Amazing stuff! What came first release of sperm or waste through a hole?
"... it was a huge development in the evolution of animals: A one-way gut was a game changer when it came to absorbing nutrients, as it allowed for sequential digestion. ... [compared to] vomit up waste ... The hole was co-opted from one used to release sperm.
The team studied Xenoturbella bocki, which belongs to a group of animals called xenacoelomorphs. These animals look so much like flatworms that they were lumped in together, until genetic analyses revealed they weren’t closely related after all. Instead, xenacoelomorphs were one of the first offshoots in the clade known as bilaterians—creatures with bilateral symmetry, like us. Another key trait of bilaterians: a one-way digestive system. ...
Xenoturbella bocki lacks a wonderful exit to its digestive tract, which researchers suspect is because xenacoelomorphs broke off from other bilaterians just before the anus evolved. That means these creatures could provide insights into how the opening first appeared. So the team studied the animals, looking for the expression of genes associated with the hindgut. Despite not having an anus themselves, the wormy critters did express genes associated with the opening—or, males did, anyway. The genes are active in a structure called the gonopore, a hole that releases sperm.
“These findings suggest a deep evolutionary relationship between the xenacoelomorph male gonopore and the bilaterian anus,” the team wrote. “What happened is likely that the hole [gonopore] existed, and the digestive system was close by,” lead author Andreas Hejnol told New Scientist. “And then they just fused. They connected to each other, and they made a common opening.” ..."
From the abstract:
"The bilaterian through gut with an anal opening is a key invention in animals, since it facilitates effective food processing, which allows animals to grow to a larger body size.
However, because non-bilaterian animals lack a through gut, the evolution of anus is still debated.
The formation of bilaterian hindgut is governed by the spatial expression of several transcription factors (e.g. Caudal and Brachyury) under the control of Wnt signaling. This conserved pattern has been used to support the homology of the anus of protostomes (insects, snails) and deuterostomes (sea urchins, humans). Here we show, that these bilaterian “hindgut” marker genes are expressed around the male gonopore of several xenacoelomorphs, which have a blind gut without an anal opening.
These findings suggest a deep evolutionary relationship between the xenacoelomorph male gonopore and the bilaterian anus. Since xenacoelomorphs are the potential sister group to all remaining Bilateria, our results suggest that the bilaterian anus evolved from a male gonopore that came in contact with the digestive endoderm to form the posterior opening."
The xenacoelomorph gonopore is homologous to the bilaterian anus (preprint, open access)
Fig. 1 The hypothetical evolutionary transition from a blind gut to a through gut.
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