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The evolution of relative trait size and shape: insights from the genitalia of dung beetles

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Abstract

Insects show relatively little genital variation within species compared to extraordinary and often rapid diversification among species. It has been suggested that selection for reproductive isolation through differences in genital shape might explain this phenomenon. This hypothesis predicts that populations diverge faster in genital shape than in genital size. We tested this prediction in males from 10 dung beetle species with known phylogenetic relationships from the genus Onthophagus (Coleoptera: Scarabaeidae), including four species for which we were able to sample multiple populations. Specifically, we compared intra- and interspecific differentiation in shape and relative sizes of genitalia and calculated their respective evolutionary rates. We compared these rates to two similarly sized non-genital traits, the head and the fore-tibia. We found significant intraspecific variation in genital shape in all four species for which multiple populations were sampled, but for three of them we also identified significant relative size variation. We also found that genital shape evolved at higher rates than relative genital size. Genital shape evolved faster than head shape, but not fore-tibia shape. However, shapes of all measured structures evolved faster than their relative size. We discuss the functional constraints that may bias the developmental evolution of relative size and shape of genitalia and other morphological traits.

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Acknowledgements

We would like to thank Tami Cruickshank who graciously calculated divergence times, Anna Macagno for statistical questions, Teiya Kijimoto for providing images for each trait, Elise Morton for helping us with the creation of the size figures, and two anonymous reviewers for constructive comments that improved the manuscript.

Funding

This study was supported in part by National Science Foundation grants IOS 0445661 and IOS 0718522 to APM.

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Correspondence to Harald F. Parzer.

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This article is part of the Special Issue “Size and Shape: Integration of morphometrics, mathematical modelling, developmental and evolutionary biology”, Guest Editors: Nico Posnien—Nikola-Michael Prpic.

Communicated by Nikola-Michael Prpic

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Parzer, H.F., David Polly, P. & Moczek, A.P. The evolution of relative trait size and shape: insights from the genitalia of dung beetles. Dev Genes Evol 228, 83–93 (2018). https://doi.org/10.1007/s00427-018-0602-2

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  • DOI: https://doi.org/10.1007/s00427-018-0602-2

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