Mechanism of an Evolutionary Change in Development

这个项目的目的是了解脊索体计划中潜在的进化变化的机制。对于这些研究，PI开发了一个模型系统，由两个密切相关的海鞘物种和不同的幼虫组成。海鞘蝌蚪幼虫表现出脊索动物的所有特征，但只有2500个细胞和6个不同的组织。眼镜蛇(有尾物种)有一条游动的蝌蚪幼虫，尾巴上有一个脊索和有条纹的肌肉细胞，而隐形眼镜蛇(无尾物种)则有一种不活动的幼虫，缺乏这些特征。无尾物种的卵子与有尾物种的精子受精后，杂交幼虫的脊索和尾巴得以恢复，这表明它们的丢失部分是由于合子尾巴形成基因的变化。在之前的资助时期，PI已经确定了一些形成尾巴的基因。该项目将重点放在Manx和山猫基因上，这两个基因分别编码一个假定的转录因子和一个RNA解旋酶。初步研究表明，在尾巴物种的基因组中，Manx和山猫以操纵子的形式存在。在这里，PI建议确定这些相关基因如何调节海鞘的进化和发育。首先，PI将描述整个Manx/山猫基因复合体的特征，以确定是否存在其他尾巴形成基因，以及这些基因是否确实是尾巴物种操纵子的一部分。其次，PI将确定Manx和山猫基因在尾巴发育中的作用，特别是在介导细胞增殖和程序性细胞死亡方面。第三，PI将确定无尾物种的Manx/山猫基因的特征，以确定可能积累了哪些突变导致尾部回归。最后，PI将研究Manx和山猫以及叉头和brachyury(T)基因之间的潜在相互作用，这些基因被认为参与了海鞘动物的轴和脊索的发育。因此，PI希望通过研究海囊幼虫的尾巴形成基因，阐明在进化过程中脊索体计划起源的机制。

The purpose of this project is to understand the mechanisms underlying evolutionary changes in the chordate body plan. For these studies, the PI has developed a model system consisting of two closely-related ascidian species with different larvae. The ascidian tadpole larva exhibits all the hallmarks of a chordate yet contains only 2500 cells and six different tissues. Molgula oculata (the tailed species) has a swimming tadpole larva with a tail containing a notochord and striated muscle cells, whereas Molgula occulta (the tailless species) has an immotile larva lacking these features. Fertilization of tailless species' eggs with tailed species' sperm results in restoration of the notochord and tail in the hybrid larvae, suggesting that they were lost in part by changes in zygotic tail-forming genes. In previous funding periods the PI has identified some of the tail forming genes. This project extends those studies in focusing on the Manx and bobcat genes, which encode a putative transcription factor and an RNA helicase respectively. Preliminary studies show that Manx and bobcat are present in an operon-like cluster in the tailed species' genome. Here the PI proposes to determine how these linked genes regulate ascidian evolution and development. First, the PI will characterize the entire Manx/bobcat gene complex to determine whether other tail-forming genes are present and whether these genes are indeed part of an operon in the tailed species. Second, the PI will determine the roles of the Manx and bobcat genes in tail development, particularly in mediating cell proliferation and programmed cell death. Third, the PI will characterize the Manx/bobcat genes in the tailless species to determine what mutations may have accumulated to result in tail regression. Finally, the PI will study the potential interactions between Manx and bobcat and the forkhead and Brachyury (T) genes, which are known to be involved in axis and notochord development in ascidians. Thus, by studying tail forming genes in ascidian larvae, the PI hopes to shed light on the mechanisms underlying the origin of the chordate body plan during evolution.