Partial co-option of the appendage patterning pathway in the development of abdominal appendages in the sepsid fly Themira biloba
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The abdominal appendages on male Themira biloba (Diptera: Sepsidae) are complex novel structures used during mating. These abdominal appendages superficially resemble the serially homologous insect appendages in that they have a joint and a short segment that can be rotated. Non-genital appendages do not occur in adult pterygote insects, so these abdominal appendages are novel structures with no obvious ancestry. We investigated whether the genes that pattern the serially homologous insect appendages have been co-opted to pattern these novel abdominal appendages. Immunohistochemistry was used to determine the expression patterns of the genes extradenticle (exd), Distal-less (Dll), engrailed (en), Notch, and the Bithorax Complex in the appendages of T. biloba during pupation. The expression patterns of Exd, En, and Notch were consistent with the hypothesis that a portion of the patterning pathway that establishes the coxopodite has been co-opted to pattern the developing abdominal appendages. However, Dll was only expressed in the bristles of the developing appendages and not the proximal–distal axis of the appendage itself. The lack of Dll expression indicates the absence of a distal domain of the appendage suggesting that sepsid abdominal appendages only use genes that normally pattern the base of segmental appendages.
KeywordsAppendage Sepsidae Abdomen Innovation Co-option
We would like to thank Rudolf Meier for sharing his culture of T. biloba, and Bill Eberhard for sharing his knowledge of sepsid behavior and rearing. Thank you to Rob White, Nipam Patel, and Sean Carroll for providing antibodies. We would like to thank Lisa Nagy, Yui Suzuki, Robin Smith, and two anonymous reviewers for their comments on the manuscript. Laura Grunert provided critical technical assistance. We would also like to thank Maple View Farm in Chapel Hill, NC and the University of Arizona Agricultural Center for providing cow dung. This work was funded by the Department of Biology at Duke University (JHB and HFN), National Science Foundation grant IBN-0315897 (HFN), and the Center for Insect Science at the University of Arizona through the National Institute of Health Training Grant 1K12 GM000708 (JHB).
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