the animal kingdom. We show that prothoracic horns derive from bilateral source tissues; that diverse wing genes are functionally required for instructing this process; and that, in the absence of Hox input, prothoracic horn primordia transform to contribute to ectopic wings. Once induced, however, the transcriptional profile of prothoracic horns diverges markedly from that of wings and other wing serial homologs. Our results substantiate the serial homology between prothoracic horns and insects wings and suggest that other insect innovations may derive similarly from wing serial homologs and the concomitant establishment of structure-specific transcriptional landscapes.

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