Fig. 5.

Evolutionary scenario for the Ox and AT precursors. Structure of the hypothetical bilaterian ancestral Ox/AT precursor. (A) The hypothetical ancestral Ox/AT bilaterian precursor is composed of an N-terminal signal peptide (blue box), an Ox or AT peptide, represented by the two logo motifs just C-terminal to the signal peptide, and a C-terminal domain of unknown function that is found in most protostomes and in a deuterotostome, the acorn worm. However, we cannot conclude whether this precursor was more closely related to the extant deuterostome Ox neuropeptides bearing prototypic cysteine patterns or to the extant protostomian AT neuropeptides. (B) Probable scenario describing Ox/AT precursor evolution. Even though Oxs (red half-circle) and ATs (yellow half-circle) display no obvious similarity, their receptors are orthologous to each other and the last common ancestor of bilaterians possessed a C-terminal domain (orange triangle) that was retained in present-day ambulacrarians and protostomes and was lost in the lineage leading to chordates.