Zakon Harold H, Jost Manda C, Zwickl Derrick J, Lu Ying, Hillis David M
Section of Neurobiology, The University of Texas, Austin, TX, USASection of Integrative Biology, The University of Texas, Austin, TX, USA.
Integr Zool. 2009 Mar;4(1):64-74. doi: 10.1111/j.1749-4877.2008.00136.x.
Voltage-dependent sodium channels are critical for electrical excitability. Invertebrates possess a single sodium channel gene; two rounds of genome duplication early in vertebrates increased the number to four. Since the teleost-tetrapod split, independent gene duplications in each lineage have further increased the number of sodium channel genes to 10 in tetrapods and 8 in teleosts. Here we review how the occurrence of multiple sodium channel paralogs has influenced the evolutionary history of three groups of fishes: pufferfish, gymnotiform and mormyriform electric fish. Pufferfish (tetraodontidae) produce a neurotoxin, tetrodotoxin, that binds to and blocks the pore of sodium channels. Pufferfish evolved resistance to their own toxins by amino acid substitutions in the pore of their sodium channels. These substitutions had to occur in parallel across multiple paralogs for organismal resistance to evolve. Gymnotiform and mormyriform fishes independently evolved electric organs to generate electricity for communication and object localization. Two sodium channel genes are expressed in muscle in most fishes. In both groups of weakly electric fishes, one gene lost its expression in muscle and became compartmentalized in the evolutionary novel electric organ, which is a muscle derivative. This gene then evolved at elevated rates, whereas the gene that is still expressed in muscle does not show elevated rates of evolution. In the electric organ-expressing gene, amino acid substitutions occur in parts of the channel involved in determining how long the channel will be open or closed. The enhanced rate of sequence evolution of this gene likely underlies the species-level variations in the electric signal.
电压依赖性钠通道对电兴奋性至关重要。无脊椎动物拥有单个钠通道基因；脊椎动物早期的两轮基因组复制将其数量增加到四个。自硬骨鱼 - 四足动物分化以来，每个谱系中的独立基因复制进一步将钠通道基因数量增加到四足动物中的10个和硬骨鱼中的8个。在这里，我们回顾了多个钠通道旁系同源物的出现如何影响了三类鱼类的进化历史：河豚、裸背电鳗目和长颌鱼目电鱼。河豚（鲀科）产生一种神经毒素——河豚毒素，它能结合并阻断钠通道的孔。河豚通过其钠通道孔中的氨基酸替代进化出对自身毒素的抗性。这些替代必须在多个旁系同源物中平行发生，生物体才能进化出抗性。裸背电鳗目和长颌鱼目鱼类独立进化出发电器官，用于交流和物体定位。大多数鱼类的肌肉中表达两种钠通道基因。在这两类弱电鱼中，一个基因在肌肉中失去了表达，并在进化出的新型发电器官（一种肌肉衍生物）中进行了区域化。然后这个基因以较高的速率进化，而仍在肌肉中表达的基因则没有显示出进化速率的提高。在表达于发电器官的基因中，氨基酸替代发生在通道中决定通道开放或关闭时间的部分。该基因序列进化速率的提高可能是电信号物种水平差异的基础。
