Zakon Harold H, Zwickl Derrick J, Lu Ying, Hillis David M
Sections of Neurobiology and Integrative Biology, The University of Texas, Austin, TX 78712, USA.
J Exp Biol. 2008 Jun;211(Pt 11):1814-8. doi: 10.1242/jeb.015982.
Animal communication systems are subject to natural selection so the imprint of selection must reside in the genome of each species. Electric fish generate electric organ discharges (EODs) from a muscle-derived electric organ (EO) and use these fields for electrolocation and communication. Weakly electric teleosts have evolved at least twice (mormyriforms, gymnotiforms) allowing a comparison of the workings of evolution in two independently evolved sensory/motor systems. We focused on the genes for two Na(+) channels, Nav1.4a and Nav1.4b, which are orthologs of the mammalian muscle-expressed Na(+) channel gene Nav1.4. Both genes are expressed in muscle in non-electric fish. Nav1.4b is expressed in muscle in electric fish, but Nav1.4a expression has been lost from muscle and gained in the evolutionarily novel EO in both groups. We hypothesized that Nav1.4a might be evolving to optimize the EOD for different sensory environments and the generation of species-specific communication signals. We obtained the sequence for Nav1.4a from non-electric, mormyriform and gymnotiform species, estimated a phylogenetic tree, and determined rates of evolution. We observed elevated rates of evolution in this gene in both groups coincident with the loss of Nav1.4a from muscle and its compartmentalization in EO. We found amino acid substitutions at sites known to be critical for channel inactivation; analyses suggest that these changes are likely to be the result of positive selection. We suggest that the diversity of EOD waveforms in both groups of electric fish is correlated with accelerations in the rate of evolution of the Nav1.4a Na(+) channel gene due to changes in selection pressure on the gene once it was solely expressed in the EO.
动物通讯系统受自然选择影响，因此选择的印记必然存在于每个物种的基因组中。电鱼通过肌肉衍生的发电器官（EO）产生电器官放电（EODs），并利用这些电场进行电定位和通讯。弱电硬骨鱼至少独立进化了两次（长颌鱼目、裸背电鳗目），这使得我们能够比较两个独立进化的感觉/运动系统的进化机制。我们聚焦于两个钠通道基因Nav1.4a和Nav1.4b，它们是哺乳动物肌肉表达的钠通道基因Nav1.4的直系同源基因。这两个基因在非电鱼的肌肉中均有表达。Nav1.4b在电鱼的肌肉中表达，但Nav1.4a在两组电鱼的肌肉中均不再表达，而是在进化中新生的发电器官中表达。我们推测，Nav1.4a可能正在进化，以优化不同感觉环境下的电器官放电，并产生物种特异性的通讯信号。我们获取了非电鱼、长颌鱼目和裸背电鳗目物种的Nav1.4a序列，构建了系统发育树，并确定了进化速率。我们观察到两组中该基因的进化速率均有所提高，这与Nav1.4a从肌肉中消失并在发电器官中分区化的现象一致。我们在已知对通道失活至关重要的位点发现了氨基酸替换；分析表明，这些变化可能是正选择的结果。我们认为，两组电鱼的电器官放电波形多样性与Nav1.4a钠通道基因进化速率的加快相关，这是由于该基因仅在发电器官中表达后选择压力发生了变化。
