Abstract
Fecundity selection, acting on traits enhancing reproductive output, is an important determinant of organismal body size. Due to a unique mode of reproduction, mating success and fecundity are positively correlated with body size in both sexes of male-pregnant Syngnathus pipefish. As male pipefish brood eggs on their tail and egg production in females occurs in their ovaries (located in the trunk region), fecundity selection is expected to affect both sexes in this species, and is predicted to act differently on body proportions of males and females during their development. Based on this hypothesis, we investigated sexual size dimorphism in body size allometry and vertebral numbers across populations of the widespread European pipefish Syngnathus typhle. Despite the absence of sex-specific differences in overall and region-specific vertebral counts, male and female pipefish differ significantly in the relative lengths of their trunk and tail regions, consistent with region-specific selection pressures in the two sexes. Male pipefish show significant growth allometry, with disproportionate growth in the brooding tail region relative to the trunk, resulting in increasingly skewed region-specific sexual size dimorphism with increasing body size, a pattern consistent across five study populations. Sex-specific differences in patterns of growth in S. typhle support the hypothesis that fecundity selection can contribute to the evolution of sexual size dimorphism.
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Acknowledgments
We would like to thank to Ingrid Ahnesjö, Murat Bilecenoglu, Iris Eigenmann, Nathalie Feiner, Jorge Gonçalves, Laurent Leveque, Federico Riccato, Valeria Rispoli, and Johan Wenngren for their help, efforts and time investments during field work. We are grateful to the Dipartimento di Scienze Ambientali (Università Ca’ Foscari), the Askö Laboratory, Klubban Biological Station, and the Roscoff Biological Station for the use of their facilities. Many thanks to Ingrid Ahnesjö, Christian Klingenberg, Marcelo Sánchez-Villagra, Lukas Rüber, and Lorenzo Tanadini for discussion and suggestions. Our special thanks go to Wolf Blanckenhorn for his statistical advice and to Jonathan Ready, INCOFISH project (EC project PL003739) for providing environmental data for sampling localities. The study was funded by the University of Zurich Forschungskredit, the Swiss Academy of Sciences and the Swiss National Science Foundation.
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Winkler, J.D., Stölting, K.N. & Wilson, A.B. Sex-specific responses to fecundity selection in the broad-nosed pipefish. Evol Ecol 26, 701–714 (2012). https://doi.org/10.1007/s10682-011-9516-4
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DOI: https://doi.org/10.1007/s10682-011-9516-4