Biology Department, University of Washington, Seattle, WA, United States; Friday Harbor Laboratories, Friday Harbor, WA, United States.
Biology Department, University of Washington, Seattle, WA, United States; Friday Harbor Laboratories, Friday Harbor, WA, United States.
Curr Top Dev Biol. 2021;141:149-171. doi: 10.1016/bs.ctdb.2020.10.001. Epub 2020 Dec 16.
The Origin of Chordates has fascinated scientists from the time of Charles Darwin's publication "Descent of Man" in 1871. For over 100 years, it was accepted that chordates evolved from tunicates, our sessile invertebrate sister group. However, genomic and embryonic analyses have shown that lancelets have a body plan and genome much more like vertebrates than do tunicates. In 2000, we proposed a worm-like hypothesis of chordate origins, and genomic and embryonic studies in the past 20 years have supported this hypothesis. This hypothesis contends that the deuterostome ancestor was worm-like, with gill slits, very much like a chordate. In contrast, tunicates have a very derived adult body plan that evolved independently. Here, we review the current understanding of deuterostome phylogeny and supporting evidence for the relationships within each phylum. Then we discuss our hypothesis for chordate origins and evidence to support it. We explore some of the evolutionary changes that ascidians have made to their adult body plan and some of the key gene regulatory networks that have been elucidated in Ciona. Finally, we end with insights that we have gained from studying tailless ascidians for the past 30 years. We've found that differentiation genes, at the end of the gene regulatory networks, become pseudogenes and nonfunctional, even though they are still expressed in tailless ascidians. We expect that eventually these pseudogenes will not be expressed and the ascidian larval body plan is abandoned, leaving the embryo to develop directly into an adult.
脊索动物的起源一直令科学家着迷，从查尔斯·达尔文 1871 年出版的《人类的由来》开始。100 多年来，人们一直认为脊索动物是从被囊动物进化而来的，被囊动物是我们无柄的无脊椎动物姐妹群。然而，基因组和胚胎分析表明，尾索动物的身体结构和基因组与脊椎动物更相似，而不是被囊动物。2000 年，我们提出了一个关于脊索动物起源的蠕虫假说，过去 20 年的基因组和胚胎研究支持了这一假说。该假说认为，后口动物的祖先是蠕虫状的，有鳃裂，非常类似于脊索动物。相比之下，被囊动物具有非常衍生的成体身体结构，是独立进化而来的。在这里，我们回顾了后口动物系统发育的当前理解以及每个门内关系的支持证据。然后，我们讨论了我们对脊索动物起源的假设及其支持证据。我们探讨了尾索动物在成体身体结构上所做的一些进化改变，以及在海鞘中阐明的一些关键基因调控网络。最后，我们以过去 30 年研究无尾尾索动物获得的一些见解作为结束。我们发现，分化基因，在基因调控网络的末端，变成假基因，失去功能，尽管它们仍然在无尾尾索动物中表达。我们预计，最终这些假基因将不再表达，尾索动物的幼虫身体结构将被放弃，使胚胎直接发育成成体。
