Mechanisms of Homoplastic Developmental Evolution in the Nematode Male Tail

线虫雄性尾部同质发育进化的机制为了探索生物体的发育结构对其进化的制约程度，这项拟议的工作利用了同质性提供的多个“进化实验”，即独立物种谱系中相似特征的进化。在没有强烈的发育限制的情况下，同质性变化很可能不会发生在相同的基因中，甚至不应该发生在相同的遗传途径中。另一方面，如果一个特征的每个独立外观都涉及相同的机制，则表明存在发展限制。实验旨在测试同质性是否存在于发育层次中的特定水平：即，形态同质性是否涉及相同的发育过程、相同的遗传途径和/或途径的相同部分？在与秀丽线虫相关的横纹线虫的系统发育研究中，在雄性尾部的两种结构中发现了同源性，这是一个研究得很好的、遗传上可获得的模式，用于阐明发育途径和参与模式和形态发生的基因。具体地说，在横纹虫的进化过程中，发生了多个独立的变化：(1)在化学感受性感受器的前后位置之间，相对于称为射线的3个机械感觉感受器；(2)在雄性尾尖的尖形细毛虫和圆形的斗鱼尾端之间。这些性状可作为形态进化的一般类别的模型，如异位和异时。为了评估同质性的程度，首先将在单个细胞(细胞谱系、融合、形态发生)的水平上检查所涉及的发育过程，以查看在形态上的每一次同质性变化期间，发育是否以相同的方式变化。其次，将测试至少一条候选遗传途径，看看它是否在控制同型形态特征方面是保守的。如果是，那么将分析指示基因的表达模式，以确定是否每次都改变途径的相同部分(即，被测试基因的上游或下游)。(确定确切的因果分子或基因变化并不是简单地解决同质性发生在什么水平上的问题；只需要“读出”途径中的几个点，就可以测试哪里的表达是保守的，或者是物种之间的差异。)初步结果表明，相粒位置的进化涉及单个原始细胞(T)不对称分裂的极性的切换，为异位(相对位置的进化变化)提供了一种新的机制。Wnt信号通路是参与其中的一个很好的候选者，因为它控制着线虫的这种极性。初步数据还表明，微小RNA控制的异时途径在尾端形态的进化中可能起到作用。虽然许多Evo-DEVO研究集中在转录调控上，但雄性尾尖进化可能为发育进化中的转录后调控提供一种新的模式。这一模型还可能为异时性(相对时间的进化变化)如何导致形态变异提供更深层次的机制理解。无论在这两个特征的进化中是否在任何特定的机制水平上存在同质性，最终(除了目前的提议之外)确定哪种基因和分子变化是进化发育变化的基础将是重要的。因此，无论发现什么，拟议的研究都将为未来理解进化发展机制的许多方面的工作提供基础。广泛影响其他科学家和教育工作者用于比较生物学的主要资源，纽约大学Rhabditid文库，将在这项研究过程中保持不增加成本。将开发更有效的基因敲除方法，以增强线虫系统的能力，供其他研究人员在比较功能基因组学和进化发展方面进行其他研究。新的试剂和抗体将被开发出来。这项工作的结果将发表在声誉良好的期刊上。在新的研究中，将招收不同的本科生，并对他们进行个别指导。一名博士生和一名博士后助理将接受进化、发育和分子遗传学方面的跨学科培训。

Mechanisms of homoplastic developmental evolution in the nematode male tailIntellectual merit To explore the degree to which the developmental architecture of an organism constrains its evolution, the proposed work exploits the multiple "evolutionary experiments" provided by homoplasy, i.e. the evolution of similar features in independent species lineages. In the absence of strong developmental constraints, chances are that homoplastic changes should not occur in the same gene or even the same genetic pathway. On the other hand, if the same mechanism is involved in each independent appearance of a feature, developmental constraint is indicated. Experiments are designed to test whether or not homoplasy exists at a particular level in the developmental hierarchy: i.e., does morphological homoplasy involve the same developmental process, the same genetic pathway, and/or the same part of the pathway? From the phylogenetic studies of rhabditid nematodes related to Caenorhabditis elegans, homoplasy is found in two kinds of structures in the male tail, a well-studied, genetically accessible model for elucidating developmental pathways and genes involved in patterning and morphogenesis. Specifically, during rhabditid evolution, multiple independent changes have occurred (1) between anterior and posterior positions of the chemoreceptive sensilla called phasmids, relative to 3 mechanosensory sensilla called rays, and (2) between the pointed leptoderan and rounded peloderan forms of male tail tips. These characters serve as models for general classes of morphological evolution, such as heterotopy and heterochrony. To assess the degree of homoplasy, the developmental processes involved will first be examined at the level of individual cells (cell lineages, fusions, morphogenesis) to see if development changes the same way during each homoplastic change in morphology. Secondly, at least one candidate genetic pathway will be tested to see if it is conserved in controlling the homoplastic morphological feature. If it is, then expression patterns of indicator genes will be analyzed to determine if the same part of the pathway (i.e., upstream or downstream of the tested gene) changed each time. (Determining the exact causal molecular or genetic change is not required simply to address at what level homoplasy occurred; only a "read-out" of a couple points in the pathway is needed to test where expression is conserved or where it differs between species.) Preliminary results suggest that the evolution of phasmid position involves switches in the polarity of the asymmetric division of a single blast cell (T), providing a novel mechanism for heterotopy (evolutionary change in relative positioning). The Wnt signaling pathway is a good candidate for being involved, as it controls this polarity in C. elegans. Preliminary data also suggest a potential role for the micro-RNA-controlled heterochronic pathway in the evolution of tail tip morphology. Although many evo-devo studies focus on transcriptional regulation, male tail tip evolution may provide a novel model for posttranscriptional regulation in developmental evolution. This model is also likely to provide a deeper mechanistic understanding into how heterochrony (evolutionary change in relative timing) can result in morphological variation. Regardless of whether or not homoplasy exists at any particular mechanistic level in the evolution of these two features, it will be important eventually (beyond the current proposal) to determine what kinds of genes and molecular changes underlie the evolutionary developmental changes. Thus, whatever is discovered, the proposed studies will provide a foundation for future work to understand many aspects of evolutionary developmental mechanisms.Broader impact A major resource used by other scientists and educators for comparative biology, the NYU Rhabditid Collection, will be maintained in the course of this research without additional cost. More efficient methods for gene knockdown will be developed to empower nematode systems for other studies by other investigators in comparative functional genomics and evolutionary development. New reagents and antibodies will be developed. Results of the work will be published in well-regarded journals. Diverse undergraduates will be recruited and individually mentored in new research. A PhD student and a postdoctoral associate will receive cross-disciplinary training in evolution, development, and molecular genetics.