Requirement for the Drosophila COE transcription factor Collier in formation of an embryonic muscle: transcriptional response to Notch signalling

During Drosophila embryogenesis, mesodermal cells are recruited to form a s tereotyped pattern of about 30 different larval muscles per hemisegment. Th e formation of this pattern is initiated by the specification of a special class of myoblasts, called founder cells, that are uniquely able to fuse wi th neighbouring myoblasts. We report here the role of the COE transcription factor Collier in the formation of a single muscle, muscle DA3([A])(DA4([T ])). Col expression is first observed in two promuscular clusters tin segme nts A1-A7), the two corresponding progenitors and their progeny founder cel ls, but its transcription is maintained in only one of these four founder c ells, the founder of muscle DA3([A]). This lineage-specific restriction dep ends on the asymmetric segregation of Numb during the progenitor cell divis ion and involves the repression of col transcription by Notch signalling. I n col mutant embryos, the DA3([A]) founder cells form but do not maintain c ol transcription and are unable to fuse with neighbouring myoblasts, leadin g to a loss-of-muscle DA3([A]) phenotype, In wild-type embryos, each of the DA3([A])-recruited myoblasts turns on col transcription, indicating that t he conversion, by the DA3([A]) founder cell, of 'naive' myoblasts to expres s its distinctive pattern of gene expression involves activation of col its elf. We find that muscles DA3([A]) and DO5([A]) (DA4([T]) and DO5([T])) der ive from a common progenitor cell. Ectopic expression of Col is not suffici ent, however, to switch the DO5([A]) to a DA3([A]) fate, Together these res ults lead us to propose that specification of the DA3([A]) muscle lineage r equires both Col and at least one other transcription factor, supporting th e hypothesis of a combinatorial code of muscle-specific gene regulation con trolling the formation and diversification of individual somatic muscles.