Smith Frank W, Bartels Paul J, Goldstein Bob
Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
Department of Biology, Warren Wilson College, PO Box 28815, Asheville, NC 9000, USA.
Integr Comp Biol. 2017 Sep 1;57(3):546-559. doi: 10.1093/icb/icx081.
Incredibly disparate brain types are found in Metazoa, which raises the question of how this disparity evolved. Ecdysozoa includes representatives that exhibit ring-like brains-the Cycloneuralia-and representatives that exhibit ganglionic brains-the Panarthropoda (Euarthropoda, Onychophora, and Tardigrada). The evolutionary steps leading to these distinct brain types are unclear. Phylogenomic analyses suggest that the enigmatic Tardigrada is a closely related outgroup of a Euarthropoda + Onychophora clade; as such, the brains of tardigrades may provide insight into the evolution of ecdysozoan brains. Recently, evolutionarily salient questions have arisen regarding the composition of the tardigrade brain. To address these questions, we investigated brain anatomy in four tardigrade species-Hypsibius dujardini, Milnesium n. sp., Echiniscus n. sp., and Batillipes n. sp.-that together span Tardigrada. Our results suggest that general brain morphology is conserved across Tardigrada. Based on our results we present a hypothesis that proposes direct parallels between the tardigrade brain and the segmental trunk ganglia of the tardigrade ventral nervous system. In this hypothesis, brain neuropil nearly circumscribes the tardigrade foregut. We suggest that the tardigrade brain retains aspects of an ancestral cycloneuralian brain, while exhibiting ganglionic structure characteristic of euarthropods and onychophorans.
在后生动物中发现了极其不同的脑类型，这就引发了这种差异是如何进化而来的问题。蜕皮动物包括呈现环状脑的类群——环神经动物，以及呈现神经节脑的类群——泛节肢动物（真节肢动物、有爪动物和缓步动物）。导致这些不同脑类型的进化步骤尚不清楚。系统基因组分析表明，神秘的缓步动物是真节肢动物+有爪动物进化枝的一个密切相关的外类群；因此，缓步动物的脑可能为蜕皮动物脑的进化提供见解。最近，关于缓步动物脑的组成出现了一些具有进化意义的问题。为了解决这些问题，我们研究了四种缓步动物——杜氏高生熊虫、新种米尔恩熊虫、新种棘影熊虫和新种巴氏棘影熊虫——的脑解剖结构，这四种缓步动物共同涵盖了缓步动物门。我们的结果表明，缓步动物门的一般脑形态是保守的。基于我们的结果，我们提出了一个假说，该假说提出了缓步动物脑与缓步动物腹侧神经系统的节段性躯干神经节之间的直接相似之处。在这个假说中，脑髓质几乎环绕着缓步动物的前肠。我们认为，缓步动物的脑保留了祖先环神经动物脑的一些特征，同时呈现出真节肢动物和有爪动物特有的神经节结构。
