Blastopore Fate: Amphistomy, Protostomy or Deuterostomy

Abstract

The bilaterian tubular gut with mouth and anus is generally believed to have evolved from the sack‐shaped gut of a gastrula‐like organism. Cell‐lineage studies show that gastrulation through epiboly and invagination follow similar patterns with the cells of the blastopore rim bordering the cells which give rise to endo‐mesoderm. Three theories for the evolution of the tubular gut prevail: (1) Protostomy in which the blastopore should become the mouth and the anus develop secondarily, (2) Deuterostomy in which the blastopore should become the anus and the mouth develop secondarily and (3) Amphistomy in which the blastopore should become divided into mouth and anus through fusion of the lateral blastopore lips. A recent review has discussed the most informative characters related to the blastopore fates, viz. fate of the actual blastoporal opening; fate of the tissues surrounding the blastoporal opening, studied both through cell‐lineage and gene expression; morphology and embryology of the central nervous systems; and morphology of larval ciliary bands according to the trochaea theory. It is concluded that the tubular gut with mouth and anus most probably evolved through amphistomy. Key Concepts The latest common ancestor of the eumetazoans (ctenophores and placozoans are not discussed here) was a gastrula‐like organism, with a sac‐shaped gut with a blastopore. The tubular gut of the bilateral animals evolved from the sack‐shaped gut of the common ancestor. Amphistomy is a condition in which the tubular gut evolved from the sack‐shaped gut through lateral blastopore closure, leaving mouth and anus. Protostomy is a condition in which the blastopore was retained as the bilaterian mouth, the anus developing as a secondary opening. Deuterostomy is a condition in which the blastopore was retained as the bilaterian anus, the mouth developing as a secondary opening. In invagination gastrula, the gastrula develops from a blastula through invagination of the endoderm forming the archenteron with the blastopore. In epibolic gastrula, there is no invagination, but the endo‐mesodermal cells become internalized through overgrowth by the ectodermal cells; the endoderm differentiates into the gut, and mouth and anus develop from ectodermal invaginations, stomodaeum and proctodaeum, respectively.

Decline and fall of the Ediacarans: late‐Neoproterozoic extinctions and the rise of the modern biosphere

“…This scenario would be reinforced by evidence for multiple, independent origins of bilaterian‐like locomotion among Ediacaran animals, including dickinsoniomorphs, ‘bilateralomorphs’, and bona fide bilaterians. Indirect support comes from the evolutionary–developmental evidence for convergent origins of a through‐gut and neural centralisation among extant bilaterians (Hejnol & Martindale, 2008; Hejnol & Pang, 2016; Gavilán et al ., 2016; Cannon et al ., 2016; Nielsen, Brunet & Arendt, 2018; Nielsen, 2019; Martín‐Durán & Hejnol, 2021; but see Kapli & Telford, 2020). Vice versa , evidence for a bilaterian‐like eumetazoan common ancestor (Finnerty, 2003) coupled with a derived status for Ediacarans (e.g.…”