Homoeosis in Drosophila: A description of the polycomb lethal syndrome☆

摘要

AdultsheterozygousfordominantmutationsatthehaploabnormalPolycomb(Pc)locusdisplaymanyhomoeotictransformations.E.B.Lewis(1978)firstdescribedthecuticularmorphologyoflethalembryoshomozygousforPolycombmutantalleles,andsuggestedthatthePc+geneproductactsasarepressorofgenesintheBithoraxgenecomplex.Inthepresentwork,wehavefurtherexaminedthePolycomblethalsyndromebyphase-contrastandscanningelectronmicroscopyofwholemounts,andshowthatthephenotypeismorecomplexthanhithertorealized.Manycuticularfeaturescharacteristicofmoreanteriorbodysegmentsarepartiallyorcompletelytransformedtoresemblethoseofmorecaudalsegments;theposteriormosthomoeoticfeatureswhichdeveloparethoseofthe8thabdominalsegment(ratherthanthe9thor10th).Involutionanddorsalclosureoftheheadareusuallyincomplete,andthelabiumanddorsalheadregularlydevelopcuticularfeaturesnormallycharacteristicofabdominalsegments.Notallhomoeoticalterationsarecaudal,however,andembryosalsodisplaypartialtransformationsofthe9th(or10th)abdominalsegmentto8thandofmeso-andmetathoraxtoprothorax.ItshouldbeemphasizedthatPolycombmutationsdonothomoeoticallytransformsegmentsasawhole;variouscuticularmarkersdifferconsiderablyintheirrelativeprobabilityandextentoftransformation,andasinglesegmentmaydisplayfeaturesnormallyspecifictoseveraldifferentsegments.Wesuggestthatthesemutationsresultinaninstabilityinthetransmissionofdeterminedstates.