Determination in Drosophila Embryos

SYNOPSIS. In this review we describe data of experiments whichinterfere with the formation of the metameric pattern duringembryogenesis. Ligating embryos before blastoderm stage leadsto a gap in the segmentation pattern of the differentiated embryo.The gap can extend up to 6 segments but terminal segments arealways recognizable. In posterior but not in anterior fragmentswe find abnormally large but fewer segments. This increase insegment size results from a different determination of blastodermcells after ligation. During nuclear multiplication stages whena gap can be produced, the zygotic genome is not yet active.Information to develop the metameric pattern in ligated embryosmust therefore have been made during oogenesis. Recently Nüisslein-Volhard and Wieschaus (1980) have describedthree zygotic mutations which form embryos with a gap of segmentssimilar to our ligated embryos. We have discussed these mutantphenotypes in connection with our experimental data. Segmentation is controlled at several levels. During oogenesisthe anterior-posterior and dorsal-ventral axes become established(Nüsslein-Volhard, 1979). Also during oogenesis, but extendinginto early embryonic life, information is generated to subdividethe embryo into blocks of cells forming the metameric pattern.At blastoderm the identity of segments becomes established.     

Determination During Early Embryogenesis in Drosophila melanogaster

Ligation of developing embryos of Drosophila melanogaster wasperformed at three different stages of nuclear multiplicationand at the cellular blastoderm stage. Egg fragments of variablesizes are able to continue development up to the hatching stage.Partial embryos differentiate larval structures, anterior fragmentsforming larval head and posterior fragments larval abdominalstructures. These fragments differentiate a variable numberof the twelve larval cuticular bands formed by intact embryos.We found that ligation at cellular blastoderm can lead to anteriorand posterior fragments which differentiate together all thetwelve bands, indicating that at this stage the embryo developsthese patterns in a mosaic fashion. Ligation of younger embryosprevents the differentiation of some intermediate larval cuticularbands, while the terminal ones are consistently differentiated.The number and position of the deleted bands is correlated withthe time and position of ligation. This indicates that the mosaicpattern present in the egg at blastoderm is not fully formedat earlier stages in development.