title: Evolutionary origin of gastrulation: insights from sponge development.
abstract: BACKGROUND
  The evolutionary origin of gastrulation--defined as a morphogenetic event that leads to the establishment of germ layers--remains a vexing question. Central to this debate is the evolutionary relationship between the cell layers of sponges (poriferans) and eumetazoan germ layers. Despite considerable attention, it remains unclear whether sponge cell layers undergo progressive fate determination akin to eumetazoan primary germ layer formation during gastrulation.
  
  
  RESULTS
  Here we show by cell-labelling experiments in the demosponge Amphimedon queenslandica that the cell layers established during embryogenesis have no relationship to the cell layers of the juvenile. In addition, juvenile epithelial cells can transdifferentiate into a range of cell types and move between cell layers. Despite the apparent lack of cell layer and fate determination and stability in this sponge, the transcription factor GATA, a highly conserved eumetazoan endomesodermal marker, is expressed consistently in the inner layer of A. queenslandica larvae and juveniles.
  
  
  CONCLUSIONS
  Our results are compatible with sponge cell layers not undergoing progressive fate determination and thus not being homologous to eumetazoan germ layers. Nonetheless, the expression of GATA in the sponge inner cell layer suggests a shared ancestry with the eumetazoan endomesoderm, and that the ancestral role of GATA in specifying internalised cells may antedate the origin of germ layers. Together, these results support germ layers and gastrulation evolving early in eumetazoan evolution from pre-existing developmental programs used for the simple patterning of cells in the first multicellular animals.
authors.forename: Nagayasu
authors.initials: N
authors.lastname: Nakanishi
authors.forename: Shunsuke
authors.initials: S
authors.lastname: Sogabe
authors.affiliation: School of Biological Sciences, University of Queensland, Brisbane, QLD 4072, Australia. b.degnan@uq.edu.au.
authors.forename: Bernard M
authors.initials: BM
authors.lastname: Degnan
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000235:genetics; D005075:Biological Evolution; D019070:Cell Lineage; D004848:Epithelium / Q000196:embryology; D050980:GATA Transcription Factors / Q000235:genetics / Q000378:metabolism; D054262:Gastrulation; D018507:Gene Expression Regulation, Developmental; D005855:Germ Layers / Q000196:embryology; D007814:Larva / Q000235:genetics / Q000648:ultrastructure; D008675:Metamorphosis, Biological / Q000235:genetics; D010587:Phagocytosis; D010802:Phylogeny; D011161:Porifera / Q000166:cytology / Q000196:embryology* / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism
chemical_list: D050980:GATA Transcription Factors; D012333:RNA, Messenger