Abstract

Axial homologies re-examined: a common ground plan at the base of Eumetazoa

Of the roughly 35 to 40 extant metazoan phyla, nearly all maintain marine representatives with biphasic lifestyles consisting of a pelagic larval dispersal phase and an adult benthic phase. Many conflicting scenarios regarding the emergence and loss of benthic forms have been proposed, but it has remained relatively unclear whether the first bilaterian animals were benthic, pelagic or ancestrally bi-phasic in regard to life history. We have assessed shared features of marine larvae and find striking similarity in the molecular topology, the cell types and the blastoporally localized signaling that form the apical organ. While conservation in wholesale patterning and cell type complement is evident for all primary larvae investigated, the similarities are most striking for species such as the annelid Platynereis, the hemichordate Saccoglossus, and the cnidarian Nematostella which undergo a gradual transition from larvae to adults. We would therefore propose that the last common ancestor of cnidarians and bilaterians had a “minimally indirect developing larvae” that underwent a similarly gradual settlement transition. We are currently investigating the gene regulatory network that gives rise to this conserved apical plate in the annelid Platynereis through the analysis of whole genome ChIP-seq datasets and are expanding our comparative analysis to include apical plate patterning in Cnidarian planula and other lophotrochozoan larvae. We have also conducted a re-examination of gut and coelom patterning in several taxa to gain insight into the evolution of endomesodermal patterning.