Evolutionary Diversity of the Mechanisms Providing the Establishment of Left-Right Asymmetry in Metazoans

Left-right asymmetry is an important feature of the body plan of bilaterian animals. Malformations of the left-right axis result in pathologies of varying severity. Therefore, studying the mechanisms of its establishment in embryonic development is important for science and medicine. The main question to answer in the course of research is how and when the symmetry breakage in a developing organism occurs. This area of research is intensely developing, and various mechanisms of the left-right axis establishment have already been discovered in animals of different phylogenetic groups. Unfortunately, comparative studies of these mechanisms often focus on a few phylogenetically distant objects of developmental biology and do not analyze the diversity that exists within the taxonomic groups. Therefore, the aims of this review are to show the diversity of mechanisms for establishing left-right asymmetry in the early development of metazoans, to make assumptions about the evolutionary conservatism of these mechanisms, and to establish which of the mechanisms is primary for the Bilateria and for the Metazoa in general. The article systematizes our knowledge about the establishment of left-right asymmetry on the basis of the Nodal signaling cascade activity, compares the morphology and functioning of the left-right organizers of different animals, and compares the role of mechanical tensions in establishing asymmetry in different Protostomia. Generalization and analysis of data on the establishment of left-right asymmetry provides a new insight into the evolution of the development of metazoans.