LADYBIRD, A NEW COMPONENT OF THE CARDIOGENIC PATHWAY IN DROSOPHILA REQUIRED FOR DIVERSIFICATION OF HEART PRECURSORS

The embryonic heart precursors of Drosophila are arranged in a repeate d pattern of segmental units, There is growing evidence that the devel opment of individual elements of this pattern depends on both mesoderm intrinsic patterning information and inductive signals from the ectod erm, In this study, we demonstrate that two homeobox genes, ladybird e arly and ladybird late, are involved in the cardiogenic pathway in Dro sophila, Their expression is specific to a subset of cardioblast and p ericardial cell precursors and is critically dependent on mesodermal t inman function, epidermal Wingless signaling and the coordinate action of neurogenic genes, Negative regulation by hedgehog is required to r estrict ladybird expression to two out of six cardioblasts in each hem isegment. Overexpression of ladybird causes a hyperplasia of heart pre cursors and alters the identity of even-skipped-positive pericardial c ells, Loss of ladybird function leads to the opposite transformation, suggesting that ladybird participates in the determination of heart li neages and is required to specify the identities of subpopulations of heart cells, We find that both early Wingless signaling and ladybird-d ependent late Wingless signaling are required for proper heart formati on, Thus, we propose that ladybird plays a dual role in cardiogenesis: (i) during the early phase, it is involved in specification of a segm ental subset of heart precursors as a component of the cardiogenic tin man-cascade and (ii) during the late phase, it is needed for maintaini ng wingless activity and thereby sustaining the heart pattern process, These events result in a diversification of heart cell identities wit hin each segment.