Modularity and Metamorphosis: the effect of complex life history on cranial integration

是什么推动了形态进化的大规模模式？自然选择作用于现有的变异，进化的原材料，塑造了我们今天看到和研究的各种有机体，但是什么塑造了变异呢？发育通常被认为是对形态变异的主要影响，但将发育效应与大规模进化模型联系起来是困难的，因为比较的发育数据相对较少。定量数据，特别是对生物体形状和生物体各部分之间相互作用的测量，有可能将从遗传学到发育和形态的不同进化研究领域统一到一个单一的框架中。在发育过程中，由于很难从小的、未成熟的标本中获得测量结果，在制备过程中往往会失去它们的形状，这阻碍了对生物形态(形态)的量化。然而，近年来，越来越多地获得高分辨率成像机，如CT扫描仪，以及先进的可视化和数字化软件，提高了收集大量数据以对开发期间的变化进行严格统计分析的可能性。形态整合的专题是那些高度定量的专题之一，并将受益于纳入比较发展数据。这些术语与复杂有机体的许多部分具有不同的相互关系这一直观概念有关。简单地说，脚部骨骼的变化对脚的其他部位的影响预计会比对鼻子等部位的影响更大。描述生物体不同部分的关系可以提供关于遗传、发育和功能对生物体形态影响的新的和不可预测的信息，这些特征关系模式被称为形态整合。许多关于形态整合的研究都集中在哺乳动物的头骨和颌骨上，并显示了一些有趣的结果。首先，哺乳动物的头骨由六个相对不同的区域组成，从猴子到老鼠，这种头骨整合的模式在哺乳动物中都能观察到。其次，这种模式可能反映了发育的相互作用，例如组织起源，因为头骨是一个发育复杂的结构。第三，头骨整合的模式随着发育而变化，从出生后的早期阶段到亚成年期或成年期。最后一个特征是复杂的，但有一些证据表明，生殖策略的差异，特别是有袋类动物(如袋鼠)和胎盘动物(如狗或人)之间的差异，反映在个体发育过程中发生的头骨整合的变化中。虽然大多数关于脊椎动物形态整合的研究都是在哺乳动物，特别是灵长类和啮齿动物身上进行的，但很少有其他脊椎动物的比较数据来判断哺乳动物的模式是不是不寻常，或者只是反映了一种更古老的条件。此外，哺乳动物的发育策略的多样性远远低于脊椎动物。在这项研究中，我们将研究一种脊椎动物头骨的形态整合，这种脊椎动物的发育策略与哺乳动物非常不同：非洲爪蛙，它经历了从幼虫蝌蚪到小狐狸再到成年青蛙的变态。我们将使用CT扫描收集来自三个发育阶段的大量标本，以测试青蛙和哺乳动物的头骨整合是否类似，头骨整合是否在青蛙发育过程中发生变化，包括变形，以及成年青蛙的头骨整合是否反映骨骼是在变形之前、期间还是之后形成的。与现有的哺乳动物数据相比，这项研究的结果将为更全面地研究四足动物的发育整合奠定基础。

What drives large-scale patterns of morphological evolution? Natural selection acts upon existing variation, the raw material of evolution, to shape the diverse organisms that we see and study today, but what shapes variation? Development is often presented as a major influence on morphological variation, but linking developmental effects into large-scale models of evolution is difficult because comparative developmental data is relatively uncommon. Quantitative data, specifically measurements of the shapes of organisms and the interactions among parts of an organism, has the potential unify disparate fields of evolutionary study, from genetics to development and morphology, into a single framework. Quantification of organismal shape (morphology), during development has been hindered by difficulties in obtaining measurements from small, immature specimens, which often can lose their shape during preparation. However, in recent years, increased access to high-resolution imaging machines, such as CT scanners, and advanced visualization and digitising software has improved the possibility of gathering large volumes for rigorous statistical analyses of changes during development. The topic of morphological integration is among those that are highly quantitative and will benefit from the incorporation of comparative developmental data. These terms relate to the intuitive idea that complex organisms have many parts with varying interrelationships. Simply, a change in a foot bone would be expected to have more effect on other parts of the foot than they do on, for example, the nose. Delineating the relationships of different parts of an organism can provide novel and unpredicted information on the genetic, developmental, and functional influences on an organism's morphology, and these patterns of trait relationships are termed morphological integration. Much research on morphological integration has focused on the mammalian skull and jaws and has shown a number of interesting results. First, mammal skulls are formed of six relatively distinct regions, and this pattern of skull integration is observed across mammals, from monkeys to mice. Second, this pattern may reflect developmental interactions, such as tissue origin, as the skull is a developmentally complex structure. Third, patterns of skull integration change through development, from the early stages immediately after birth until subadulthood or adulthood. This last feature is complicated, but there is some evidence that differences in reproductive strategies, specifically between marsupials (such as kangaroos) and placentals (such as dogs or humans) are reflected in the changes in skull integration that occur during ontogeny. While most research on morphological integration in vertebrates has been conducted on mammals, especially primates and rodents, there is little comparative data on other vertebrates with which to judge whether mammalian patterns are unusual, or simply reflect a more ancestral condition. Furthermore, the diversity of developmental strategies in mammals is far less than that observed across vertebrates. In this study, we will examine morphological integration in the skull of a vertebrate with a very different developmental strategy to that seen in mammals: the African clawed frog, which undergoes metamorphosis from a larval tadpole to a froglet to an adult frog. We will use CT scanning to gather large numbers of specimens from three stages of development to test if skull integration is similar in frogs and mammals, if skull integration changes during frog development, including metamorphosis, and if skull integration in adult frogs reflects whether bones are formed before, during or after metamorphosis. The results of this study, in comparison with existing data for mammals, will form the basis for a more comprehensive study of developmental integration across tetrapods.