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Intensity of sexual selection along the anisogamy–isogamy continuum

Abstract

Research into the evolution of giant sperm has uncovered a paradox within the foundations of sexual selection theory. Postcopulatory sexual selection on males (that is, sperm competition and cryptic female choice) can lead to decreased sperm numbers by favouring the production of larger sperm1. However, a decline in sperm numbers is predicted to weaken selection on males and increase selection on females2,3. As isogamy is approached (that is, as investment per gamete by males approaches that by females), sperm become less abundant, ova become relatively less rare, and competition between males for fertilization success is predicted to weaken. Sexual selection for longer sperm, therefore, is expected to be self limiting. Here we examine this paradox in Drosophila along the anisogamy–isogamy continuum using intraspecific experimental evolution techniques and interspecific comparative techniques. Our results confirm the big-sperm paradox by showing that the sex difference in sexual selection gradients4 decreases as sperm size increases. However, a resolution to the paradox is provided when this finding is interpreted in concert with the ‘opportunity for selection’ and the ‘opportunity for sexual selection’5,6. Furthermore, we show that most of the variation in measures of selection intensity is explained by sperm length and relative investment in sperm production.

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Figure 1: Drosophila bifurca sperm and egg.
Figure 3: Mating system measures in relation to investment in sperm production.
Figure 2: Intraspecific and interspecific sexual selection gradients in Drosophila.

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Acknowledgements

We thank R. Dallai for contributing the scanning electron micrographs of Drosophila bifurca for Fig. 1, W. T. Starmer for statistical advice, B. A. Byrnes for technical assistance, W. J. Etges for directions to the D. bifurca collection site in Mexico, and C. Hubbell and SUNY Upstate Medical University for providing access to the gamma irradiator. We are also indebted to S. M. Shuster, M. J. Wade, S. J. Arnold, M. Kirkpatrick, G. A. Parker, R. Lande, R. A. Schmedicke, L. L. Wolf, W. T. Starmer, W. D. Brown, J. A. C. Uy and G. T. Miller for discussion of our data and/or comments on an earlier draft of this manuscript. This work was supported by a National Science Foundation Grant to S.P. and A.B.

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Correspondence to Adam Bjork.

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Supplementary information

Supplementary Table 1

Summary of the slopes of the sexual selection gradients presented in Figure 2. (PDF 40 kb)

Supplementary Methods

Full description of methods and analyses used in this study. (PDF 66 kb)

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Bjork, A., Pitnick, S. Intensity of sexual selection along the anisogamy–isogamy continuum. Nature 441, 742–745 (2006). https://doi.org/10.1038/nature04683

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