Studies on the maternal effect of the semilethal gene, v, in the Mexican axolotl. I. Influence of temperature on the expression of the effect. II. Cytological changes in the affected embryos☆

摘要

Thegene,v,intheMexicanaxolotlhasrecentlybeenshowntoaffecttheeggcytoplasmduringoogenesisinsuchawayastoleadtoabnormalitiesindevelopmentfollowingfertilization.Affectedembryosfrequentlyarearrestedinblastulastagesanddisplayaremarkableappearanceresultingfromastratificationofthecontentsoftheblastomeres.Ortheymaycontinuetolaterstagesbeforethematernaleffectbecomesapparent(Humphrey,1962).Thepresentpaperisconcernedwiththeinfluenceoftemperatureontheexpressionofthismaternaleffectandwiththecytologicalchangesoccurringintheaffectedembryos.Theinfluenceoftemperatureonthematernaleffectwasinvestigatedbyrearingtheembryosat25?°Candat14a€“20?°C.(Bothtemperaturesarewellwithintherangepermittingnormaldevelopmentofcontroleggs.)At25?°alargemajority(88%)oftheembryoswerearrestedinmid-blastulastagesandexhibitedastrikingappearanceduetoastratificationofthecontentsoftheblastomeres.Atlowertemperatures(14a€“20?°)theexpressionofthematernaleffectwasdelayed.Lessthan1%oftheembryoswerearrestedinmid-blastulastages.Approximately70%developedtogastrulaandneurulastagesand30%tomoreadvancedembryonicstages.Stratificationofcellcontentswasobservedinthegastrulaeandsomeneurulaebutwaslesspronouncedthanthatseeninarrestedblastulae.Thematernaleffectresultsinastrikingrearrangementofcellcomponentsinembryosarrestedinblastulaandgastrulastages.ThefirstcytologicallydetectablechangeconsistsinaconcentrationoftheRNA-containingcytoplasmintotheupper(animal)partsofthecells.Nucleiaresimilarlydisplacedfromtheirnormalcentrallocationandarefoundintheupperpartsofthecells,whollyorpartlywithintheRNA-containingcytoplasm.ThesechangesinthedistributionofnucleiandofcytoplasmicRNAarefollowedbyasedimentationofyolkplateletsandthenofpigmentgranules.Eventuallythecellstakeonahighlystratifiedappearance.Foratimeafterthebeginningofthecytoplasmicstratificationthenucleicontinuemitoticactivitywhich,however,isusuallyofanabnormaltype.Themostcommonabnormality(at25?°)isanextensivepolyploidization,indicatingthatchromosomalreplicationcontinuesforatimeafternuclearandcelldivisionhavebeensuppressed.