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

是什么驱动了大规模的形态进化模式？自然选择作用于现存的变异，进化的原材料，塑造了我们今天看到和研究的各种生物，但是是什么塑造了变异呢？发育通常被认为是形态变化的主要影响因素，但由于比较发育数据相对较少，将发育效应与大规模进化模型联系起来是困难的。定量数据，特别是生物体形状的测量和生物体各部分之间的相互作用，有可能将进化研究的不同领域，从遗传学到发育和形态学，统一到一个单一的框架中。在发育过程中，由于难以从小的、未成熟的标本中获得测量结果而阻碍了生物形状（形态学）的量化，这些标本在制备过程中往往会失去形状。然而，近年来，越来越多的高分辨率成像机器，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的可能性，以便对开发过程中的变化进行严格的统计分析。形态整合的主题是高度量化的主题之一，将受益于比较发展数据的结合。这些术语与一个直观的概念有关，即复杂生物体有许多具有不同相互关系的部分。简单地说，脚骨的变化对脚的其他部位的影响比对鼻子的影响更大。描述生物体不同部分之间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颖和不可预测的信息，这些特征关系的模式被称为形态整合。许多形态学整合的研究都集中在哺乳动物的头骨和颌骨上，并显示了许多有趣的结果。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可以观察到。其次，这种模式可能反映了发育的相互作用，如组织起源，因为颅骨是一个发育复杂的结构。第三，颅骨整合的模式随着发育而改变，从出生后的早期阶段一直到亚成年期或成年期。最后一个特征很复杂，但有证据表明，生殖策略的差异，特别是有袋动物（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颅骨整合的变化上。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，特别是灵长类动物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寻常的，还是仅仅反映了一个更古老的条件。此外，哺乳动物发育策略的多样性远不如脊椎动物。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截然不同：非洲爪蛙，它经历了从蝌蚪幼虫到小青蛙再到成年青蛙的蜕变。我们将利用CT扫描收集三个发育阶段的大量标本，以测试青蛙和哺乳动物的颅骨整合是否相似，在青蛙发育包括变态过程中颅骨整合是否发生变化，成年青蛙的颅骨整合是否反映了骨骼是在变态前、变态中还是变态后形成的。这项研究的结果，与现有的哺乳动物数据进行比较，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.