Abstract
Symmetry in the arrangement of body parts is a distinctive phylogenetic feature of animals. Cnidarians show both bilateral and radial symmetries in their internal organs, such as gastric pouches and muscles. However, how different symmetries appear during the developmental process remains unknown. Here, we report intraspecific variations in the symmetric arrangement of gastric pouches, muscles, and siphonoglyphs, the Anthozoan-specific organ that drives water into the organism, in D. lineata (Diadumenidae, Actiniaria). We found that the positional arrangement of the internal organs was apparently constrained to either biradial or bilateral symmetries depending on the number of siphonoglyphs. Based on the morphological observations, a mathematical model of internal organ positioning was employed to predict the developmental backgrounds responsible for the biradial and bilateral symmetries. In the model, we assumed that the specification of gastric pouches is orchestrated by lateral inhibition and activation, which results in different symmetries depending on the number of siphonoglyphs. Thus, we propose that a common developmental program can generate either bilateral or biradial symmetries depending on the number of siphonoglyphs formed in the early developmental stages.
Highlights
Symmetry in the arrangement of external and internal organs is a distinctive phylogenetic feature of animals [1,2,3,4,5]
D. lineata pigmentation at the body wall and oral surface is an externally visible indicator of internal organs In D. lineata, orange-striped pigmentations only appear on the body column at the levels of first gastric pouch (P1) and second gastric pouch (P2) (Fig. 2a) [27]
Histological analysis confirmed that the orange stripes are associated with all P1s and P2s but not third gastric pouch (P3) and further revealed that they are restricted to the endodermal cell layer (Figs. 2c, d)
Summary
Symmetry in the arrangement of external and internal organs is a distinctive phylogenetic feature of animals [1,2,3,4,5]. Bilaterians (e.g., vertebrates) are defined by bilateral symmetry, in which paired organs are arranged in a mirror image to a single symmetry plane (Fig. 1a). Symmetrical arrangement is apparent in internal organs such as gastric pouches and muscles that arise perpendicular to the oral–aboral axis In scyphozoans (e.g., A. aurita), Both biradial and bilateral symmetries are found in anthozoan species [7, 12] (Fig. 1a). In addition to the mesenteries and muscles, there are siphonoglyph organs placed at longitudinal termini of the actinopharynx to control the (2021) 7:12 a b c d e
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