Fate and nature of the onychophoran mouth–anus furrow and its contribution to the blastopore

The ancestral states of bilaterian development, and which living groups have conserved them the most, has been a controversial topic in biology for well over a hundred years. In recent years, the idea that gastrulation primitively proceeded via the formation of a slit-like blastopore that then evolved into either protostomy or deuterostomy has gained renewed attention and some molecular developmental support. One of the key pieces of evidence for this 'amphistomy' theory comes from the onychophorans, which form a clear ventral groove during gastrulation. The interpretation of this structure has, however, proved problematic. Based on expression patterns of forkhead (fkh), caudal (cad), brachyury (bra) and wingless (wg/Wnt1), we show that this groove does not correspond to the blastopore, even though both the mouth and anus later develop from it. Rather, the posterior pit appears to be the blastopore; the posterior of the groove later fuses with it to form the definitive anus. Onychophoran development therefore represents a case of 'concealed' deuterostomy. The new data from the onychophorans thus remove one of the key pieces of evidence for the amphistomy theory. Rather, in line with other recent results, it suggests that ancestral bilaterian development was deuterostomic.