Bashey Farrah
Department of Biology, University of California, Riverside, California 92521, USA.
Evolution. 2006 Feb;60(2):348-61.
The existence of adaptive phenotypic plasticity demands that we study the evolution of reaction norms, rather than just the evolution of fixed traits. This approach requires the examination of functional relationships among traits not only in a single environment but across environments and between traits and plasticity itself. In this study, I examined the interplay of plasticity and local adaptation of offspring size in the Trinidadian guppy, Poecilia reticulata. Guppies respond to food restriction by growing and reproducing less but also by producing larger offspring. This plastic difference in offspring size is of the same order of magnitude as evolved genetic differences among populations. Larger offspring sizes are thought to have evolved as an adaptation to the competitive environment faced by newborn guppies in some environments. If plastic responses to maternal food limitation can achieve the same fitness benefit, then why has guppy offspring size evolved at all? To explore this question, I examined the plastic response to food level of females from two natural populations that experience different selective environments. My goals were to examine whether the plastic responses to food level varied between populations, test the consequences of maternal manipulation of offspring size for offspring fitness, and assess whether costs of plasticity exist that could account for the evolution of mean offspring size across populations. In each population, full-sib sisters were exposed to either a low- or high-food treatment. Females from both populations produced larger, leaner offspring in response to food limitation. However, the population that was thought to have a history of selection for larger offspring was less plastic in its investment per offspring in response to maternal mass, maternal food level, and fecundity than the population under selection for small offspring size. To test the consequences of maternal manipulation of offspring size for offspring fitness, I raised the offspring of low- and high-food mothers in either low- or high-food environments. No maternal effects were detected at high food levels, supporting the prediction that mothers should increase fecundity rather than offspring size in noncompetitive environments. For offspring raised under low food levels, maternal effects on juvenile size and male size at maturity varied significantly between populations, reflecting their initial differences in maternal manipulation of offspring size; nevertheless, in both populations, increased investment per offspring increased offspring fitness. Several correlates of plasticity in investment per offspring that could affect the evolution of offspring size in guppies were identified. Under low-food conditions, mothers from more plastic families invested more in future reproduction and less in their own soma. Similarly, offspring from more plastic families were smaller as juveniles and female offspring reproduced earlier. These correlations suggest that a fixed, high level of investment per offspring might be favored over a plastic response in a chronically low-resource environment or in an environment that selects for lower reproductive effort.
适应性表型可塑性的存在要求我们研究反应规范的进化，而不仅仅是固定性状的进化。这种方法不仅需要考察单一环境中性状之间的功能关系，还需要考察跨环境以及性状与可塑性本身之间的功能关系。在本研究中，我考察了特立尼达孔雀鱼（Poecilia reticulata）后代大小的可塑性与局部适应性之间的相互作用。孔雀鱼对食物限制的反应是生长和繁殖减少，同时也会产出更大的后代。后代大小的这种可塑性差异与不同种群间进化出的遗传差异处于同一数量级。在某些环境中，较大的后代大小被认为是对新生孔雀鱼所面临的竞争环境的一种适应。如果对母体食物限制的可塑性反应能够带来相同的适合度益处，那么孔雀鱼的后代大小为何还会进化呢？为了探究这个问题，我考察了来自经历不同选择环境的两个自然种群的雌性对食物水平的可塑性反应。我的目标是检验不同种群对食物水平的可塑性反应是否存在差异，测试母体对后代大小的操控对后代适合度的影响，并评估是否存在可塑性成本，这可能解释了不同种群间平均后代大小的进化。在每个种群中，全同胞姐妹被分别置于低食物或高食物处理条件下。两个种群的雌性在食物受限的情况下都会产出更大、更瘦的后代。然而，被认为有选择较大后代历史的种群，在根据母体质量、母体食物水平和繁殖力对每个后代的投入方面，比选择较小后代大小的种群可塑性更低。为了测试母体对后代大小的操控对后代适合度的影响，我在低食物或高食物环境中饲养低食物和高食物母体的后代。在高食物水平下未检测到母体效应，这支持了在非竞争环境中母体应增加繁殖力而非后代大小的预测。对于在低食物水平下饲养的后代，母体对幼体大小和成熟雄性大小的影响在不同种群间有显著差异，这反映了它们在母体对后代大小操控方面的初始差异；然而，在两个种群中，每个后代投入的增加都提高了后代适合度。确定了几个可能影响孔雀鱼后代大小进化且与每个后代投入可塑性相关的因素。在低食物条件下，来自可塑性更强家族的母体对未来繁殖投入更多，对自身身体投入更少。同样，来自可塑性更强家族的后代幼体更小，雌性后代繁殖更早。这些相关性表明，在长期低资源环境或选择较低繁殖努力的环境中，相对于可塑性反应，固定的、高投入水平的每个后代投入可能更受青睐。
