Abstract
In this review, we consider transformations of axial symmetry in metazoan evolution and development, the genetic basis, and phenotypic expressions of different axial body plans. In addition to the main symmetry types in metazoan body plans, such as rotation (radial symmetry), reflection (mirror and glide reflection symmetry), and translation (metamerism), many biological objects show scale (fractal) symmetry as well as some symmetry-type combinations. Some genetic mechanisms of axial pattern establishment, creating a coordinate system of a metazoan body plan, bilaterian segmentation, and left–right symmetry/asymmetry, are analysed. Data on the crucial contribution of coupled functions of the Wnt, BMP, Notch, and Hedgehog signaling pathways (all pathways are designated according to the abbreviated or full names of genes or their protein products; for details, see below) and the axial Hox-code in the formation and maintenance of metazoan body plans are necessary for an understanding of the evolutionary diversification and phenotypic expression of various types of axial symmetry. The lost body plans of some extinct Ediacaran and early Cambrian metazoans are also considered in comparison with axial body plans and posterior growth in living animals.
Highlights
The symmetry of an animal body is one of the basic features characterizing the body plan (Bauplan) in different metazoan clades [1,2,3,4,5,6,7,8,9,10,11,12,13]
Body axes form the basis of animal body plans; polarity establishment requires a symmetry-breaking event, so body polarity is a breaking of symmetry [8,24,25]
Locomotion in three-dimensional macro-world space is itself sufficient to explain the maintenance of bilateral symmetry in animal evolution, and bilaterality is the only type of symmetry that can maximize this force; an actively moving bilateral body can have the maximal maneuverability as compared to other symmetry types, providing obvious selective advantages in the bilateral animal [25,60]
Summary
The symmetry of an animal body is one of the basic features characterizing the body plan (Bauplan) in different metazoan clades [1,2,3,4,5,6,7,8,9,10,11,12,13]. Biological morphogenesis has a dynamical character involving qualitative discontinuities as topological bifurcations tightly coupled with symmetry breaking, so developmental and evolutionary transformations of symmetry are discrete steps in biological morphogenesis inevitably disrupting a preexisting pattern of symmetry [17,18,19,20,29,30] Both phylogeny and ontogeny include transitions from symmetry to dissymmetry, and the process is generally shifted towards symmetries decreasing [8,20,31]. It is possible to establish the relation between morphogenetic fields with positional information provided by gradients of different morphogens (transcription and translation factors, components of signaling systems including Wnt, BMP, Nodal, and Hedgehog) and integral developmental patterns to understand many genetic and cellular mechanisms of symmetry breakdown in metazoan development and evolution [9,13,41,42]. We try to provide a better understanding of correlations between genetic basis and some phenotypic expression of axial body plans in different metazoans, taking into consideration recent advances in this research field
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
