The xenacoelomorph gonopore is homologous to the bilaterian anus

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.