Department of Animal Physiology, University of Bayreuth, Bayreuth 95440, Germany.
Proc Natl Acad Sci U S A. 2024 Dec 3;121(49):e2415854121. doi: 10.1073/pnas.2415854121. Epub 2024 Nov 18.
The ability to follow the evolutionary trajectories of specific neuronal cell types has led to major insights into the evolution of the vertebrate brain. Here, we study how cave life in the Mexican tetra () has affected an identified giant multisensory neuron, the Mauthner neuron (MN). Because this neuron is crucial in driving rapid escapes, the absence of predation risk in the cave forms predicts a massive reduction in this neuron. Moreover, the absence of functional eyes in the Pachón form predicts an even stronger reduction in the cell's large ventral dendrite that receives visual inputs in sighted fish species. We succeeded in recording in vivo from this neuron in the blind cavefish and two surface tetra ( and ), which offers unique chances to simultaneously study evolutionary changes in morphology and function in this giant neuron. In contrast to the predictions, we find that cave life, while sufficient to remove vision, has neither affected the cell's morphology nor its functional properties. This specifically includes the cell's ventral dendrite. Furthermore, cave life did not increase the variance in morphological or functional features. Rather, variability in surface and cave forms was the same, which suggests a complex stabilizing selection in this neuron and a continued role of its ventral dendrite. We found that adult cavefish are potent predators that readily attack smaller fish. So, one of the largely unknown stabilizing factors could be using the MN in such attacks and, in the young fish, escaping them.
研究特定神经元细胞类型的进化轨迹的能力，使我们对脊椎动物大脑的进化有了重大的了解。在这里，我们研究了墨西哥钝口螈（Astyanax mexicanus）的洞穴生活如何影响已鉴定的大型多感觉神经元，即玛氏巨神经元（Mauthner neuron，MN）。由于这种神经元在驱动快速逃避中至关重要，因此在洞穴中没有捕食风险，这预示着这种神经元会大量减少。此外，在有眼形式的 Pachón 中，这种神经元接收视觉输入的大型腹侧树突的功能缺失甚至预示着更强烈的减少，而在有视力的鱼类中这种树突是存在的。我们成功地在盲眼洞穴鱼和两种有眼钝口螈（Astyanax fasciatus 和 Astyanax mexicanus）中对这种神经元进行了活体记录，这为同时研究这种巨大神经元在形态和功能上的进化变化提供了独特的机会。与预测相反，我们发现洞穴生活虽然足以消除视觉，但既没有影响细胞的形态，也没有影响其功能特性。这具体包括细胞的腹侧树突。此外，洞穴生活并没有增加形态或功能特征的变异性。相反，表面形式和洞穴形式的变异性是相同的，这表明这种神经元存在复杂的稳定选择，其腹侧树突仍在继续发挥作用。我们发现成年洞穴鱼是强大的捕食者，它们会轻易地攻击较小的鱼类。因此，一个很大程度上未知的稳定因素可能是在这些攻击中使用 MN，以及在幼鱼中逃避这些攻击。
