Molecular evidence from Ciona intestinalis for the evolutionary origin of vertebrate sensory placodes

摘要

Cranialsensoryplacodesarefocusedareasoftheheadectodermofvertebratesthatcontributetothedevelopmentofthecranialsenseorgansandtheirassociatedganglia.Placodeshavelongbeenconsideredakeycharacterofvertebrates,andtheirevolutionisproposedtohavebeenessentialfortheevolutionofanactivepredatorylifestylebyearlyvertebrates.Despitetheirimportanceforunderstandingvertebrateorigins,theevolutionaryoriginofplacodeshasremainedobscure.Here,weuseapanelofmolecularmarkersfromtheSix,Eya,Pax,Dach,FoxI,COEandPOUIVgenefamiliestoexaminethetunicateemCionaintestinalis/emforevidenceofstructureshomologoustovertebrateplacodes.OurresultsidentifytwodomainsofemCiona/emectodermthataremarkedbythegeneticcascadethatregulatesvertebrateplacodeformation.Thefirstisjustanteriortothebrain,andwesuggestthisterritoryisequivalenttotheolfactory/adenohypophysealplacodesofvertebrates.ThesecondisabilateraldomainadjacenttotheposteriorbrainandincludescellsfatedtoformtheatriumandatrialsiphonofadultemCiona/em.Weshowthisbaresmostsimilaritytoplacodesfatedtoformthevertebrateacoustico-lateralissystem.Weinterpretthesedataassupportforthehypothesisthatsensoryplacodesdidnotarisedenovoinvertebrates,butevolvedfrompre-existingspecialisedareasofectodermthatcontributedtosensoryorgansinthecommonancestorofvertebratesandtunicates./p/div