Although the genetics of dorsal-ventral polarity which leads to mesoderm formation in DrosophiΓa are understood in considerable detail, subsequent molecular mechanisms involved in patterning the mesoderm primordium into individual mesodermal subtypes are poorly understood. Two papers published recently('>*) suggest strongly that an inductive signal from dorsal ectoderm is involved in subdividing the underlying mesoderm, and present evidence that one of the signalling factors is Decapentaplegic (Dpp), a member of the bone morphogenetic protein subgroup of the Transforming Growth Factor$ (TGF-p) super family of proteins. lntroduction Specification of cell fate during developrnent is a rnultistep process. Cells often receive different signals frorn neighboring cells at different stages during their developrnent, which play a significant role in determining their developrnental fate. Members of the TGF-P superfarnily, which are widely conserved in different organisrns, constitute a rnajor group of signalling molecules. They appear to rnediate key events in normal growth and developrnent and exhibit diverse activities like specification of body axis and induction of rnesoderrn in Xenopus, and control sexual developrnent and creation of bones and cartilage in rnamrnals (reviewed in ref. 4). In Drosophila, decapentaplegic (dpp) is required for formation of the ernbryonic dorsoventral axis of ectoderrnal cuticular structures, cornrnunication between ticsue layers in gut developrnent and correct proxirnal dista1 axis of adult ap~endages(~-~). The two papers reviewed here dernonstrate a new inductive role for dpp expressed in the dorsal ectoderm, to pattern the underlying mesoderrn during early embryogenesis(1,2).
Mecoderm formation in Drosophila
Over the past two decades, enorrnous progress has been rnade in understanding the molecular rnechanisrns of cell fate specification in Drosophila. Most of these discoveries have concerned the specification of body axis, segrnental patterning of the ectoderrn and diversification in nervous systern. Not much has been known about the rnolecular basis of rnesoderrnal pattern forrnation and specification of rnesodermal derivatives, with the exception of initial deterrnination of the rnesodermal anlagen. Mesoderrn forrnation at blastoderm stage is controlled by a cascade of maternally active genes that generate dorso-ventral polarity. This process cul-