Background The larval nervous system of the solitary tunicate Ciona is a simple model for the study of chordate neurodevelopment. The development and connectivity of the Ciona motor ganglion have been studied in fine detail, but how this important structure develops in other tunicates is not well known. Methods and Results By comparing gene expression patterns in the developing MG of the distantly related tunicate Molgula occidentalis, we found that its patterning is highly conserved compared to the Ciona MG. MG neuronal subtypes in Molgula were specified in the exact same positions as in Ciona, though the timing of subtype-specific gene expression onset was slightly shifted to begin earlier, relative to mitotic exit and differentiation. In transgenic Molgula embryos electroporated with Dmbx reporter plasmids, we were also able to characterize the morphology of the lone pair of descending decussating neurons (ddNs) in Molgula, revealing the same unique contralateral projection seen in Ciona ddNs and their putative vertebrate homologs the Mauthner cells. Although Dmbx expression labels the ddNs in both species, cross-species transgenic assays revealed significant changes to the regulatory logic underlying Dmbx transcription. We found that Dmbx cis-regulatory DNAs from Ciona can drive highly specific reporter gene expression in Molgula ddNs, but Molgula sequences are not active in Ciona ddNs. Conclusions This acute divergence in the molecular mechanisms that underlie otherwise functionally conserved cis-regulatory DNAs supports the recently proposed idea that the extreme genetic plasticity observed in tunicates may be attributed to the extreme rigidity of the spatial organization of their embryonic cell lineages.

中文翻译：

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远缘被囊类动物运动神经节模式的发育系统漂移。

背景 独居被囊动物 Ciona 的幼虫神经系统是研究脊索动物神经发育的简单模型。海鞘运动神经节的发育和连接性已被详细研究，但这一重要结构在其他被囊动物中如何发育尚不清楚。方法和结果通过比较远缘被囊类 Molgula occidentalis 发育中 MG 的基因表达模式，我们发现与 Ciona MG 相比，其模式高度保守。Molgula 中的 MG 神经元亚型与 Ciona 中的位置完全相同，但相对于有丝分裂退出和分化，亚型特异性基因表达开始的时间略有改变。在用 Dmbx 报告质粒电穿孔的转基因 Molgula 胚胎中，我们还能够表征 Molgula 中孤对下降交叉神经元 (ddNs) 的形态，揭示了在 Ciona ddNs 及其假定的脊椎动物同源物 Mauthner 细胞中看到的相同独特的对侧投射。尽管 Dmbx 表达标记了两个物种的 ddN，但跨物种转基因测定揭示了 Dmbx 转录基础调控逻辑的显着变化。我们发现来自 Ciona 的 Dmbx 顺式调控 DNA 可以驱动 Molgula ddN 中高度特异性的报告基因表达，但 Molgula 序列在 Ciona ddN 中不活跃。结论 功能保守的顺式调控 DNA 分子机制的这种严重分歧支持了最近提出的观点，即在被囊类动物中观察到的极端遗传可塑性可能归因于其胚胎细胞谱系的空间组织的极端刚性。