Background Understanding the evolution and development of morphological traits of the last common bilaterian ancestor is a major goal of the evo-devo discipline. The reconstruction of this "urbilaterian" is mainly based on comparative studies of common molecular patterning mechanisms in recent model organisms. The NK homeobox genes are key players in many of these molecular pathways, including processes regulating mesoderm, heart and neural development. Shared features seen in the expression patterns of NK genes have been used to determine the ancestral bilaterian characters. However, the commonly used model organisms provide only a limited view on the evolution of these molecular pathways. To further investigate the ancestral roles of NK cluster genes, we analyzed their expression patterns in the onychophoran Euperipatoides rowelli. Results We identified nine transcripts of NK cluster genes in E. rowelli, including single copies of NK1, NK3, NK4, NK5, Msx, Lbx and Tlx, and two copies of NK6. All of these genes except for NK6.1 and NK6.2 are expressed in different mesodermal organs and tissues in embryos of E. rowelli, including the anlagen of somatic musculature and the heart. Furthermore, we found distinct expression patterns of NK3, NK5, NK6, Lbx and Msx in the developing nervous system. The same holds true for the NKL gene NK2.2, which does not belong to the NK cluster but is a related gene playing a role in neural patterning. Surprisingly, NK1, Msx and Lbx are additionally expressed in a segment polarity-like pattern early in development-a feature that has been otherwise reported only from annelids. Conclusion Our results indicate that the NK cluster genes were involved in mesoderm and neural development in the last common ancestor of bilaterians or at least nephrozoans (i.e., bilaterians to the exclusion of xenacoelomorphs). By comparing our data from an onychophoran to those from other bilaterians, we critically review the hypothesis of a complex "urbilaterian" with a segmented body, a pulsatile organ or heart, and a condensed mediolaterally patterned nerve cord.

中文翻译：

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NK 簇基因在 onychophoran Euperipatoides rowelli 中的表达：对肾虫中 NK 家族基因进化的影响。

背景 了解最后一个共同祖先的形态特征的进化和发展是 evo-devo 学科的主要目标。这种“urbilaterian”的重建主要基于对近期模式生物中常见分子模式机制的比较研究。NK 同源框基因是许多这些分子途径的关键参与者，包括调节中胚层、心脏和神经发育的过程。在 NK 基因的表达模式中看到的共同特征已被用于确定祖先的双边特征。然而，常用的模式生物仅对这些分子途径的进化提供了有限的看法。为了进一步研究 NK 簇基因的祖先作用，我们分析了它们在 onychophoran Euperipatoides rowelli 中的表达模式。结果 我们在 E. rowelli 中鉴定了 9 个 NK 簇基因的转录本，包括 NK1、NK3、NK4、NK5、Msx、Lbx 和 Tlx 的单拷贝，以及 NK6 的两个拷贝。除 NK6.1 和 NK6.2 外，所有这些基因都在 E. rowelli 胚胎的不同中胚层器官和组织中表达，包括体细胞肌肉组织的原基和心脏。此外，我们在发育中的神经系统中发现了 NK3、NK5、NK6、Lbx 和 Msx 的不同表达模式。NKL 基因 NK2.2 也是如此，它不属于 NK 簇，而是在神经模式中发挥作用的相关基因。令人惊讶的是，NK1、Msx 和 Lbx 在发育早期还以类似片段的极性模式表达——这一特征仅在环节动物中报道过。结论 我们的研究结果表明，NK 簇基因参与了两栖动物或至少是肾虫（即不包括异体腔动物的两栖动物）的最后共同祖先的中胚层和神经发育。通过将我们的甲骨动物数据与其他双侧动物的数据进行比较，我们批判性地审查了具有分段身体、搏动器官或心脏以及浓缩的中间外侧图案神经索的复杂“乌比动物”的假设。