Animal left–right asymmetry

Abstract

Symmetry is appealing, be it in architecture, art or facial expression, where symmetry is a key feature to finding someone attractive or not. Yet, asymmetries are widespread in nature, not as an erroneous deviation from the norm but as a way to adapt to the prevailing environmental conditions at a time. Asymmetries in many cases are actively selected for: they might well have increased the evolutionary fitness of a species. Even many single-celled organisms are built asymmetrically, such as the pear-shaped ciliate Paramecium, which may depend on its asymmetry to navigate towards the oxygen-richer surface of turbid waters, at least based on modeling. Everybody knows the lobster with its asymmetric pair of claws, the large crusher usually on the left and the smaller cutter on the right. Snail shells coil asymmetrically, as do the organs they house. Organ asymmetries are found throughout the animal kingdom, referring to asymmetric positioning, asymmetric morphology or both, with the vertebrate heart being an example for the latter. Functional asymmetries, such as that of the human brain with its localization of the language center in one hemisphere, add to the complexity of organ asymmetries and presumably played a decisive role for sociocultural evolution. The evolutionary origin of organ asymmetries may have been a longer than body length gut, which allows efficient retrieval of nutrients, and the need to stow a long gut in the body cavity in an orderly manner that ensures optimal functioning. Vertebrate organ asymmetries (situs solitus) are quite sophisticated: in humans, the apex of the asymmetrically built heart points to the left; the lung in turn, due to space restrictions, has fewer lobes on the left than on the right side (two versus three in humans), stomach and spleen are found on the left, the liver on the right, and small and large intestine coil in a chiral manner (Figure 1A). In very rare cases (1:10,000), the organ situs is inverted (situs inversus), while heterotaxia refers to another rare situation (about 1:1,000), in which subsets of organs show normal or aberrant positioning or morphology (Figure 1B). Individuals with situs solitus or situs inversus are healthy, whereas heterotaxia presents severe congenital malformations. Many human syndromes are known in which patients suffer from laterality defects, such as Katagener syndrome, in which the organ situs is inverted in one half of patients and males are sterile. Snail shells and vertebrate organs are examples of biased asymmetries with on average only one inversion in every 10,000 cases. Other asymmetries such as the coiling of the tails of piglets occur randomly with a 50:50 distribution. This primer exclusively deals with organ asymmetries in the animal kingdom, specifically with the mechanisms that ensure the development of biased asymmetries during embryogenesis.

The rodent hippocampus as a bilateral structure: A review of hemispheric lateralization

“…How does lateralization happen in the first place? The genetics of organismal lateralization may be conserved in invertebrates and in vertebrates (Blum & Ott, 2018;Yuan & Brueckner, 2018). In mammals, thoracic, and abdominal organs have a ubiquitous asymmetric arrangement and the development of this lateralization is dependent upon leftward flow of growth factors generated by the chiral beating of cilia during embryonic development (Nonaka, Shiratori, Saijoh, & Hamada, 2002).…”

E and ID proteins regulate cell chirality and left–right asymmetric development in Drosophila

“…Many animals show directional LR asymmetry in their body structures and functions, and the mechanism of LR asymmetric development has been a central question in Developmental Biology (Levin, ). These molecular mechanisms have been well studied in vertebrates (Blum & Ott, ; Kimelman & Martin, ; Riechmann & Ephrussi, ). For example, in some vertebrates, the LR symmetry is first broken by an LR directional flow of extra‐embryonic fluid, which is induced by ciliary rotation in the node or its equivalent tissue, in early embryos (Yoshiba & Hamada, ).…”

“…For example, in some vertebrates, the LR symmetry is first broken by an LR directional flow of extra‐embryonic fluid, which is induced by ciliary rotation in the node or its equivalent tissue, in early embryos (Yoshiba & Hamada, ). In contrast, in invertebrates the mechanisms of LR asymmetric formation remain unclear, although a few excellent studies have unveiled basic concepts behind the directional LR asymmetric formation in nematodes, snails and Drosophila (Blum & Ott, ; Kuroda, Endo, Abe, & Shimizu, ; Naganathan, Fürthauer, Nishikawa, Jülicher, & Grill, ; Okumura et al, ; Spéder & Noselli, ). Some invertebrate species develop LR asymmetric body structures using mechanisms arising from the intrinsic chirality of blastomeres or cells in tissues, which is distinct from the mechanism used in vertebrates, indicating that the processes for directional LR symmetric development diverged in evolution (Blum & Ott, ; Inaki, Sasamura, & Matsuno, ; Okumura et al, ).…”

The rodent hippocampus as a bilateral structure: A review of hemispheric lateralization

“…How does lateralization happen in the first place? The genetics of organismal lateralization may be conserved in invertebrates and in vertebrates (Blum & Ott, 2018;Yuan & Brueckner, 2018). In mammals, thoracic and abdominal organs have a ubiquitous asymmetric arrangement and the development of this lateralization is dependent upon leftward flow of growth factors generated by the chiral beating of cilia during embryonic development (Nonaka et al, 2002).…”