title: Evolutionary conservation of the placodal transcriptional network during sexual and asexual development in chordates.
abstract: BACKGROUND
  An important question behind vertebrate evolution is whether the cranial placodes originated de novo, or if their precursors were present in the ancestor of chordates. In this respect, tunicates are of particular interest as they are considered the closest relatives to vertebrates. They are also the only chordate group possessing species that reproduce both sexually and asexually, allowing both types of development to be studied to address whether embryonic pathways have been co-opted during budding to build the same structures.
  
  
  RESULTS
  We studied the expression of members of the transcriptional network associated with vertebrate placodal formation (Six, Eya, and FoxI) in the colonial tunicate Botryllus schlosseri. During both sexual and asexual development, each transcript is expressed in branchial fissures and in two discrete regions proposed to be homologues to groups of vertebrate placodes.
  
  
  CONCLUSIONS
  Results reinforce the idea that placode origin predates the origin of vertebrates and that the molecular network involving these genes was co-opted in the evolution of asexual reproduction. Considering that gill slit formation in deuterostomes is based on similar expression patterns, we discuss possible alternative evolutionary scenarios depicting gene co-option as critical step in placode and pharynx evolution.
authors: Dipartimento di Biologia, Università degli Studi di Padova, Padova, Italy. fabio.gasparini@unipd.it
authors: Fabio
authors: F
authors: Gasparini
authors: Valentina
authors: V
authors: Degasperi
authors: Sebastian M
authors: SM
authors: Shimeld
authors: Paolo
authors: P
authors: Burighel
authors: Lucia
authors: L
authors: Manni
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001921:Brain / Q000196:embryology; D043344:Chordata / Q000235:genetics* / Q000254:growth & development; D018076:DNA, Complementary / Q000378:metabolism; D020869:Gene Expression Profiling; D018507:Gene Expression Regulation, Developmental; D053263:Gene Regulatory Networks; D017403:In Situ Hybridization; D008213:Lymphocyte Activation; D010802:Phylogeny; D014158:Transcription, Genetic; D014561:Urochordata / Q000235:genetics* / Q000254:growth & development
chemical_list: D018076:DNA, Complementary