Mechanisms of Homoplastic Developmental Evolution in the Nematode Male Tail

为了探索生物体的发育结构在多大程度上限制了其进化，本研究利用了同质性提供的多个“进化实验”，即在独立物种谱系中相似特征的进化。在缺乏强大的发育限制的情况下，可能在同一基因甚至同一遗传途径中不会发生同质性变化。另一方面，如果相同的机制涉及到一个特征的每一个独立的外观，则表明发育限制。实验的目的是测试是否同质性存在于发育层次的特定水平上：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？从与秀丽隐杆线虫（Caenorhabditis elegans）相关的横纹肌线虫的系统发育研究来看，在雄性尾巴的两种结构中发现了同质性，这是一种研究得很好的遗传学模型，用于阐明发育途径和参与模式和形态发生的基因。具体来说，在横纹肌的进化过程中，发生了多个独立的变化(1)在化学感受器（称为phasmids）的前后位置之间，相对于3个机械感觉感受器（称为rays），以及(2)在雄性尾尖的尖尾类和圆尾类之间。这些性状可以作为一般形态进化的模型，如异位和异时性。为了评估同质性的程度，所涉及的发育过程将首先在单个细胞（细胞系、融合、形态发生）的水平上进行检查，以查看在每次形态学同质性变化期间发育是否以相同的方式变化。其次，将测试至少一个候选遗传途径，看看它是否在控制同质形态特征方面是保守的。如果是，则分析指示基因的表达模式，以确定每次通路的同一部分（即被测基因的上游或下游）是否发生了变化。（确定确切的分子或遗传变化的因果关系，并不需要简单地解决同源性发生在什么水平上；只需要“读出”通路中的几个点，就可以测试物种之间哪些地方的表达是保守的，哪些地方是不同的。）初步结果表明，相粒位置的进化涉及单个胚细胞(T)不对称分裂的极性开关，为异位（相对位置的进化变化）提供了一种新的机制。Wnt信号通路是参与的一个很好的候选，因为它控制秀丽隐杆线虫的这种极性。初步数据还表明，微rna控制的异时通路可能在尾尖形态的进化中发挥作用。尽管许多进化研究都集中在转录调控上，但雄性尾尖的进化可能为发育进化中的转录后调控提供了一种新的模型。该模型还可能提供更深层次的机制理解，以了解异时性（相对时间的进化变化）如何导致形态变异。无论在这两种特征的进化过程中是否存在任何特定的机制水平上的同质性，最终（超越目前的建议）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，无论发现了什么，所提出的研究将为未来的工作提供基础，以了解进化发展机制的许多方面。NYU Rhabditid Collection是其他科学家和教育工作者在比较生物学方面使用的一个主要资源，将在本研究过程中保留，不收取额外费用。将开发更有效的基因敲除方法，使其他研究人员能够在比较功能基因组学和进化发展方面对线虫系统进行其他研究。将开发新的试剂和抗体。这项工作的结果将发表在知名期刊上。不同的本科生将被招募，并在新的研究中得到个别指导。一名博士生和一名博士后将接受进化、发育和分子遗传学的跨学科培训。

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.