Evolution of deuterostomy - and origin of the chordates

Abstract

The chordates are usually characterized as bilaterians showing deuterostomy, i.e. the mouth developing as a new opening between the archenteron and the ectoderm, serial gill pores/slits, and the complex of chorda and neural tube. Both numerous molecular studies and studies of morphology and embryology demonstrate that the neural tube must be considered homologous to the ventral nerve cord(s) of the protostomes, but the origin of the 'new' mouth of the deuterostomes has remained enigmatic. However, deuterostomy is known to occur in several protostomian groups, such as the chaetognaths and representatives of annelids, molluscs, arthropods and priapulans. This raises the question whether the deuterostomian mouth is in fact homologous with that of the protostomes, viz. the anterior opening of the ancestral blastopore divided through lateral blastopore fusion, i.e. amphistomy. A few studies of gene expression show identical expression patterns around mouth and anus in protostomes and deuterostomes. Closer studies of the embryology of ascidians and vertebrates show that the mouth/stomodaeum differentiates from the anterior edge of the neural plate. Together this indicates that the chordate mouth has moved to the anterior edge of the blastopore, so that the anterior loop of the ancestral circumblastoporal nerve cord, which is narrow in the protostomes, has become indistinguishable. In the vertebrates, the mouth has moved further around the anterior pole to the 'ventral' side. The conclusion must be that the chordate mouth (and that of the deuterostomes in general) is homologous to the protostomian mouth and that the latest common ancestor of protostomes and deuterostomes developed through amphistomy, as suggested by the trochaea theory.

The developmental basis for the recurrent evolution of deuterostomy and protostomy

“…2a), as the planuloid/acoeloid scenario would imply [15][16][17][18] . Although the amphistomy concept [7][8][9][10][11][12][13] could conceptually explain the different blastoporal fates of T. transversa and N.…”

“…1c). The amphistomy concept assumes that the ancestral pre-bilaterian condition was a 'Gastraea'-like organism with a single opening to the gut, which later formed a slit-like blastoporal opening that closed laterally with its ends forming the mouth and anus simultaneously [7][8][9][10][11][12][13][14] . Modifications of blastopore closure, e.g.…”

“…Modifications of blastopore closure, e.g. by convergent extension 11 , in this ancestral pattern of amphistomic gastrulation [7][8][9][10]12,13 would secondarily generate both deutero-and Unformatted accepted version 3 protostomy (Fig. 1c).…”

Duplicated FoxA genes in the leech Helobdella: Insights into the evolution of direct development in clitellate annelids

“…In these species, blastopore is fated to become the mouth. Although some investigators argued for the possibility of amphistomy in spiralians rather than protosomy (Arendt and Nubler-Jung, 1997;Arendt et al, 2001;Lartillot et al, 2002b;Nielsen, 2017), the mouth opening is definitely a blastopore derivative in these polychaete species.…”

Neurons and Glia Cells in Marine Invertebrates: An Update

“…The marine arrow worm Chatognatha is an example of an organism with ancestral plexus and centralized nervous structures (Harzsch and Wanninger, 2010). It is speculated that these organisms are the earliest protostome, and its study contributes to the understanding of the NS evolution in Bilateria (Fröbius and Funch, 2017;Nielsen, 2017).…”