Symmetry diversification in animals and plants: Exploration from phenotypic polymorphisms

Abstract

Symmetry in animals and plants is a fundamental aspect of the body morphology. No matter if it is animal or plant, symmetry types in individuals can be evaluated based on organ properties such as types, numbers, and arrangements. There are mainly two types of symmetry, defined by the number of symmetry axes that divide the body into two same parts. Bilateral symmetry is characterized by a single symmetry axis and radial symmetry is characterized by multiple symmetry axes. How are symmetry types diversified? To explore these mechanisms, we mainly focused on intraspecific phenotypic variations. This review broadens its scope to include animals and plants, exploring the mechanisms of symmetry diversification. By comparing animals and plants revealed both commonalities and differences in symmetry diversity mechanisms. Transition between radial symmetry and bilateral symmetry suggests need for elements that establish a new axis, often involving asymmetric positional information along the adaxial-abaxial (directive) axis, as seen in the expression of CYCLOIDEA (CYC) homologues in angiosperm flowers. In the case of sea anemones, intraspecific polymorphism suggested the siphonoglyph establishment as a potential source of positional information, similar to the role of CYC in plants. In hydrozoans with radial symmetry, the number of symmetry axes generally increases as body size expands. Nevertheless, when we focus on larger clades such as cnidarians or eudicots, the number of symmetry axes between species is maintained, suggesting additional mechanism(s) that ensure the robustness of specific numbers. Understanding these mechanisms could provide valuable insights into the evolution and robustness of symmetry in multicellular organisms.