Unwinding snail chirality by a massive subtractive linkage analysis (MSLA)

虽然许多动物的外表是对称的，但它们中的许多是内在不对称的，或手性的：脊椎动物（包括我们自己）有一个或几个器官向一侧移位，蠕虫有一个不对称的神经系统，甚至果蝇，长期以来被认为具有完美的“镜像”对称，也有一个不对称的肠道。对于一种动物来说，要想变得不对称，对称性必须在发育过程中以某种方式被打破。这就提出了一个有趣的问题，即我们称之为“正确”的一方本质上是任意的，那么如何始终将一方与另一方区分开来。解决方案是，在开发的早期阶段，一个假设的、不对称的“F分子”与前/后和顶/底平面排成一行，从而产生左右不对称。后来的器官不对称性是因为F-分子向一侧或另一侧（即向右或向左）发送某些物质。为了试图理解不对称是如何建立的，科学家们将研究重点放在了小鼠、小鸡和斑马鱼等模式生物上。在这些动物中，已经发现，早期胚胎中的小毛发（“纤毛”）的旋转跳动产生了不对称的流体运动，从而暗示这是关键的破坏性步骤：纤毛周围的马达蛋白的不对称导致定向流体运动，最终决定了分子和生物体的不对称性。虽然这些发现是优雅的，但一些最近的和更古老的研究表明，打破沉默的事件要早得多，使上述研究的最终相关性受到怀疑。例如，在线虫中，左右不对称是在六细胞阶段建立的，可能更早。在池塘中，蜗牛的手性是由母亲在未受精卵中沉积的物质决定的。在青蛙中，分子的不对称性是由四细胞阶段建立的，甚至在斑马鱼和小鸡中，也有迹象表明在纤毛阶段之前存在差异。总之，这些结果表明，在许多动物中，包括我们的近亲，分子不对称性在胚胎发育的早期就建立了，形态不对称性只是在后来才变得明显。我们相信，蜗牛可能是理解破蛹步骤的关键模式生物，因为它们的不对称性很早就建立起来了，但近年来几乎完全被忽视了。因此，该项目的目标是利用新的DNA测序技术的力量，直接识别决定蜗牛卵手性的基因序列。由于一种想法是，这个基因也是F分子，那么这项工作将在未来导致对蜗牛中的破囊事件的理解。这些结果将为其他生物（包括脊椎动物）中相同或相关分子的分析提供动力。最后，由于我们将使用的方法是超高通量DNA测序的全新应用，因此该项目的成功将是使用相同方法识别其他基因的跳板。

Although many animals are symmetric on the outside, very many of them are inwardly asymmetric, or chiral: vertebrates (including ourselves) have one or several organs displaced to one side, nematode worms have an asymmetric nervous system, and even fruitflies, long-supposed to have perfect 'mirror image' symmetry, have an asymmetric intestine. For an animal to become asymmetric, symmetry must somehow be broken during development. This raises the interesting problem of how one side is consistently distinguished from the other, given that the side that we call 'right' is essentially arbitrary. The solution is that in the early stages of development a hypothetical, asymmetric 'F-molecule' lines up with the front/back and top/bottom planes, so creating a left-right asymmetry. Later organ asymmetry comes about because the F-molecule sends some substance(s) toward one side or the other (i.e. to the right, or, to the left). To attempt to understand how asymmetry is established, scientists have focussed their research on model organisms such as the mouse, chick and zebrafish. In these animals, it has been found that rotational beating of small hairs ('cilia') in the early embryo create a fluid movement that is asymmetric, leading to a suggestion that this is the critical symmetry-breaking step: the asymmetry of motor proteins around the cilia leads to directional fluid movement, ultimately determining the molecular and organismal asymmetry. While these findings are elegant, some recent and also much older research indicates that the symmetry-breaking event is much earlier, putting the ultimate relevance of the above research into doubt. For instance, in the nematode worm, left-right asymmetry is established by the six-cell stage, and probably earlier. In the pond snail chirality is determined by a substance that the mother deposits in the unfertilised egg. In the frog, molecular asymmetry is established by the four-cell stage, and even in zebrafish and chick, there are indications of differences prior to the cilial stage. Together, the results suggest that in many animals, including our close relatives, molecular asymmetry is established early in the development of embryos, with morphological asymmetry only becoming apparent later. We believe that snails may be a crucial model organism in coming to understand the symmetry-breaking step, because their asymmetry is established very early, yet they have been almost completely neglected in recent years. The objective of this project, therefore, is to utilise the power of new DNA sequencing technologies to directly identify the gene sequence that determines chirality in snail eggs. As one idea is that this gene is also the F-molecule, then this work will lead in the future to an understanding of the symmetry-breaking event in snails. The results will then invigorate analyses of the same or related molecules in other organisms, including vertebrates. Finally, as the methodology that we will use is an entirely new application of ultrahigh-throughput DNA sequencing, then success in this project would be a springboard towards using the same method to identify other genes with the same methodology.