Univ Lyon 1, ENSL, UJM, CNRS, LGL-TPE (Laboratoire de Géologie de Lyon: Terre, Planète, Environnement), 69622 Villeurbanne, France;
Mathematical Institute, University of Oxford, Oxford OX2 6GG, United Kingdom.
Proc Natl Acad Sci U S A. 2021 Nov 30;118(48). doi: 10.1073/pnas.2109210118.
Snails are model organisms for studying the genetic, molecular, and developmental bases of left-right asymmetry in Bilateria. However, the development of their typical helicospiral shell, present for the last 540 million years in environments as different as the abyss or our gardens, remains poorly understood. Conversely, ammonites typically have a bilaterally symmetric, planispiraly coiled shell, with only 1% of 3,000 genera displaying either a helicospiral or a meandering asymmetric shell. A comparative analysis suggests that the development of chiral shells in these mollusks is different and that, unlike snails, ammonites with asymmetric shells probably had a bilaterally symmetric body diagnostic of cephalopods. We propose a mathematical model for the growth of shells, taking into account the physical interaction during development between the soft mollusk body and its hard shell. Our model shows that a growth mismatch between the secreted shell tube and a bilaterally symmetric body in ammonites can generate mechanical forces that are balanced by a twist of the body, breaking shell symmetry. In gastropods, where a twist is intrinsic to the body, the same model predicts that helicospiral shells are the most likely shell forms. Our model explains a large diversity of forms and shows that, although molluscan shells are incrementally secreted at their opening, the path followed by the shell edge and the resulting form are partly governed by the mechanics of the body inside the shell, a perspective that explains many aspects of their development and evolution.
蜗牛是研究两侧对称动物中左右不对称的遗传、分子和发育基础的模式生物。然而，它们典型的螺旋壳的发育仍然知之甚少，这种螺旋壳在过去的 5.4 亿年中存在于深渊或我们的花园等不同的环境中。相比之下，菊石通常具有双侧对称的平面螺旋壳，只有 3000 个属中的 1%显示出螺旋或蜿蜒的不对称壳。比较分析表明，这些软体动物中手性壳的发育是不同的，与蜗牛不同，具有不对称壳的菊石可能具有双侧对称的身体，这是头足类动物的特征。我们提出了一个用于贝壳生长的数学模型，考虑了在发育过程中软体动物身体和硬壳之间的物理相互作用。我们的模型表明，菊石中分泌的贝壳管与双侧对称身体之间的生长不匹配会产生机械力，这些机械力被身体的扭曲所平衡，从而打破了贝壳的对称性。在腹足类动物中，身体的扭曲是内在的，同样的模型预测螺旋壳是最有可能的贝壳形式。我们的模型解释了大量的形式，并表明，尽管软体动物的贝壳是在开口处逐渐分泌的，但贝壳边缘所遵循的路径和产生的形状部分受到壳内身体力学的控制，这种观点解释了它们发育和进化的许多方面。
