Finnerty John R
Department of Biology, Boston University, 5 Cummington Street, Boston, MA 02215, USA.
Bioessays. 2005 Nov;27(11):1174-80. doi: 10.1002/bies.20299.
The standard explanation for the origin of bilateral symmetry is that it conferred an advantage over radial symmetry for directed locomotion. However, recent developmental and phylogenetic studies suggest that bilateral symmetry may have evolved in a sessile benthic animal, predating the origin of directed locomotion. An evolutionarily feasible alternative explanation is that bilateral symmetry evolved to improve the efficiency of internal circulation by affecting the compartmentalization of the gut and the location of major ciliary tracts. This functional design principle is illustrated best by the phylum Cnidaria where symmetry varies from radial to tetraradial, biradial and bilateral. In the Cnidaria, bilateral symmetry is manifest most strongly in the internal anatomy and the disposition of ciliary tracts. Furthermore, the bilaterally symmetrical Cnidaria are typically sessile and, in those bilaterally symmetrical cnidarians that undergo directed locomotion, the secondary body axis does not bear a consistent orientation to the direction of locomotion as it typically does in Bilateria. Within the Cnidaria, the hypothesized advantage of bilateral symmetry for internal circulation can be tested by experimental analysis and computer modeling of fluid mechanics. The developmental evolution of symmetry within the Cnidaria can be further explored through comparative gene expression studies among species whose symmetry varies.
对于两侧对称起源的标准解释是，相较于辐射对称，它在定向运动方面具有优势。然而，最近的发育学和系统发育学研究表明，两侧对称可能在一种固着底栖动物中就已演化出来，早于定向运动的起源。一种在进化上可行的替代解释是，两侧对称的演化是为了通过影响肠道的分隔和主要纤毛束的位置来提高内部循环的效率。这种功能设计原则在刺胞动物门中体现得最为明显，该门动物的对称性从辐射对称到四辐射对称、两侧辐射对称和两侧对称不等。在刺胞动物中，两侧对称在内部解剖结构和纤毛束的排列中表现得最为强烈。此外，两侧对称的刺胞动物通常是固着的，而在那些进行定向运动的两侧对称刺胞动物中，第二体轴与运动方向的取向并不像在两侧对称动物中那样保持一致。在刺胞动物门内，可以通过流体力学的实验分析和计算机建模来检验两侧对称对内部循环的假设优势。通过对对称性不同的物种进行比较基因表达研究，可以进一步探索刺胞动物门内对称性的发育演化。
