Acoel development provides insights into anal evolution

By | May 7, 2021
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Acoels ,a bilaterally symmetric triploblastic worms possess a mid-ventral mouth and lack an anus. Morphology and recent molecular phylogenies place the Acoela and the nemertodermatid flatworms as basal branches of the Bilateria.

The evolutionary origin of the bilaterian mouth and anus, and their relationship to the blastopore can be explained by many hypothesis ,but the most famous is the one ,which begin with either a radially symmetric larva or a cnidarian polyp-like organism that elongates its body along the future anterior–posterior axis, followed by a lateral closure of the slit-like blastoporal opening with the ends giving rise to a mouth and anus simultaneously.

Few years back, Andreas Hejnol & Mark Q. Martindale published their results of developmental gene expression of Convolutriloba longifissura in Nature journal , which deny the leading theory of anal evolution claiming that anuses evolve independently from mouth and in some lineages, the anus may have formed through a fusion of the gut with the reproductive organs ( probably getting a hint from the anuses of amphibians, reptiles and birds ,which also works as channels of reproduction).

Studying the developmental expression of genes responsible for formation of mouth like brachyury and goosecoid in higher organisms supports the homology of the single body opening of acoels with the mouth of protostomes and ambulacrarians (echinoderms and hemichordates).But the gene brachyury also shows expression in posterior region and in order to understand better this feature Andreas looked into the expression of further genes expressed in the hindgut of some bilaterians, caudal (cdx), orthopedia (otp), forkhead (FoxA, also known as HNF3) and NK2.1.They also looked into the the embryonic fate map of acoel flatworms.

Important conclusions from Expression and lineage analysis:

1) Acoel mouth opening is homologous to the mouth of protostomes and deuterostomes and that the last common ancestor of the Bilateria “Urbilateria”.
2) The presence of hindgut genes in coordinated manner can be explained in two ways

a) A through gut( two openings) was present is urbilateria (last common ancestor of bilaterians) but anus is lost independently in Acoels and Nemertodermatida lineages.
b) The metazoan mouth evolved first and the anal opening arose independently.

Objections :

Detlev Arendt, a researcher at the European Molecular Biology Laboratory in Heidelberg, Germany ,who was leading author in the work which suggested that that over time, the mouth elongated and then separated into a mouth and anus. Once the body included a gut with two ends, the anus could migrate to the far end of animal.

“I’m not convinced at all,” “The issue remains open.”–Detlev Arendt

Detlev finding this study difficult to digest as C. longifissura are fast-evolving creatures and what seems to be reproductive organs now could have be started as anus.The work on slowly evolving creatures might help to conclude many things and can further help to understand evolution of gut in metazoans. But whatever be the doubts it still is a great study and further experiments in this direction might solve the puzzle.

“It’s interesting and it makes you think,” — Claus Nielsen

Reference :

Andreas Hejnol & Mark Q. Martindale
Acoel development indicates the independent evolution of the bilaterian mouth and anus.
Nature , doi:10.1038/nature07309 September 2008.