Among animals, diploblasts contain two germ layers, endoderm and ectoderm, while triploblasts have a distinct third germ layer called the mesoderm. Spiralians are a group of triploblast animals that have highly conserved development: they share the distinctive spiralian cleavage pattern as well as a unique source of mesoderm, the ectomesoderm. This population of mesoderm is distinct from endomesoderm and is considered a hallmark of spiralian development, but the regulatory network that drives its development is unknown. Here we identified ectomesoderm-specific genes in the mollusc Tritia (aka Ilyanassa) obsoleta through differential gene expression analyses comparing control and ectomesoderm-ablated embryos, followed by in situ hybridization of identified transcripts. We identified a Tritia serpin gene (ToSerpin1) that appears to be specifically expressed in the ectomesoderm of the posterior and head. Ablation of the 3a and 3b cells, which make most of the ectomesoderm, abolishes ToSerpin1 expression, consistent with its expression in these cells. Morpholino knockdown of ToSerpin1 causes ectomesoderm defects, most prominently in the muscle system of the larval head. This is the first gene identified that is specifically implicated in spiralian ectomesoderm development.

在动物中，双胚细胞包含两个胚层，内胚层和外胚层，而三胚细胞具有一个独特的第三胚层，称为中胚层。螺旋虫是一群高度保守发育的三倍体动物：它们具有独特的螺旋虫卵裂模式以及中胚层，外胚层的独特来源。中胚层的这一群体不同于内胚层，被认为是螺旋状发展的标志，但是驱动其发展的调控网络是未知的。在这里软体动物我们鉴定ectomesoderm特异性基因Tritia（又名Ilyanassa）obsoleta通过差异基因表达分析比较控制和ectomesoderm消融胚胎，随后在原位鉴定的转录本的杂交。我们确定了Tritia serpin基因（ToSerpin1），该基因似乎在后部和头部的外胚层中特异性表达。消融了大部分外胚层的3a和3b细胞，废除了ToSerpin1表达，与其在这些细胞中的表达一致。ToSerpin1的Morpholino抑制导致外胚层缺陷，最明显的是幼虫头部的肌肉系统。这是鉴定出的第一个与螺旋外胚层发育特别相关的基因。