Bouton numbers are unaffected at the NMJ of HemJ4-48/+ third instar larvae.
Mutant stage 16 embryos show a severe myoblast fusion defect in the dorsal pharyngeal muscle.
HemJ4-48/HemG1-37 transheterozygote embryos display defects in the number, migration and morphology of visceral longitudinal muscle founder cells.
Defects in longitudinal muscle fusion are observed in HemJ4-48 mutant embryos: mainly mononucleated and only a few binucleated muscle cells are found at late embryonic stages.
Approximately 13% ofHemJ4-48 mutant embryos display a duplication of the RP2 neuron in one hemisegment and have one missing in the contralateral segment. The GMC is formed correctly in both hemisegments and divides normally into an RP2 and a sib cell. The RP2 duplication occurs due to aberrant crossing of the midline at 11hr of development. The sib cell remains in the correct hemisegment. In 4% of HemJ4-48 embryos the RP2 neurons fail to migrate and are found in the position of a newly formed GMC-1. No axon projections are seen in these cells. The axonal projections in the mis-migrated RP2 cells have a contra-ipsilateral axon projection, fasciculating correctly with the intersegmental nerve bundle. No defects are seen in the commissural tracts.
HemJ4-48/HemC3-20 mutant embryos have a duplication of the RP2 neuron in one hemisegment and have one missing in the contralateral segment.
HemJ4-48/Df(3L)ED230 mutant embryos have a duplication of the RP2 neuron in one hemisegment and have one missing in the contralateral segment.
17% of embryos expressing HemScer\UAS.ΔJ4-48 under the control of Scer\GAL4nos.UTR.T:Hsim\VP16 in a HemJ4-48/Df(3L)ED230 mutant background display an RP2 neuron defect in which a duplication is seen in one hemisegment and have one missing in the contralateral segment. The GMC is formed in both hemisegments and divides normally into an RP2 and a sib cell. The RP2 duplication occurs due to aberrant crossing of the midline at 11hr of development. The sib cell remains in the correct hemisegment.
HemJ4-48 mutants exhibit normal visceral mesoderm development, resulting in normal gut development.
Stage 15 HemJ4-48 mutant embryos contain enlarged F-actin foci. These actin foci are found in fusion competent myoblasts (FCMs), but are not seen in founder cells. The percentage of invasive foci and depth of invasion are both comparable to wild type. Myoblast fusion is blocked before fusion pore formation as cytoplasmic material exchange does not occur between the founder cell and FCM.
HemJ4-48 mutants show enlarged foci, as well as increased numbers of actin foci. Enlarged foci are seen in these mutants from the earliest stages of fusion and foci persist after fusion would be complete in wild-type.
Homozygous HemJ4-48 mutant embryos exhibit a myoblast fusion phenotype.
HemJ4-48/HemΔ2-6 larvae have reduced neuromuscular junctions with bulging, irregular boutons. About 15% of the heads of HemJ4-48/HemΔ2-6 flies have one or more bent macrochaetae.
HemJ4-48 mutant embryos contain many unfused myoblasts, even at stage 16, which cluster close to stretched mini-muscles. Marker analysis indicates that both founder cells and fusion competent myoblasts form normally at stage 13 in these embryos. By stage 14 mini-muscles with 3-4 nuclei are apparent at the site of the developing DA1 and DO1 muscles. As in wild-type, these 3-4 nuclei precursor cells establish contacts with fusion-competent myoblasts and plaques form at the sites of contact. Unlike wild-type, there is no sign of membrane breakdown in the vicinity of these plaques which remain visible into stage 15. Further fusion fails to occur. By contrast stage 15-16 wild type DA1 contains up to 14 nuclei. Dorsal closure is normal in HemJ4-48 mutant embryos, as is formation of the dorsal vessel and cardioblasts.
Commissures are fused in stage 16 mutant embryos, and the two longitudinal connectives are closer to the midline than in normal embryos.