Modularity and developmental patterning in the evolution of early jawed vertebrates

大约5.4亿年前，所有头骨动物的祖先产生了今天的八目鳗、七鳃鳗、软骨鱼(鼠鱼、溜冰鱼和鲨鱼)和硬骨鱼(鳍鱼、腔棘鱼、肺鱼和四足动物)，获得了独特的胚胎学特征--神经脊和表皮胎盘(在器官发生早期形成的专门的胚胎细胞组)。这两种发育的新颖性是颅骨的许多解剖学特征的原因，例如大脑和颅面软骨和骨骼。没有其他发育上的新奇发现与有颌脊椎动物或颌口动物(棘纲、安慰皮、软骨和骨鱼)的早期进化有关。其结果是，基本的颌口之间的相互关系没有得到很好的解决。最有可能的是，对新的发育特征的认识，如发育模块(作为相对独立的发育单位的相互作用的元件网络)和模式(这些元件的形成顺序)可以帮助解决颌口动物的系统发育问题。先前提出的所有解剖模块都将被汇编，以重建下颌骨模块化的理论模型。这些基本模块的存在将通过观察(1)它们的内部变异模式和(2)它们的形成模式在活的颌口动物身上进行测试。这些模块和模式将在化石分类群中进行评估，并随后用于重建系统发育。换句话说，我们将解决三个问题：(1)脊椎动物起源时的模块化组织是什么？(2)我们还能在活的和化石的脊椎动物中识别这些模块及其相关的形成模式吗？(3)我们能基于新的模块和模式特征重建脊椎动物的生命树吗？这项研究计划将有助于理解导致颌骨脊椎动物进化的发育机制，我们所属的分类学群体继承了我们的基本解剖学和发育特征。

Approximately 540 million years ago, the ancestor of all craniates, which gave rise to today's hagfishes, lampreys, cartilaginous (ratfishes, skates and sharks) and bony fishes (ray-fined fishes, coelacanths, lungfishes and tetrapods), acquired distinctive embryological features - the neural crest and the epidermal placodes (specialized groups of embryonic cells forming early during organogenesis). These two developmental novelties are accountable for numerous anatomical characteristics of craniates, such as the brain and craniofacial cartilages and bones. No other developmental novelties have been associated to the early evolution of jawed vertebrates, or gnathostomes (acanthodians, placoderms, cartilaginous and bony fishes). As a result, basal gnathostome interrelationships are poorly resolved. Most likely, the recognition of new developmental features such as developmental modules (a network of interacting elements behaving as relatively independent units of development) and patterning (the order of formation of these elements) can help solving gnathostome phylogeny. All anatomical modules that have been previously proposed will be compiled to reconstruct a theoretical model of gnathostome modularity. The presence of these fundamental modules will be tested in living gnathostomes by looking at (1) their internal pattern of variation and (2) their pattern of formation. These modules and patterning will be evaluated in fossil taxa and subsequently used to reconstruct phylogeny. In other words, we will be addressing three questions about the evolution of modularity and patterning in jawed vertebrate history: (1) What was the modular organization of gnathostomes at their origin? (2) Can we still recognize these modules and their associated patterns of formation in living and fossil gnathostomes? (3) Can we reconstruct the gnathostomes' tree of life based on new modular and patterning characters? This research program will help understanding developmental mechanisms that have been responsible for the evolution of jawed vertebrates, the taxonomic group we belong to and from which we inherited fundamental anatomical and developmental characteristics.