Asymmetry in a pterobranch hemichordate and the evolution of left–right patterning

Abstract

To investigate the possible origin of vertebrate asymmetry, we reexamined the long-described asymmetry of pterobranch hemichordates, a group of tiny, mostly colonial marine filter feeders. As first described over a century ago, we found left-right (LR) asymmetry in localization of the gonad. However, contrary to early descriptions, the direction of asymmetry is random, displaying antisymmetry rather than directional asymmetry. We found that the direction of gonad localization is significantly related to the shape of the tube in which the zooid lives. We also reexamined other anatomical features described to have directional asymmetry, the shape of the oral lamella and localization of the gonopore; however, we did not confirm asymmetry in these examples. Together with previous studies, our results suggest that hemichordates have undergone degeneration in the precision of asymmetry from an ancestral asymmetrical condition that existed before the divergence of deuterostomes. Developmental Dynamics 237:3634 -3639, 2008.

A detailed description of the development of the hemichordate Saccoglossus kowalevskii using SEM, TEM, Histology and 3D-reconstructions

“…However, a recent statistical analysis of the position of the gonads done by Sato & Holland [76] could furthermore show that the position of the gonad displays antisymmetry and not directional asymmetry.…”

Right across the tree of life: The evolution of left–right asymmetry in the Bilateria

“…At least some adult hemichordates have subtle asymmetries, for example, in the location of an anterior pore (Kaul‐Strehlow and Stach, ). Whether pterobranch hemichordates are asymmetric is less clear; classical descriptions of these animals as having an asymmetric gonad have recently been shown to be antisymmetric rather than directionally asymmetric (Sato and Holland, ). As of yet, the regulation of hemichordate asymmetry has not been described.…”

“…1). Most hemichordates are long, thin, and worm-shaped animals, although one hemichordate lineage, the pterobranchs, are tube dwellers (Sato and Holland, 2008). At least some adult hemichordates have subtle asymmetries, for example, in the location of an anterior pore (Kaul-Strehlow and Stach, 2013).…”

Filter feeding, deviations from bilateral symmetry, developmental noise, and heterochrony of hemichordate and cephalochordate gills

“…Our finding is limited by the number of species analyzed, but in the context of the hypothesis above, our results provide a robust tendency within the study groups and bring evidence that gills of species that have experienced a relaxation of the filter feeding trait exhibit elevated FA. This finding is significant because it rejects hypotheses that the deuterostome ancestor was asymmetrical (Jefferies et al, 1996;Sato & Holland, 2008). Instead, the directional asymmetry of stem group fossil echinoderm (Smith, 2008;Zamora & Rahman, 2014;Zamora et al, 2012) and cephalochordates evolved independently (Igawa et al, 2017;Kaji et al, 2016).…”

“…The first four genes are transcription factors that are arranged in a synteny unique to the deuterostomes, including echinoderms (Simakov et al, 2015). The symmetry of the ancestral deuterostome is a subject of debate because living and fossil species may be bilateral or directionally asymmetrical (Cameron, 2016;Jefferies, 2001;Sato & Holland, 2008;Zamora & Rahman, 2014). Deuterostomes are comprised of two major branches, and the acorn worms are thought to most closely resemble the ancestral Ambulacraria (including echinoderms) whereas the cephalochordates the ancestral Chordata (including tunicates and vertebrates).…”