Data from: Convergence of ion channel genome content in early animal evolution

Multicellularity has evolved multiple times, but animals are the only multicellular lineage with nervous systems. This fact implies that the origin of nervous systems was an unlikely event, yet recent comparisons among extant taxa suggest that animal nervous systems may have evolved multiple times independently. Here, we use ancestral gene content reconstruction to track the timing of gene family expansions for the major families of ion-channel proteins that drive nervous system function. We find that animals with nervous systems have broadly similar complements of ion-channel types but that these complements likely evolved independently. We also find that ion-channel gene family evolution has included large loss events, two of which were immediately followed by rounds of duplication. Ctenophores, cnidarians, and bilaterians underwent independent bouts of gene expansion in channel families involved in synaptic transmission and action potential shaping. We suggest that expansions of these family types may represent a genomic signature of expanding nervous system complexity. Ancestral nodes in which nervous systems are currently hypothesized to have originated did not experience large expansions, making it difficult to distinguish among competing hypotheses of nervous system origins and suggesting that the origin of nerves was not attended by an immediate burst of complexity. Rather, the evolution of nervous system complexity appears to resemble a slow fuse in stem animals followed by many independent bouts of gene gain and loss.

多细胞性（Multicellularity）已多次独立演化，但动物是唯一拥有神经系统的多细胞演化支系。该事实暗示神经系统的起源本是一桩低概率事件，然而近期针对现存分类群（extant taxa）的比较研究表明，动物神经系统或曾多次独立起源。本研究借助祖先基因含量重建（ancestral gene content reconstruction）技术，追踪了驱动神经系统功能的主要离子通道蛋白（ion-channel proteins）家族的基因扩张时序。研究发现，拥有神经系统的动物所携带的离子通道类型组成大体相似，但这类组成极有可能是独立演化而来的。我们还观察到，离子通道基因家族的演化过程中存在多起大规模基因丢失事件，其中两起事件发生后随即伴随了多轮基因复制事件。栉水母动物（Ctenophores）、刺胞动物（cnidarians）与两侧对称动物（bilaterians），在参与突触传递（synaptic transmission）和动作电位（action potential）塑造的通道基因家族中，均经历了独立的基因扩张阶段。我们提出，此类基因家族的扩张或可作为神经系统复杂度提升的基因组特征标记。当前学界针对神经系统起源的假说所指向的祖先节点，并未发生大规模基因扩张，这使得我们难以甄别关于神经系统起源的多种竞争性假说（competing hypotheses），同时表明神经系统的起源并未伴随即时的复杂度爆发式增长。相较而言，神经系统复杂度的演化更像是在干群动物（stem animals）类群中缓慢引燃的导火索，后续伴随了多轮独立的基因得失事件。