The evolution of developmental system drift in axial specification in Spiralia

不同动物的相似器官在卵子或母亲子宫的发育过程中往往形成略有不同。例如，人类和鱼类的背部都有脊椎，由椎骨和神经组成。然而，在这些动物的脊柱发育过程中，细胞的行为和构成我们遗传信息的核心单位——基因——的相互作用并不完全相同。这是自相矛盾的，因为通常的期望是，相等器官形成方式的差异应该导致它们出生时的样子的变化。类似器官通过不同机制形成的现象被称为“发育系统漂移”，研究人员并不完全了解这种现象发生的原因和方式。各种各样的动物，如贻贝、蜗牛和蚯蚓，在它们发育的最初阶段看起来都很相似。事实上，我们已经证明，在许多情况下，当它们在卵中发育时，它们使用相似的分子和信号来定义它们成年形态的蓝图。然而，我们也发现不同的蠕虫可以使用两种不同的信号来定义沿着身体轴形成的器官，从它们的背部到腹部，即所谓的背腹轴。我们的项目希望利用这一令人困惑的观察结果作为研究系统来调查“发育系统漂移”的过程，从而解决进化和发育生物学中一个基础的和长期存在的知识鸿沟。在这个项目中，我们将研究海洋节段蠕虫背腹轴形成的变化是如何起源的。我们将通过应用先进的方法来做到这一点，这些方法可以记录单个细胞水平上的基因活动。我们将在两种形成背向腹部轴的蠕虫身上使用这些技术，并用化学药物干扰它们的发育，以重建它们的基因在形成身体轴的关键阶段是如何相互作用的。通过对这两个物种的比较，我们将在单个基因和单个细胞的水平上发现它们发育过程中的异同，从而推断出在这些物种的发育过程中导致“发育系统漂移”的基因活性和调控的变化。此外，我们将研究一种新型蠕虫的背向腹部轴的形成，以验证这样一种假设，即控制该轴的信号的变化与一种依赖于母亲将分子沉积到卵中的繁殖模式的转变有关。我们的项目将共同为动物发育研究建立新的方法和物种，并产生无与伦比的数据集，揭示支撑“发育系统漂移”的原理，这是动物中普遍而基本的现象。因此，我们将产生新的概念、预测和方法，这些概念、预测和方法可能适用于许多其他动物，从而更好地理解支配我们生命中最关键阶段的规则。

Similar organs in different animals often form slightly differently during development in the egg or the mother's womb. For example, humans and fishes have a spine on their backs, made of vertebrae with a nerve running through them. Yet, how cells behave and how the core units that comprise our genetic information--the genes--interact during the development of the spine in these animals are not entirely identical. This is paradoxical because the common expectation is that differences in how equivalent organs form should result in changes in how they look at birth. The phenomenon by which similar organs form through varying mechanisms is called 'developmental system drift', and researchers do not fully understand why and how it happens.Animals as diverse as mussels, snails and earthworms look alike at the very first steps of their development. Indeed, we have shown that, in many cases, they use similar molecules and signals to define the blueprint of their adult morphology while they develop in their eggs. However, we have also found that different worms can use two distinct signals to define the organs that form along the body axis that goes from their backs to their bellies, the so-called dorsoventral axis. Our project wants to use this puzzling observation as a study system to investigate the process of 'developmental system drift' and, thereby, solve a fundamental and long-standing knowledge gap in Evolution and Developmental Biology.In this project, we will investigate how changes in the formation of the back-to-belly axis in marine segmented worms originated. We will do this by applying advanced methodologies that allow recording the activity of genes at the level of individual cells. We will use these techniques in two worms that form the back-to-belly axis differently and perturb their development with chemical drugs to reconstruct how their genes interact during the critical phase of forming that body axis. By comparing the two species, we will identify similarities and differences at the level of single genes and single cells during their development, thereby inferring the changes in gene activity and regulation that cause 'developmental system drift' during the development of these species. Moreover, we will study the formation of the back-to-belly axis in a new species of worm to test the hypothesis that changes in the signals controlling this axis correlate with transitions to a mode of reproduction that relies on molecules that the mother deposits into the eggs.Together, our project will establish new methods and species for the study of animal development and produce unparalleled datasets that will reveal the principles underpinning 'developmental system drift', a widespread and fundamental phenomenon in animals. We will thus generate new concepts, predictions and approaches that will likely apply to many other animals, providing a better understanding of the rules that govern the most critical phase of our lives.