Origin of chromosomal abnormalities in nuclear transplants—A reevaluation of nuclear differentiation and nuclear equivalence in amphibians☆

[摘要] Previoustransplantationtestsofamphibiannucleiderivedfromadvancedsomaticandgerminalcelltypeshavedemonstratedthatmostofthesenucleihaveacquireddevelopmentalrestrictionsduringembryogenesis.Thenatureoftheserestrictionshasbeenrevealedinpartthroughchromosomalstudiesofabnormalnucleartransplants.Inthesestudies,ithasbeenfoundthatthedevelopmentalrestrictionsinthenucleartransplantsareaccompaniedinmostcasesbykaryotypicalterations.Inordertodeterminetheoriginofthesechromosomalaberrationsandtherebygainsomeunderstandingofthechromosomalchangesaccompanyingnormalembryogenesis,wehavestudiedthenuclearandchromosomalmorphologyofnucleartransplantswithinthefirst8hoursafternucleartransplantation.Thesestudiescarriedoutonnucleiofdeterminedendodermalcellsdemonstratethatinmostcasestheoriginofthesechromosomalaberrationsistraceabletotheinitialresponsesofthetransplantednucleustothecytoplasmoftherecipientegg.Manytransplantedendodermalnucleifailtoundergosufficientnuclearenlargementandchromatindecondensation.Asaresultoftheseevents,chromatidbridgesariseduringthefirstmitoticcleavageandsubsequenttochromatidbreakageatlateanaphase,variabledeletionsinchromosomesoccur.Thesedifferentamountsofgeneticlossesthenleadtovariableabnormaldevelopmentinnucleartransplantsandultimatelyarethecauseofdevelopmentalarrest.Theresultsofthesestudiesindicatethatduringembryogenesisthechromosomesofmanyendodermalnucleihaveacquiredrestrictionsintheformofcondensedorcompactchromatin(heterochromatin).Furthermore,theserestrictionsarenotreversedcompletelybytransplantationintoyoungeggcytoplasm;andconsequently,thecompleteexpressionofthetransplantednucleusisprevented.Thesignificanceofthesefindingsisdiscussedinrelationtoourcurrentunderstandingofnucleardifferentiationinamphibians.

[学科分类] 生物科学（综合）