DIFFERENTIAL RESCUE OF VISCERAL AND CARDIAC DEFECTS IN DROSOPHILA BY VERTEBRATE TINMAN-RELATED GENES

tinman, a mesodermal NK2-type homeobox gene, is absolutely required fo r the subdivision of the early Drosophila mesoderm and for the formati on of the heart as well as the visceral muscle primordia, Several vert ebrate relatives of tinman, many of which are predominately expressed in the very early cardiac progenitors (and pharyngeal endoderm), also seem to promote heart development, Here, we show that most of these ve rtebrate tinman-related genes can readily substitute for Drosophila ti nman function in promoting visceral mesoderm-specific marker gene expr ession, but much less in promoting cardiac-specific gene expression in dicative of heart development, In addition, another mesodermal NK2-typ e gene from Drosophila, bagpipe, which is normally only needed for vis ceral mesoderm but not heart development, cannot substitute for tinman at all. These data indicate that the Functional equivalence of the ti nman-related subclass of NK2-type genes (in activating markers of visc eral mesoderm development in Drosophila) is specific to this subclass and distinct from other homeobox genes, Despite the apparent overall c onservation of heart development between vertebrates and invertebrates , the differential rescue of visceral mesoderm versus heart developmen t suggests that some of the molecular mechanisms of organ formation ma y have diverged during evolution.