Genetic control of cellular differentiation in Ulva mutabilis. Gene effects in early development

[摘要] Thespontaneousmutantbu(bubble)ofthemulticellulargreenalgaUlvamutabilisdevelopsinitsmostextremeformasahollowsphereconsistingofonlyonecelltype,whilethewildtypehasadifferentiatedthallusconsistingofrhizoidcells,giantstemcells,andbladecells.Intheearlydevelopmentofthewildtype,theplanesofdivisionareperpendiculartothelongaxisoftheplant.Later,therestrictionsofthedivisionplanesarepartlyreleasedandtheplanesareperpendiculartothesurfaceoftheplantandotherwisearbitrarilyoriented.Themutant,throughoutitsdevelopment,hasonlythelatterrestrictionsonitsdivisionplanes.Theearlycelldivisionsofthemutantaresynchronizedinadiurnallight-darkcycle,incontrasttotheearlydevelopmentofthewildtype,butinagreementwithwhatisfoundinitsblade.Inaddition,newlydifferentiatedbladecells,whenisolated,developintoplantsphenotypicallysimilartothemutant.Theseresultssuggestthatthemutantcellsareequivalenttothebladecellsofwildtype.Thebugametophytesderivedfrombu+busporophytesmaydevelopeithertheirownbubblephenotype,orphenotypesintermediatebetweenbubbleandwildtype,orthewild-typephenotype.Thefrequencyofthelatterislowerbytreatmentofthesporemothercellswithdactinomycinduringaperiodimmediatelyprecedingmeiosisandsporulation.Itisalsodependentonthelengthofthefirstperiodoflighttowhichthezoosporesareexposed.Accordingly,itisapremeiotictranscriptionofthebu+gene,andaproductofthegeneistransferredfromtheheterozygous(bu+bu)sporemothercellstothezoospores.Whenthezoosporesareexposedtolightforthefirsttime,thebu+geneproductbecomesinvolvedinprocesseswhichproceedduringthelight-darksensitiveperiod,andwhencompleted,channelthedevelopmentofthemutantprogenyintothedevelopmentalpathofthewildtype.Thepredeterminingeffectofthebu+productisalsoobservedamonggametophytesderivedfromsporophytesheterozygousforbuandthesemidominantmutantSl(Slender).ThepresenceofSlintheheterozygoussporophytehasnoeffectonpredetermination,evenifSlisphenotypicallyexpressedintheheterozygoteitself.ButthepresenceofSlinthezoosporesreducestheexpressionofthepredetermination,andthisimpliesthattheSl+geneisofimportanceforthecompletionofthebu+processesofthezoospores.Thebladecellsformthereproductiveunits(zoosporesandgametes)inUlva,andthepropertiesofthemutantcanbeexplainedifthebu+geneisconcernedwiththeremovalofrestrictionsimposedonthedifferentiatedbladecells,sothatthetotalinformationinthegenomecanagainbeusedtodirectanormaldevelopment.

[学科分类] 生物科学（综合）