A key role of Pox meso in somatic myogenesis of Drosophila.

The Pax gene Pox meso (Poxm) was the first and so far only gene whose initial expression was shown to occur specifically in the anlage of the somatic mesoderm, yet its role in somatic myogenesis remained unknown. Here we show that it is one of the crucial genes regulating the development of the larval body wall muscles in Drosophila. It has two distinct functions expressed during different phases of myogenesis. The early function, partially redundant with the function of lethal of scute [l(1)sc], demarcates the ;Poxm competence domain', a domain of competence for ventral and lateral muscle development and for the determination of at least some adult muscle precursor cells. The late function is a muscle identity function, required for the specification of muscles DT1, VA1, VA2 and VA3. Our results led us to reinterpret the roles of l(1)sc and twist in myogenesis and to propose a solution of the 'l(1)sc conundrum'.