Abstract
Male genitalia often show remarkable differences among related species in size, shape and complexity. Across poeciliid fishes, the elongated fin (gonopodium) that males use to inseminate females ranges from 18 to 53% of body length. Relative genital size therefore varies greatly among species. In contrast, there is often tight within-species allometric scaling, which suggests strong selection against genital–body size combinations that deviate from a species' natural line of allometry. We tested this constraint by artificially selecting on the allometric intercept, creating lines of males with relatively longer or shorter gonopodia than occur naturally for a given body size in mosquitofish, Gambusia holbrooki. We show that relative genital length is heritable and diverged 7.6–8.9% between our up-selected and down-selected lines, with correlated changes in body shape. However, deviation from the natural line of allometry does not affect male success in assays of attractiveness, swimming performance and, crucially, reproductive success (paternity).
Highlights
Male genitalia often show remarkable differences among related species in size, shape and complexity
Recent selection analyses of poeciliids find that relative genital size and shape is associated with male mating and/or reproductive success in both G. holbrooki[6,7] and a related species, the guppy, Poecilia reticulata[8,9]
How do we determine whether relative genital size in poeciliids is under selection, which might account for its precise relationship with body size? One approach, especially in sexual selection studies, is to experimentally manipulate a focal trait[11,12]
Summary
Male genitalia often show remarkable differences among related species in size, shape and complexity. There is often tight within-species allometric scaling, which suggests strong selection against genital–body size combinations that deviate from a species’ natural line of allometry We tested this constraint by artificially selecting on the allometric intercept, creating lines of males with relatively longer or shorter gonopodia than occur naturally for a given body size in mosquitofish, Gambusia holbrooki. The only experimental evidence for direct selection on male genitals comes from gross manipulation of genital features by ablating or surgically removing major components These studies demonstrate that certain genital traits affect fertilization success[13,14,15,16,17,18,19]. Artificial selection should reduce fitness if the focal trait is under strong natural and/or sexual selection[21]
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