Expression of Vrp1^Scer\UAS.cMa under the control of Scer\GAL4^Mef2.PR in a Vrp1^S1946/Vrp1^D30 mutant background produces male sterile flies. The testis contain mature cysts that do not undergo appreciable coiling, cysts do not occupy the base of the testis tube and the seminal vesicle is completely empty of released sperm. The microfilaments and nuclei of late-stage cysts show significant structural alterations. There is a consistent failure to achieve the intertwined and tightly packed organisation of head cyst cell F-actin arrays and associated spermatid nuclei. The microfilament arrays appear sparse and misshapen, and the normally tight and uniformly oriented bundles of nuclei are loosely packed and partially split.

Vrp1^S1946 mutant embryos show a partial loss of myoblast fusion at stage 14 and, unlike in wild type, myoblast actin foci persist in stage 15 Vrp1^S1946 mutant embryos. The F-actin foci are found in fusion competent myoblasts (FCMs), but are not seen in founder cells.

Only 9% of actin foci in Vrp1^S1946 mutant embryos invade into founder cells, compared to 35% in wild type, and the depth of invasion is also reduced. The foci exhibit non-invasive F-actin enriched fingers, which often fold upon each other without extending towards the apposing founder cell. Occasional actin rich fingers deform the founder cell membrane, but these are fewer in number and shorter and wider than in wild type.

Cytoplasmic material exchange does not occur between the founder cell and fusion competent myoblast (FCM); all actin foci observed in Vrp1^S1946 mutant embryos are associated with intact plasma membranes of founder cells and apposing FCMs.

Homozygous embryos contain many mononucleated myocytes. In late stage mutant embryos, unfused myoblasts are seen attached to elongated muscle precursors. The Kr-positive founder cells appear to be properly specified in the mutant embryos, although they are mononucleated at stage 13, but contain one to three nuclei at stage 14 (in wild-type embryos these founder cells are bi- or tri-nucleated by stage 13). eve-positive dorsal acute muscle 1 is mostly binucleated in stage 14 mutant embryos, whereas in wild-type embryos it contains approximately ten nuclei at this stage.

Analysis of pairs of adjacent muscle founder and fusion-competent cells shows that F-actin foci are greatly diminished in fusion-competent cells in mutant embryos, whereas F-actin gradually accumulates to an abnormally high level in the adjacent founder cell along the apposing membranes.

Mutant myoblasts contain electron-dense vesicles that are seen in wild-type myoblasts during fusion, but the mutant vesicles show a profound defect in membrane targeting, being found not only at sites of fusion between the apposing founder and fusion-competent cell, but also between neighbouring fusion-competent cells (in wild-type embryos, these vesicles are only found at the contact sites between the founder and fusion-competent cells). The vesicles also persist much longer than normal in the mutant myoblasts.

Vrp1^S1946 has embryonic myoblast phenotype, enhanceable by SCAR^k13811

Vrp1^S1946 has actin cytoskeleton phenotype, enhanceable by SCAR^k13811

Vrp1^S1946 is an enhancer of embryonic myoblast phenotype of SCAR^k13811

Vrp1^S1946 is an enhancer of actin cytoskeleton phenotype of SCAR^k13811

Act5C^UASp.GFP, Scer\GAL4^{kirre-rP298-G4}, Vrp1^S1946 has muscle founder cell phenotype

Act5C^UASp.GFP, Scer\GAL4^{kirre-rP298-G4}, Vrp1^S1946 has actin cytoskeleton phenotype