Fitness consequences of artificial selection on relative male genital size

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 ...

亲缘关系相近的物种间，雄性外生殖器在大小、形态与复杂程度上往往存在显著差异。在花鳉科（Poeciliidae）鱼类中，雄性用于给雌性授精的延长型鳍肢——生殖足（gonopodium），其长度占体长的比例介于18%至53%之间，因此不同物种间的相对生殖器大小差异显著。与之相反，同一物种内部通常存在严格的异速生长缩放（allometric scaling）关系，这表明存在较强的选择压力，会淘汰那些偏离物种自然异速生长线的生殖器-体型大小组合。本研究以东部食蚊鱼（Gambusia holbrooki）为实验对象，通过对异速生长截距进行人工选择，构建了在特定体型下，生殖足长度相较于自然状态更长或更短的雄性品系。研究结果表明，相对生殖器长度具有可遗传性；在人工向上选择和向下选择的品系之间，该性状的分化幅度达7.6%至8.9%，同时伴随体型的相关性变化。然而，偏离物种自然异速生长线，并不会对雄性在吸引力、游泳能力等测试中的表现产生影响。