Flies have a number of brain defects, the exact phenotype depending on the genetic background. In the original genetic background in which it was induced, mbd¹ produces the following phenotype; the calyx is greatly enlarged and misshapen, the Kenyon cell axons are wound into 'turbans', and the peduncle and lobes are absent in most flies. The mushroom body defects are highly penetrant and strongly expressed. The ellipsoid body is open ventrally or flattened. The noduli are misshapen. When placed in a Canton S background, the mushroom body phenotype is less severe than in the original genetic background, but the central complex defects persist in most flies.

Kenyon fibres form a large 'turban' at the site of the calyx. No peduncle, α, β or γ lobes can be detected. Defect develops during metamorphosis. Deficient in conditioning behaviour, learning behaviour is lower than wild type. The degree of morphological defect is not correlated to learning performance. Response to odours is lower than wild type. Visual learning is not impaired.

Late third instar larvae show wild type mushroom bodies but imagos exhibit mutant phenotype: the lobe and stalk system of the mushroom bodies is missing and calyces are enlarged due to failure of the Kenyon cell axons to find their normal way through the central brain neuropil as the neuropil is completely degraded.

Most axons in peduncles of mushroom bodies in dorsal brain absent; no detectable lobes associated with these bodies (cf. α, β and γ lobes of wild-type); posterior calyces of mushroom bodies enlarged. During post-larval mushroom body regeneration (on the first day of pupation), axons roll up at periphery of brain; other parts of mutant brains appear normal. Mushroom body abnormalities are variable; most behaviors of mutant adults are normal, including basic ability to discriminate between different odors; in tests using olfactory stimuli, learning in third larval instar is normal, but olfactory learning in mutant adults is aberrant (Heisenberg, Borst, Wagner and Byers, 1985). Penetrance of the morphological phenotype diminishes rapidly under normal culturing, but does not wane when maintained in heterozygous condition.