Table of Contents:
The adult male reproductive system of Drosophila comprises paired testes, seminal vesicles, paragonia as well as a single ejaculatory duct and the sperm pump. All organs except the testes are surrounded by muscle layers, which consist of mono- or multinucleated striated muscles. In contrast, testes are surrounded by multinucleated, smooth-like muscles (Susic-Jung et al., 2012). These muscles originate from the genital imaginal disc, where 28 hours after puparium formation testes-relevant myoblasts fuse on the prospective seminal vesicles. The resulting nascent myotubes then migrate onto the pupal testes und migrate further towards the apical testis tip (Kuckwa et al., 2016). Only little is known about this migration process. Therefore, the aim of the present study was to search for proteins und signaling pathways which might regulate the migration.
Studies concerning canonical Wnt signaling suggest that some components are involved in the development of the testis musculature. Most likely, the determination of testes-relevant myoblasts on the genital disc is DWnt2-dependent. Analyses concerning non-canonical Wnt signaling did not indicate a role in migration. Nevertheless, some components could trigger cell polarity of the testis muscles.
First studies of the Focal adhesion kinase do not point towards a regulation of testes myotube migration. Contrastingly, the kinase Misshapen seems to regulate the migration of nascent testes myotubes towards the testis tip as well as the arrangement of F-actin filaments in testis muscles.
Analyses concerning FGF signaling revealed that the FGF receptor Heartless as well as the adaptor protein Stumps are required to cover the testis completely with muscles. In the absence of Stumps, nascent myotubes can reach the testes but fail to migrate towards the apical testis tip. The other FGF receptor Breathless is most likely not involved in the migration of testes myotubes, but could possibly induce fusion of testes-relevant myoblasts.
The results implicate that the migration of testes myotubes can be divided into two phases: during phase one, nascent myotubes migrate collectively onto the testis, phase two shows the migration of single testes myotubes towards the testis tip.