Molecular evidence from xenacoelomorph gonopore formation supports homology with the bilaterian anus.

The bilaterian through gut with an anal opening is a major evolutionary innovation in animals. It facilitates effective food processing, which allows animals to grow to a larger body size. However, because non-bilaterian animals (such as cnidarians) lack a through gut, the evolution of the anus in bilaterians (such as insects and humans) remains unresolved. The formation of the bilaterian hindgut is driven by the spatial expression of several transcription factors (for example, Caudal and Brachyury) under the control of Wnt signalling. Here we show that this bilaterian 'hindgut' molecular signature is expressed around the male gonopore of several xenacoelomorphs, which possess a blind gut without an anal opening. Since xenacoelomorphs are the potential sister group to all remaining Bilateria, our results suggest that the bilaterian anus shares a deep evolutionary relationship with the xenacoelomorph male gonopore. We therefore propose that the bilaterian anus evolved from a male gonopore that secondarily connected to the digestive tract.