Direct regulation of the muscle-identity gene apterous by a Hox protein inthe somatic mesoderm

Hox genes control segment identity in the mesoderm as well as in other tiss ues. Most evidence indicates that Hox genes act cell-autonomously in muscle development, although this remains a controversial issue. We show that apt erous expression in the somatic mesoderm is under direct Hox control. We ha ve identified a small enhancer element of apterous (apME680) that regulates reporter gene expression in the LT1-4 muscle progenitors. We show that the product of the Hox gene Antennapedia is present in the somatic mesoderm of the second and third thoracic segments. Through complementary alterations in the Antennapedia protein and in its binding sites on apME680, we show th at Antennapedia positively regulates apterous in a direct manner, demonstra ting unambiguously its cell-autonomous role in muscle development. Finally, we determine that LT1-4 muscles contain more nuclei in the thorax than in the abdomen and we propose that one of the segmental differences under Hox control is the number of myoblasts allocated to the formation of specific m uscles in different segments.