Development of the annelid axochord: insights into notochord evolution

The origin of chordates has been debated for more than a century, with one key issue being the emergence of the notochord. In vertebrates, the notochord develops by convergence and extension of the chordamesoderm, a population of midline cells of unique molecular identity. We identify a population of mesodermal cells in a developing invertebrate, the marine annelid Platynereis dumerilii, that converges and extends toward the midline and expresses a notochord-specific combination of genes. These cells differentiate into a longitudinal muscle, the axochord, that is positioned between central nervous system and axial blood vessel and secretes a strong collagenous extracellular matrix. Ancestral state reconstruction suggests that contractile mesodermal midline cells existed in bilaterian ancestors. We propose that these cells, via vacuolization and stiffening, gave rise to the chordate notochord.

脊索动物（chordates）的起源问题已争论逾百年，其中一个核心争议点便是脊索（notochord）的起源与演化。在脊椎动物中，脊索由脊索中胚层（chordamesoderm）的会聚延伸发育而来，脊索中胚层是一群具有独特分子标识的中线细胞群。本研究在一种发育中的无脊椎动物——海洋环节动物杜氏异沙蚕（Platynereis dumerilii）——中鉴定出一群向中线会聚延伸的中胚层细胞，这群细胞表达一套脊索特异性的基因组合。这些细胞最终分化为一条纵行肌肉——轴索（axochord），该结构定位于中枢神经系统与轴向血管之间，并分泌致密的胶原样细胞外基质。祖先状态重建分析显示，两侧对称动物的祖先类群中已存在具有收缩能力的中胚层中线细胞。本研究据此提出，这类细胞通过空泡化与硬化过程，演化形成了脊索动物的脊索。