Background Segmentation, the subdivision of the major body axis into repeated elements, is considered one of the major evolutionary innovations in bilaterian animals. In all three segmented animal clades, the predominant segmentation mechanism is sequential segmentation, where segments are generated one by one in anterior-posterior order from a posterior undifferentiated zone. In vertebrates and arthropods, sequential segmentation is thought to arise from a clock-and-wavefront-type mechanism, where oscillations in the posterior growth zone are transformed into a segmental prepattern in the anterior by a receding wavefront. Previous evo-devo simulation studies have demonstrated that this segmentation type repeatedly arises, supporting the idea of parallel evolutionary origins in these animal clades. Sequential segmentation has been studied most extensively in vertebrates, where travelling waves have been observed that reflect the slowing down of oscillations prior to their cessation and where these oscillations involve a highly complex regulatory network. It is currently unclear under which conditions this oscillator complexity and slowing should be expected to evolve, how they are related and to what extent similar properties should be expected for sequential segmentation in other animal species. Results To investigate these questions, we extend a previously developed computational model for the evolution of segmentation. We vary the slope of the posterior morphogen gradient and the strength of gene expression noise. We find that compared to a shallow gradient, a steep morphogen gradient allows for faster evolution and evolved oscillator networks are simpler. Furthermore, under steep gradients, damped oscillators often evolve, whereas shallow gradients appear to require persistent oscillators which are regularly accompanied by travelling waves, indicative of a frequency gradient. We show that gene expression noise increases the likelihood of evolving persistent oscillators under steep gradients and of evolving frequency gradients under shallow gradients. Surprisingly, we find that the evolutions of oscillator complexity and travelling waves are not correlated, suggesting that these properties may have evolved separately. Conclusions Based on our findings, we suggest that travelling waves may have evolved in response to shallow morphogen gradients and gene expression noise. These two factors may thus also be responsible for the observed differences between different species within both the arthropod and chordate phyla.

中文翻译：

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全天候：梯度形状和噪声影响振荡分段动力学的演变。

背景分割，将主要身体轴细分为重复元素，被认为是两侧对称动物的主要进化创新之一。在所有三个分段动物进化枝中，主要的分段机制是顺序分段，其中分段是从后部未分化区域按照前后顺序一一生成的。在脊椎动物和节肢动物中，顺序分割被认为是由时钟和波前型机制产生的，其中后生长区的振荡通过后退的波前转化为前部的节段预模式。先前的进化-进化模拟研究表明，这种分割类型反复出现，支持了这些动物进化枝中平行进化起源的想法。顺序分割在脊椎动物中得到了最广泛的研究，其中观察到行波反映了振荡在停止之前的减慢，并且这些振荡涉及高度复杂的调节网络。目前尚不清楚这种振荡器的复杂性和减慢应该在什么条件下进化，它们是如何相关的，以及在其他动物物种中顺序分割应该在多大程度上预期类似的特性。结果为了研究这些问题，我们扩展了先前开发的用于分割演化的计算模型。我们改变后形态发生素梯度的斜率和基因表达噪声的强度。我们发现，与浅梯度相比，陡峭的形态发生素梯度允许更快的进化，并且进化的振荡器网络更简单。此外，在陡峭的梯度下，阻尼振荡器经常会演化，而浅梯度似乎需要持续的振荡器，这些振荡器经常伴随着行波，表明频率梯度。我们表明，基因表达噪声增加了在陡梯度下进化持久振荡器和在浅梯度下进化频率梯度的可能性。令人惊讶的是，我们发现振荡器复杂性和行波的演化并不相关，这表明这些特性可能是单独演化的。结论根据我们的发现，我们认为行波可能是响应浅形态发生素梯度和基因表达噪声而进化的。因此，这两个因素也可能是造成节肢动物门和脊索动物门内不同物种之间观察到的差异的原因。