Department of Craniofacial Development, King's College London, Floor 27, Guy's Hospital, London Bridge, London SE1 9RT, UK.
Dev Biol. 2013 May 15;377(2):428-48. doi: 10.1016/j.ydbio.2013.02.022. Epub 2013 Mar 7.
The acquisition of jaws constitutes a landmark event in vertebrate evolution, one that in large part potentiated their success and diversification. Jaw development and patterning involves an intricate spatiotemporal series of reciprocal inductive and responsive interactions between the cephalic epithelia and the cranial neural crest (CNC) and cephalic mesodermal mesenchyme. The coordinated regulation of these interactions is critical for both the ontogenetic registration of the jaws and the evolutionary elaboration of variable jaw morphologies and designs. Current models of jaw development and evolution have been built on molecular and cellular evidence gathered mostly in amniotes such as mice, chicks and humans, and augmented by a much smaller body of work on the zebrafish. These have been partnered by essential work attempting to understand the origins of jaws that has focused on the jawless lamprey. Chondrichthyans (cartilaginous fish) are the most distant group to amniotes within extant gnathostomes, and comprise the crucial clade uniting amniotes and agnathans; yet despite their critical phylogenetic position, evidence of the molecular and cellular underpinnings of jaw development in chondrichthyans is still lacking. Recent advances in genome and molecular developmental biology of the lesser spotted dogfish shark, Scyliorhinus canicula, make it ideal for the molecular study of chondrichthyan jaw development. Here, following the 'Hinge and Caps' model of jaw development, we have investigated evidence of heterotopic (relative changes in position) and heterochronic (relative changes in timing) shifts in gene expression, relative to amniotes, in the jaw primordia of S. canicula embryos. We demonstrate the presence of clear proximo-distal polarity in gene expression patterns in the shark embryo, thus establishing a baseline molecular baüplan for branchial arch-derived jaw development and further validating the utility of the 'Hinge and Caps' model in comparative studies of jaw development and evolution. Moreover, we correlate gene expression patterns with the absence of a lambdoidal junction (formed where the maxillary first arch meets the frontonasal processes) in chondrichthyans, further highlighting the importance of this region for the development and evolution of jaw structure in advanced gnathostomes.
颌的获得是脊椎动物进化中的一个里程碑事件，在很大程度上促进了它们的成功和多样化。颌的发育和模式形成涉及到头部上皮和颅神经嵴（CNC）以及头部中胚层间充质之间复杂的时空相互诱导和反应性相互作用。这些相互作用的协调调控对于颌的发生注册和可变颌形态和设计的进化演变都至关重要。当前的颌发育和进化模型是基于在羊膜动物（如老鼠、小鸡和人类）中收集的分子和细胞证据构建的，并通过对斑马鱼的少量研究工作进行了补充。这些模型与试图理解无颌鱼的起源的重要工作相结合，重点研究了无颌鱼。软骨鱼类（软骨鱼）是现存颌类动物中与羊膜动物最遥远的群体，是将羊膜动物和无颌动物联合在一起的关键分支；然而，尽管它们具有关键的系统发育地位，但软骨鱼类颌发育的分子和细胞基础的证据仍然缺乏。斑点狗鲨（Scyliorhinus canicula）的基因组和分子发育生物学的最新进展使其成为研究软骨鱼颌发育的理想模型。在这里，我们遵循颌发育的“铰链和帽”模型，研究了在斑点狗鲨胚胎的颌原基中，相对于羊膜动物，基因表达的异位（位置相对变化）和异时（时间相对变化）变化的证据。我们证明了鲨鱼胚胎中基因表达模式存在明显的近-远极性，从而为鳃弓衍生的颌发育建立了一个基本的分子基础，并进一步验证了“铰链和帽”模型在颌发育和进化比较研究中的实用性。此外，我们将基因表达模式与软骨鱼类中缺乏矢状关节（上颌第一弓与额鼻突相遇处形成）联系起来，进一步强调了该区域对高级颌类动物颌结构发育和进化的重要性。
