Glide-reflection symmetry in deuterostomes: an evolutionary perspective

Abstract

Abstract Alternation of left and right antimeres of segmental structures, generally known as the glide-reflection symmetry, was found to be unexpectedly widespread in deuterostomes. It occurs in both somatic and visceral organs. Its distribution in echinoderms, acorn worms, cephalochordates, ascidians, hagfishes, and chimaeras might suggest its ancestral nature in deuterostomes. It could have been inherited from Precambrian ancestors. There is a lead of the left segments against their right counterparts in all deuterostomes with glide-reflection symmetry, where the leading side is known. This allows us to presume an inherent left-to-left-side and right-to-right-side homology across all deuterostomes, contradicting a hypothesis of the whole-body dorsoventral inversion in chordates. The features of inversion could be alternatively explained by a transformation of the gastrulation process, which has caused an inversion of dorsoventral polarity in some parts of the body, but did not affect the left–right polarity. It is suggested that this process had involved the animal-vegetal extension of the blastopore along embryonic dorsal side in basal deuterostomes that was followed by the shift of the left–right organizer from the dorsal lip of the blastopore to the anterior tip of archenteron in non-chordate deuterostomes, which has caused roof-to-floor relocation of the notochordal area in their archenteron.