The Evolutionary Genetics of an Adaptive Maternal Effect: Egg Size Plasticity in a Seed Beetle

在许多生物中，雌性的环境为其后代将遇到的环境条件提供了可靠的指标。在这种情况下，母体效应可能演变为跨代表型可塑性的机制，即，根据可预测的环境线索（如高或低宿主密度，短或长光周期），母亲可以改变她所产卵的类型，或者可以在她的后代中编程发育开关，使后代为线索所预测的环境条件做好准备。这些母系效应通常具有遗传基础，并且在许多生物体中似乎具有适应性。然而，人们对母体效应的进化遗传学或选择对母体效应的性质和后果知之甚少，因此，它们对自然选择的反应潜力在很大程度上仍然是推测性的。该提案描述了一系列实验，旨在测试解释种子甲虫定子边缘甲虫的卵大小、卵数量和卵大小可塑性（一种明显的适应性母体效应）进化的假设。具体来说，本研究将(1)量化这种表型变异的来源（遗传、母系、父系和环境），在S. limbatus种群中卵的大小和卵的大小可塑性；(2)验证了在花尾Cercidium floridum （blue paloverde）种子上发育的幼虫为提高存活率而进行的选择导致了卵大的进化的假设；(3)检验为提高繁殖力而进行的选择会导致卵尺寸变小的假设；(4)验证卵大小可塑性的进化受到响应金合欢（catclaw Acacia）产蛋大小与产蛋大小的遗传相关性的制约；(5)验证寄主间遗传相关性的进化是对卵大小和卵大小可塑性选择的响应。母性效应在自然界中无处不在；它们在产生我们观察到的许多表型模式和种群动态方面很重要，并且通常是种群对自然选择反应的重要组成部分。事实上，对自然选择的进化反应可能通过母体效应表现出来，因此母体效应可能提供适应多变环境的机制。然而，母体效应的进化动力学，以及受母体效应影响的性状，很少被探索。在实验室和野外实验中，可以证明选择有利于或反对母体效应的进化。然而，证明选择有利于母性效应的进化并不能充分证明母性效应是作为一种适应而进化的。应用于行为、形态学和生命史研究的严格技术，现在也需要应用于母性效应的研究，以确定适应性母性效应在自然界中有多普遍，以及母性效应作为亲代和后代特征进化的结果而进化的频率有多高。这些实验的目的是：(a)通过严格的定量遗传实验设计，研究适应性母体效应（种子甲虫卵大小可塑性）的进化遗传学；(b)模拟自然选择对母体效应的影响，因为它被认为发生在自然界中，以研究母体效应和相关生活史特征的进化。

Fox 9807315 In many organisms, a female's environment provides a reliable indicator of the environmental conditions that her progeny will encounter. In such cases, maternal effects may evolve as mechanisms for trans-generational phenotypic plasticity, whereby, in response to a predictive environmental cue (such high or low host density, short or long photoperiod), a mother can change the type of eggs that she makes, or can program a developmental switch in her offspring, producing offspring prepared for the environmental conditions predicted by the cue. These maternal effects often have a genetic basis, and appear to be adaptive in many organisms. Yet little is understood about the evolutionary genetics of maternal effects or the nature and consequences of selection on maternal effects, such that the potential for them to respond to natural selection remains largely speculative. This proposal describes a series of experiments designed to test hypotheses that explain the evolution of egg size, egg number, and egg size plasticity (an apparently adaptive maternal effect) in a seed beetle, Stator limbatus. Specifically, this research will (1) quantify the sources of this phenotypic variation (genetic, maternal, paternal, and environmental) in egg size and egg size plasticity within populations of S. limbatus; (2) test the hypothesis that selection for improved survivorship when larvae develop on seeds of Cercidium floridum (blue paloverde) results in the evolution of larger egg size; (3) test the hypothesis that selection for increased fecundity results in the evolution of smaller egg size; (4) test the hypothesis that the evolution of egg size plasticity is constrained by the genetic correlation between the size of eggs laid in response to Acacia greggii (catclaw acacia) and the size of eggs laid on C. floridum; (5) test the hypothesis that the genetic correlation across hosts evolves in response to selection on egg size and egg size plasticity. Maternal effe cts are ubiquitous in nature; they are important in generating many of the phenotypic patterns and population dynamics that we observe, and are often important components of population responses to natural selection. In fact, evolutionary responses to selection may be manifest through maternal effects, such that maternal effects may provide mechanisms for adapting to variable environments. However, the evolutionary dynamics of maternal effects, and characters influenced by maternal effects, have been little explored. In lab and field experiments, selection can be demonstrated to favor or oppose the evolution of maternal effects. However, demonstrating selection favoring the evolution of a maternal effect is not sufficient evidence to demonstrate that a maternal effect has evolved as an adaptation. The same rigorous techniques applied to studies of behavior, morphology, and life history need now be applied to the study of maternal effects to determine how pervasive adaptive maternal effects are in nature and how often maternal effects evolve simply as consequences of the evolution of parental and offspring characters. The objectives of these experiments are (a) to study the evolutionary genetics of an adaptive maternal effect (egg size plasticity in the seed beetle, Stator limbatus), using rigorous quantitative genetic experimental designs, and (b) to simulate natural selection on the maternal effect as it is thought to occur in nature to study the evolution of both the maternal effect and correlated life history characters.