幼态持续与异时发育：中国化石与现生蝾螈类演化发育生物学综合研究

Evolutionary Developmental Biology (evo-devo) is an interdisciplinary research field by its nature, whereas heterochrony—evolutionary change in the timing of developmental events—has been the central concept in this field since the term was first coined in 1875. This proposal is planned to conduct research on evolutionary development in salamanders by comparative study of superbly preserved fossils from, and extant species endemic to, China. The planned research aims to investigate neotenic evolution of salamander clades, and heterochronic ossification of hyobranchium and mesopodium in extant hynobiids and their fossil relatives. Our study will stress comparison of shifts in the timing of ossification sequence rather than allometric growth of bony structures. Ossification sequence data will be obtained by means of Micro CT-scan of both fossil and extant salamander specimens; iodine enhanced CT-scan techniques will also be used to process samples of extant species. By comparative study changes in the timing of onset ossification of the same structure in different species of extant hynobiids and their fossil relatives, the proposed research is expected to detect heterochronic events in post-embryonic ontogeny of the sampled salamanders, and to discuss what roles these events played in phylogenetic diversification of hynobiid salamanders. Neoteny (retention of larval morphology at reproductive maturity) has been recognized as one type of heterochrony and a key life-history feature in relevant to salamander diversity. The proposed research will include rich samples of superbly preserved Mesozoic fossils from China in our study of neotenic evolution of salamander clades. Neotenes in fossil forms can be distinguished from metamorphic salamanders by identification of larval features (internal and external gills, larval type of hyobranchium) in adults with ossification of articular as an indication of sexual maturity. By color coding of neotenic taxa with geologic range, this sifinificant life-history feature will be plotted into a phylogenetic framework at hand, for tracing the evolution of neoteny in deep time, and for developing a discussion on the roles that neoteny played in the evolution of salamanders. Such a research has not been done before largely owing to the lack of adequate data from the fossil record. Results of the research are expected to produce publications of at least one paper in a high-profile journal including PNAS/RSPB, 3-5 technical papers in PeerJ, PLoS One, or Journal of Anatomy, and 2-3 technical papers in the journal of China Scientific Data.

本项目拟应用中国特有的中生代化石和现生蝾螈类标本进行演化发育生物学研究。研究内容包括化石与现生小鲵类骨化序列的异时发育和蝾螈类幼态持续的宏观演化。异时发育的研究侧重于骨化序列时间上的差异性比较，而非某些结构的异速生长。项目设计拟采用高精度CT扫描和碘增强扫描技术，分别从化石与现生标本中获取舌器及中足骨骼胚后发育过程中的骨化序列。通过不同属种间相同结构在骨化时间上的差异性比较，识别小鲵类胚后发育过程中的异时发育现象并探究其在分支演化中的作用。幼态持续研究中，化石属种幼态持续的识别依据成年个体保留幼年特征区别于变态发育类型。通过幼态持续在分支系统中的标定，追索其深度时间上的演化，进而分析讨论蝾螈类幼态持续的起源及宏观分支系统框架下的演化形式。预期成果包括至少一篇PNAS/RSPB级别的论文，3-5篇PeerJ/PLoSONE级别的常规论文，及2-3篇国内核心刊物《中国科学数据》的技术性论文。

演化发育生物学（简称evo-devo）就其科学属性而言应视为一个交叉学科领域，而异时发育（heterochrony）—发育事件在时间或速率上的差异性演化 —自其1875年被正式造词命名以来，已经成为当代演化生物学领域的一个极为重要的科学概念。最近几十年中该领域的研究进展主要反映在基因与发育过程在时间和速率上的差异性演化方面，即基因与胚胎发育过程的演化产生形态学多样性方面。然而，国际上已有学者疾呼大量研究工作急需在胚后发育过程中的形态发生事件（morphogenetic events）方面展开比较分析研究。就此而言，古生物学基于化石证据进行比较发育形态学分析研究在搜寻和发现特定演化支系潜在的祖先显性特征（ancestral phenotypic features）方面有其独特的优势。.本项目实施过程中按计划采用Micro-CT扫描技术手段，从选取的一批中国早期蝾螈类化石以及特有现生物种标本上成功获取了大量高精度CT扫描图像数据。本项目对这些图像数据的分析研究揭示出相关蝾螈类骨骼系统中个体发育/系统演化方面具有重要科学意义的资料信息。在此基础上，本项目进行并已完成的研究内容包括：1-现生蝾螈类骨骼解剖研究； 2-化石/现生小鲵类系统分支与骨骼形态的演化序列；3-化石/现生蝾螈类腭区解剖构造的生态-形态学研究；4-蝾螈类肢骨骨化模式研究及其对轴前优势发育假说的影响。其中肢骨骨化模式的研究成果突出，在Science Advances上刊出。然而，原计划中的一批选样标本的CT扫描工作在过去三年中被迫中断，致使蝾螈类幼态持续演化研究未能如期完成，留待后续研究发表。