Cell Lineage, Axis Formation, and the Origin of Germ Layers in the Amphipod Crustacean Orchestia cavimana

摘要

EmbryosoftheamphipodcrustaceanOrchestiacavimanaareexaminedduringcleavage,gastrulation,andsegmentationbyusinginvivolabelling.Singleblastomeresofthe8-and16-cellstageswerelabelledwithDiItotracecelllineages.Earlycleavagefollowsadistinctpatternandthea/pandd/vbodyaxesarealreadydeterminedatthe4-and8-cellstages,respectively.Inthesestages,thegerminalrudimentandthenaupliarmesodermcanbetracedbacktoasingleblastomereeach.Inaddition,theectodermandthepostnaupliarmesodermareseparatedintorightandleftcomponents.Atthe16-cellstage,naupliarectodermisdividedfromthepostnaupliarectoderm,andextraembryoniclineagesareseparatedfrompostnaupliarmesodermandendoderm.Fromourinvestigation,itisevidentthatthecleavagepatternandcelllineageofOrchestiacavimanaarenotofthespiraltype.Furthermore,theresultsofthelabellingshowmanydifferencestocleavagepatternsandcelllineagesinothercrustaceans,inparticular,otherMalacostraca.ThecleavageandcelllineagepatternsoftheamphipodOrchestiaarecertainlyderivedwithinMalacostraca,whoseancestralcleavagemodewasmostlikelyofthesuperficialtype.Ontheotherhand,Orchestiaexhibitsastereotypedcelldivisionpatternduringformationanddifferentiationofthegermbandthatistypicalformalacostracans.Hence,aderived(apomorphic)earlycleavagepatternistheontogeneticbasisforanevolutionarilyoldercelldivisionpatternofadvanceddevelopmentalstages.O.cavimanaoffersthepossibilitytotracethelineagesandthefatesofcellsfromearlydevelopmentalstagesuptotheformationofsegmentalstructures,includingneurogenesisatalevelofresolutionthatisnotmatchedbyanyotherarthropodsystem.