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Antagonistic selection on body size and sword length in a wild population of the swordtail fish, Xiphophorus multilineatus: potential for intralocus tactical conflict
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  • Melissa Liotta,
  • Jessica Abbott,
  • Molly Morris,
  • Oscar Rios-Cardenas
Melissa Liotta
Ohio University
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Jessica Abbott
Lund University
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Molly Morris
Ohio University
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Oscar Rios-Cardenas
Instituto de EcologĂ­a A.C.
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Abstract

Alternative reproductive tactics (ARTs) have provided valuable insights into how sexual selection and life history tradeoffs can lead to variation within a sex. However, the possibility that tactics may constrain evolution through intralocus tactical conflict (IATC) is rarely considered. In addition, when IATC has been considered, the focus has often been on the genetic correlations between the ARTs, while evidence that the ARTs have different optima for associated traits and that at least one of the tactics is not at its optima is often missing. Here we investigate selection on three traits associated with the ARTs in the swordtail fish Xiphophorus multilineatus; body size, body shape and the sexually selected trait for which these fishes were named, sword length (elongation of the caudal fin). All three traits are tactically dimorphic, with courter males being larger, deeper bodied and having longer swords, and the sneaker males being smaller, more fusiform and having shorter swords. Using measures of reproductive success in a wild population we calculated selection differentials, linear and quadratic gradients, demonstrate that the tactics have different optima and at least one of the tactics is not at its optima for body size and sword length. Our results provide the first evidence of selection in the wild on the sword, an iconic trait for sexual selection. In addition, given the high probability that these traits are genetically correlated to some extent between the two tactics, our study suggests that IATC is constraining both body size and the sword from reaching their phenotypic optima. We discuss the importance of considering the role of IATC in the evolution of tactical dimorphism, how this conflict can be present despite tactical dimorphism, and how it is important to consider this conflict when explaining not only variation within a species but differences across species as well.

Peer review status:ACCEPTED

15 Dec 2020Submitted to Ecology and Evolution
16 Dec 2020Submission Checks Completed
16 Dec 2020Assigned to Editor
18 Dec 2020Review(s) Completed, Editorial Evaluation Pending
23 Dec 2020Editorial Decision: Revise Minor
09 Jan 20211st Revision Received
09 Jan 2021Submission Checks Completed
09 Jan 2021Assigned to Editor
09 Jan 2021Review(s) Completed, Editorial Evaluation Pending
18 Jan 2021Editorial Decision: Accept