LADYBIRD DETERMINES CELL FATE DECISIONS DURING DIVERSIFICATION OF DROSOPHILA SOMATIC MUSCLES

In the mesoderm of Drosophila embryos, a defined number of cells segre gate as progenitors of individual body wall muscles. Progenitors and t heir progeny founder cells display lineage-specific expression of tran scription factors but the mechanisms that regulate their unique identi ties are poorly understood. Here we show that the homeobox genes ladyb ird early and ladybird late are expressed in only one muscle progenito r and its progeny: the segmental border muscle founder cell and two pr ecursors of adult muscles. The segregation of the ladybird-positive pr ogenitor requires coordinate action of neurogenic genes and an interpl ay of inductive Hedgehog and Wingless signals from the overlying ectod erm. Unlike so far described progenitors but similar to the neuroblast s, the ladybird-positive progenitor undergoes morphologically asymmetr ic division. We demonstrate that the ectopic ladybird expression is su fficient to change the identity of a subset of progenitor/founder cell s and to generate an altered pattern of muscle precursors. When ectopi cally expressed, ladybird transforms the identity of neighbouring, Kru ppel-positive progenitors leading to the formation of giant segmental border muscles and supernumerary precursors of lateral adult muscles. In embryos lacking ladybird gene function, specification of two ladybi rd-expressing myoblast lineages is affected. The segmental border musc les do not form or have abnormal shapes and insertion sites while the number of lateral precursors of adult muscles is dramatically reduced. Altogether our results provide new insights into the genetic control of diversification of muscle precursors and indicate a further similar ity between the myogenic and neurogenic pathways.