Development and patterning of adult thoracic muscles in Drosophila melanogaster

Two sets of muscles are formed during the lifecycle of the fruit fly. The first set of muscles is formed in the embryo. In the pupa, most of these muscles degenerate and are replaced with a second set of adult muscles. Although much is known about the mechanisms that underlie the patterning of muscles in the embryo; the extent to which similar mechanisms are utilized in the patterning of adult muscles has not been clearly established. Each muscle in Drosophila can be identified by its size, orientation, attachment points and fiber type. A major challenge is to understand how myoblasts are organized during development to give rise to muscles with distinct cellular and molecular identities. Two approaches were used here to investigate how adult muscles are patterned in the Drosophila thorax: (1) the use of Ultrabithorax (Ubx) mutants to investigate the role of ectodermal cues in muscle patterning, and (2) the use of myoblast- and muscle-specific markers to examine the cellular strategies employed in adult myogenesis.; Mutations in Ubx result in the homeotic transformation of the third thoracic segment (T3) into the second thoracic segment (T2). The pattern of adult muscles in these two segments is highly divergent. Because Ubx is only expressed in the ectoderm and not in the mesoderm, any effect on the underlying muscle pattern must be due to the inductive influence of the ectoderm. An analysis of Ubx alleles and allelic combinations revealed that induction plays a major role in the patterning of homeotic muscles in the transformed T3 segment.; A histological analysis of adult muscle development in the thorax revealed that specialized mesodermal cells called imaginal muscle pioneers prefigure the formation of at least three adult muscles in T2, the tergal depressor of the trochanter (TDT) and dorsoventral muscles I and II (DVM-I and -II). This suggests that adult myoblasts are organized to form individual muscles in a manner that is similar to that observed in the embryo. While each TDT pioneer gives rise to two adult fibers, each DVM fiber is prefigured by a single pioneer. An understanding of the mechanisms that underlie the patterning of imaginal pioneers will therefore lead to an understanding of how adult fiber number is regulated.