4-5 day old Mhc^E1570K mutant homozygotes show severe dorsal longitudinal muscle (DLM) abnormality, with intact fibres attached through thin strands at both ends. Muscle deformity (reduced birefringence) is also present, although the tearing and bundling of fibres associated with a typical hypercontraction phenotype is not observed. Sarcomeres show some of the repetitive structure seen in wild type but there is considerable disarray, with severe disruption of the filament lattice. The lattice is almost totally absent except for small patches where myofilaments are seen, and individual scattered thick and thin filaments are present. In some areas, clumps of Z-disc-like structures are linked by what appear to be thin filaments. These Z-discs are smaller and more variable in shape, with short spacing compared to wild type. DLM defects are also detected during development. Z discs of an irregular shape and size are first detectable at 42-46h after puparium formation and the myofibrils appear wider at this stage.

The dorsal longitudinal muscle (DLM) fibres in newly eclosed Mhc^E1570K/+ flies appear similar to wild type, with areas of reduced birefringence. However, by two days these heterozygous flies show a more typical complete hypercontraction phenotype; the fibres have detached and bundled to one side as a result of tearing, especially at the anterior muscle attachment site. The thick/thin filament lattice is clearly present, but is irregular and loosely packed. Repeating sarcomeric structures are seen but are short and irregular compared to wild type. M-bands and Z-discs are irregular, and I-bands and H-zones are not seen. No defects are detected in the DLMs of pupal stage Mhc^E1570K/+ mutants

The dorsal longitudinal muscles of Mhc^E1570K/Mhc^L1736P transheterozygotes show fibre thinning at both the anterior and posterior ends. The flies are flightless.

Mhc^E1570K interacts with Mhc¹, Mhc⁷ or Mhc¹³ in trans to give muscle phenotypes that are intermediate between the two or show severe indirect flight muscle degeneration.