The fate of the blastopore during development in the bilaterian ancestor is currently not well understood. In deuterostomes, the blastopore forms the anus, but its fate in protostome groups is variable. This variability, combined with an absence of information from key taxa, hampers the reconstruction of the ancestral developmental mode of the Protostomia and the Bilateria. The blastopore fate of the bilaterian ancestor plays a crucial role in understanding the transition from radial to bilateral symmetric organisms. Priapulids have a conservative morphology, an abundant Cambrian fossil record, and a phylogenetic position that make them a key group in understanding protostome evolution. Here, we characterize gastrulation and the embryonic expression of genes involved in bilaterian foregut and hindgut patterning in Priapulus caudatus. We show that the blastopore gives rise to the anus at the vegetal pole and that the hindgut markers brachyury and caudal are expressed in the blastopore and anus, whereas the foregut markers foxA and goosecoid are expressed in the mouth in the animal hemisphere. Thereby, gastrulation in the conservatively evolving protostome P. caudatus follows strictly a deuterostomic pattern. These results are more compatible with a deuterostomic rather than protostomic (blastopore forms the mouth) or amphistomic (mouth and anus are formed simultaneously) mode of development in the last common bilaterian ancestor.
A study on the development of priapulids or ‘penis’ worms throws doubt on a feature that has been thought for more than 100 years to define the largest branch of the animal tree of life. Members of this branch — the protostomes — have historically been defined by the order in which they develop a mouth and an anus as embryos. But gene-expression data suggest that this definition is incorrect, researchers report this week in Current Biology.
Evolutionary biologists will need to rename the protostomes. To do that, “we need to rethink how our earliest ancestors developed,” says Andreas Hejnol, an evolutionary developmental biologist at the University of Bergen in Norway and lead author on the report.
'Penis worm' pokes holes in evolutionary dogma: Priapulid highlights the need to rename the largest animal group
José M. Martín-Durán, Ralf Janssen, Sofia Wennberg, Graham E. Budd, Andreas Hejnol. 2012. Deuterostomic Development in the Protostome Priapulus caudatus. Current Biology. DOI: http://dx.doi.org/10.1016/j.cub.2012.09.037
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