Protostomy, Deuterostomy? - Does it really matter?

Speaker: Groupleader Andreas Hejnol, SARS centeret i Norge

Host: Associate Professor Katrine Worsaae, Marine Biology Section

Abstract
Generations of biology students learn that all bilaterally symmetrical animals (e.g. humans and flies) belong to either Deuterostomia or Protostomia, a fundamental grouping that was originally based on whether the primary embryonic opening, called the blastopore, becomes the mouth or the anus of the adult. This division has prevailed for over 100 years, and has influenced nearly all views on animal evolution. However, gastrulation in Protostomia is vastly variable. For instance, most arthropods, priapulids and chaetognaths, as well as some spiralian lineages, exhibit deuterostomic development.  The phylogenetic distribution of both modes of gastrulation indicates a recurrent evolution of protostomy and deuterostomy in the Protostomia - e.g. amphipod crustaceans evolved protostomy from a deuterostomic embryo.  What are the developmental mechanisms that underly these evolutionary switches from one mode to the other?  To identify the mechanisms underlying the recurrent evolution of these two embryonic patterns, we compared the development of two related species of brachiopods that have similar ecological and reproductive strategies, but surprisingly display deuterostomic and protostomic development respectively. The investigation of the establishment of the axial polarity and fate identity during embryogenesis demonstrated that the protostomic species undergoes an extensive re-patterning of the blastoporal rim that relates to the cooption of the blastoporal orifice into the mouth opening. The differential deployment of Wnt signaling around the vegetal pole, together with the timing and location of mesoderm formation, is sufficient to influence the differential behavior and fate of the blastopore in these two species of brachiopods. Our findings leverage the current knowledge of animal embryogenesis to mechanistically explain the evolution of deuterostomy and protostomy, demonstrating that the blastoporal behavior is a developmental by-products (i.e. ‚spandrels‘). Our study thus challenges the long-standing evolutionary emphasis on extant blastoporal behaviors to explain the origin and diversification of bilaterian animals and renders the blastoporal behavior as irrelevant for understanding animal evolution.