253 — Getting to the bottom of anal evolution

Biologists, don't try to come up with pun titles for your papers because that game has already been won and it's over:https://t.co/PIr5JrkR3P

— ℙ𝕙𝕚𝕝 𝕊𝕡𝕖𝕒𝕣 (@Duke_of_neural) April 28, 2018

¡Incredible!

After the giggling subsides, this turns out to be a really interesting question both in terms of evolution as well as in terms of understanding our relationship to the rest of the animal kingdom. Based upon the enormous amount of variation in gut architecture (most Bilateria have some sort of alimentary canal), it’s clear that anal evolution took place a long phylogenetic time ago.

For example: Acoelomorphs, or bilateral flatworms, lack coeloms and a “through gut” — the terminology for the gut architecture in which material passes end-to-end continuously. Instead, they use a single gut opening. But species more closely related to through-gut-wielding anuses don’t all necessarily also have through guts; instead, it appears that expression of a certain subset of hindgut developmental markers vary even within subgroups of Bilateria.

This through-gun discussion becomes a question of the origin of the anus, and by proxy, the fate of the blastopore (the developmental invagination of the preembryonic cell mass). Different hypotheses place anal evolution in, generally, one of three categories: Either genital-derived; hindgut-derived; or endoderm-derived.

This phrase from the paper stood out to me:

Advancing live imaging technologies in three dimensions allow to trace embryonic event is much greater detail.

Certainly sundays’s paper will shed light on the embryonic development of the anus — and teach us about the evolutionary origins of bilateral body plan in general.