Approximately 80% of homozygous slow¹ flies are non-viable. Approximately 75-80% of homozygous mutant embryos hatch. However, approximately 65% of the resulting homozygous mutant first instar larvae die either immediately after hatching or within 1-2 hours. The larvae that live for 1-2 hours show a significantly lower rate of body contractions per minute and their locomotion speed is reduced. Lethality occurs throughout the course of development, including the period before, as well as during, pupation. Importantly, more of the homozygous escaper flies are unable to fly and die within 1-2 days of eclosion.

Homozygous slow¹ mutant live third instar larvae exhibit torn and/or detached somatic muscles in most segments.

Homozygous slow¹ mutant larval muscles are rounded up and still associated with torn tendons.

In slow¹ mutant embryos the muscle anterior migrating edge, as well as the posterior edge, does not appear to spread upon arrival at the tendon at the segmental border.

Analysis of MTJ (myotendinous junction) architecture at stage 16 reveals abnormal MTJ morphology, in which the surface area of the muscle-tendon junction is smaller than in wild-type, leading to an altered overall morphology of the muscle ends. Three-dimensional reconstruction of optical sections taken at the z-axis of the MTJ reveals that slow¹ mutant embryos exhibit smaller MTJs that do not appear as a continuous straight line, consistent with the pointed morphology observed at the muscle ends (whereas in wild-type embryos the MTJ appears as a straight line arranged along the smoothened muscle ends).

Irregular muscle ends are found at the muscle-tendon junction of slow¹ mutant larvae.

slow¹ has viable phenotype, suppressible by if^k27e/if[+]

slow¹ has larval somatic muscle cell phenotype, enhanceable by if^k27e/if[+]

if^k27e/if[+], slow¹ has flight muscle cell phenotype