Crossing of the pleurovisceral nerve cords in gastropods has supported the view that gastropods evolved by 180 degrees rotation between the ventral and dorsal body regions. Indeed, a rotation of this type occurs as a dramatic morphogenetic movement ("ontogenetic torsion") during the development of basal gastropods. According to a long-standing hypothesis, ontogenetic torsion in basal gastropods preserves an ancient developmental aberration that generated the contorted gastropod body plan. It follows from this reasoning that crossing of the pleurovisceral nerve cords during gastropod development should be mechanically coupled to ontogenetic torsion. The predicted mechanical coupling can now be examined because of the discovery of an early differentiating neuron in Haliotis kamtschatkana (Vetigastropoda) that expresses 5-hydroxytryptamine-like immunoreactivity. The neuron appeared to delineate the trajectory of the pleurovisceral nerve cords beginning before ontogenetic torsion. Before torsion, the neuronal soma is embedded in mantle epithelium at the ventral midline and two neurites extend anteriorly toward the apical sensory organ. Contrary to expectation, the two neurites of this cell did not cross-over during ontogenetic torsion because the soma of this mantle neuron shifted in the same direction as the rotating head and foot. Full crossing of the pleurovisceral nerve cords occurred gradually during later development as the mantle cavity deepened and expanded leftward. These results are consistent with a generalization emerging from comparative studies indicating a conserved developmental stage for gastropods in which the mantle cavity is localized to one side, despite a fully "post-torsional" orientation for other body components. Developmental morphology before this stage is much more variable among different gastropod clades.

译文

腹足动物胸膜内脏神经索的交叉支持了腹足动物通过腹侧和背侧身体区域之间180度旋转而进化的观点。实际上,在基础腹足类动物的发育过程中,这种类型的旋转是一种戏剧性的形态发生运动 (“个体发生扭转”)。根据长期存在的假设,基础腹足动物的个体发育扭转保留了古老的发育畸变,该畸变产生了扭曲的腹足动物身体计划。从这种推理可以得出,腹足动物发育过程中胸膜内脏神经索的交叉应与个体发生扭转机械耦合。现在可以检查预测的机械耦合,因为在Haliotis kamtschatkana (Vetigastropoda) 中发现了表达5-羟色胺样免疫反应性的早期分化神经元。神经元似乎描绘了个体发生扭转之前开始的胸膜内脏神经索的轨迹。在扭转之前,神经元躯体嵌入腹侧中线的套膜上皮中,两个神经突向前向顶端感觉器官延伸。与预期相反,该细胞的两个神经突在个体发生扭转期间没有交叉,因为该套神经元的躯体与旋转的头和脚沿相同的方向移动。在后来的发育过程中,随着地幔腔的加深和向左扩展,胸膜内脏神经索的完全交叉逐渐发生。这些结果与比较研究得出的概括一致,该比较研究表明腹足类动物处于保守的发育阶段,尽管其他身体成分完全处于 “扭转后” 方向,但地幔腔仍位于一侧。不同腹足纲进化枝在此阶段之前的发育形态变化更大。

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