The homeodomain transcription factors six3 and otx are involved in patterning the anterior body and parts of the central nervous system (CNS) in bilaterians. Their similar expression patterns have been used as an argument for homology of heads, brains, segmentation, and ciliated larvae. We investigated the developmental expression of six3 and otx in the aplacophoran mollusk Wirenia argentea. Six3 is expressed in subepithelial cells delimiting the apical organ of the solenogaster pericalymma larva. Otx is expressed in cells of the prototroch and adjacent regions as well as in posterior extensions of the prototrochal expression domain. Advanced larvae also show pretrochal otx expression in the developing CNS. Comparative analysis of six3 and otx expression in bilaterians argues for an ancestral function in anterior‐posterior body axis patterning but, due to its presence in animals lacking a head and/or a brain, not necessarily for the presence of these morphological structures in the last common ancestor (LCA) of bilaterians. Likewise, the hypothesis that the posterior border of otx expression corresponds to the border between the unsegmented head and the segmented trunk of the LCA of protostomes is not supported, since otx is extensively expressed in the trunk in W. argentea and numerous other protostomes.

中文翻译：

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在腹足目（Solenogastres（Mollusca））中six3和otx的表达支持双侧前后轴模式中的祖先作用。

同源结构域转录因子six3和otx参与了Bilaterians的前体和中枢神经系统（CNS）的部分模式。它们相似的表达模式已被用作头部，大脑，分段和纤毛幼虫的同源性的论据。我们研究了六面体软体动物Wirenia argentea中six6和otx的发育表达。Six3在上皮下细胞中表达，该上皮下细胞限定了狼尾草蝶ym幼虫的顶端器官。奥克斯在原生动物及其邻近区域的细胞中以及在原生动物表达域的后延伸中表达的“α”表达。晚期幼虫在发育中的中枢神经系统中也显示出滑行前的otx表达。对双语者中Six3和otx表达的比较分析认为，祖先功能在前后体轴模式中起作用，但是由于它在缺乏头部和/或大脑的动物中存在，因此不一定在最后一个存在这些形态结构双语者的共同祖先（LCA）。同样，不支持otx表达的后边界对应于原虫LCA的未分段头部与分段主干之间的边界的假设，因为otx在W. argentea和许多其他原生动物的躯干中广泛表达。