1. Division of Biology, Chalmers Hall, Kansas State University, Manhattan, Kansas 66506, USA
2. Department of Genetics, Washington University School of Medicine, 4566 Scott Avenue, St Louis, Missouri 63110, USA
3. Present address: Program in Molecular Biology and Biotechnology, The University of North Carolina at Chapel Hill, Campus Box 3280 - Fordham Hall, Chapel Hill, North Carolina 27599-3280, USA

Correspondence to: Yoshinori Tomoyasu¹ Correspondence and requests for materials should be addressed to Y.T. (Email: tomoyasu@ksu.edu).
The sequences are available in GenBank under the following accession numbers. Tc-sal: AY600513; Tc-iro: AY600514; Tc-omb: AY600516.

Abstract

The two pairs of wings that are characteristic of ancestral pterygotes (winged insects) have often undergone evolutionary modification. In the fruitfly, Drosophila melanogaster, differences between the membranous forewings and the modified hindwings (halteres) depend on the Hox gene Ultrabithorax (Ubx). The Drosophila forewings develop without Hox input, while Ubx represses genes that are important for wing development, promoting haltere identity^{1, 2}. However, the idea that Hox input is important to the morphologically specialized wing derivatives such as halteres, and not the more ancestral wings, requires examination in other insect orders. In beetles, such as Tribolium castaneum, it is the forewings that are modified (to form elytra), while the hindwings retain a morphologically more ancestral identity. Here we show that in this beetle Ubx 'de-specializes' the hindwings, which are transformed to elytra when the gene is knocked down. We also show evidence that elytra result from a Hox-free state, despite their diverged morphology. Ubx function in the hindwing seems necessary for a change in the expression of spalt, iroquois and achaete-scute homologues from elytron-like to more typical wing-like patterns. This counteracting effect of Ubx in beetle hindwings represents a previously unknown mode of wing diversification in insects.

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