Campbell Diane R, Brody Alison K, Price Mary V, Waser Nickolas M, Aldridge George
Am Nat. 2017 Dec;190(6):818-827. doi: 10.1086/694116. Epub 2017 Sep 27.
Individual differences in fecundity often serve as proxies for differences in overall fitness, especially when it is difficult to track the fate of an individual's offspring to reproductive maturity. Using fecundity may be biased, however, if density-dependent interactions between siblings affect survival and reproduction of offspring from high- and low-fecundity parents differently. To test for such density-dependent effects in plants, we sowed seeds of the wildflower Ipomopsis aggregata (scarlet gilia) to mimic partially overlapping seed shadows of pairs of plants, one of which produced twice as many seeds. We tested for differences in offspring success using a genetic marker to track offspring to flowering multiple years later. Without density dependence, the high-fecundity parent should produce twice as many surviving offspring. We also developed a model that considered the geometry of seed shadows and assumed limited survivors so that the number of juvenile recruits is proportional to the area. Rather than a ratio of 2∶1 offspring success from high- versus low-fecundity parents, our model predicted a ratio of 1.42∶1, which would translate into weaker selection. Empirical ratios of juvenile offspring and of flowers produced conformed well to the model's prediction. Extending the model shows how spatial relationships of parents and seed dispersal patterns modify inferences about relative fitness based solely on fecundity.
繁殖力的个体差异通常可作为整体适应性差异的指标，尤其是当难以追踪个体后代直至其达到生殖成熟时。然而，如果同胞之间的密度依赖相互作用对高繁殖力和低繁殖力亲本的后代存活和繁殖产生不同影响，那么使用繁殖力可能会产生偏差。为了测试植物中的这种密度依赖效应，我们播种了野花聚合草（猩红色吉莉草）的种子，以模拟成对植物部分重叠的种子阴影，其中一种植物产生的种子数量是另一种的两倍。我们使用遗传标记追踪后代多年后开花情况，以测试后代成功的差异。如果没有密度依赖，高繁殖力亲本应该产生两倍数量的存活后代。我们还开发了一个模型，该模型考虑了种子阴影的几何形状，并假设幸存者数量有限，以便幼年新植株数量与面积成正比。我们的模型预测，高繁殖力与低繁殖力亲本的后代成功比例不是2∶1，而是1.42∶1，这意味着选择作用较弱。幼年后代和所开花朵的实际比例与模型预测非常吻合。扩展该模型表明，亲本的空间关系和种子传播模式如何改变仅基于繁殖力对相对适应性的推断。
