Mcm3^smu adult mutants display structural defects in the brain mushroom body (MB) and reduced size of the MB neuropil: both the dorsal projecting α/α'-lobe system and the medial projecting β/β'-lobe system are either severely reduced or absent (the &ggr-lobe is less affected) and the calyx volume and the number of Kenyon cells are significantly decreased compared to controls. The asymmetric cell division, mitotic spindle positioning or asymmetric protein localization are all unaffected in third instar larval neuroblasts of Mcm3^smu mutants but the average size of larval neuroblasts is significantly increased compared to controls (except for the mushroom body neuroblasts, whose size is not significantly changed). The number of Kenyon cells is normal in early but reduced in late third instar larvae. The number of ganglion mother cell (GMC) clusters per hemisphere as well the number of GMCs associated with each MBNBs is also significantly reduced in Mcm3^smu late third instar larval brains. Correspondingly, the number of mushroom body neuroblasts (MBNB) is normal at the onset of larval development but these are prematurely lost at the pupal stage, whereas additional neuroblasts are observed outside the Kenyon cell region in first instar larval as well as pupal brains, a phenotype never observed in wild-type. Mcm3^smu mutant MBNBs progressively decrease their proliferation during third instar larval stage: Mcm3^smu homozygous single mushroom body neuroblast somatic MARCM clones (induced in first instar larvae and analyzed in adults) contain decreased number of &grr;-neurons and only a few, if any, α'/β'-neurons.

Mcm3^smu mutant adults display severe learning and memory defects in an olfactory memory assay: short-term memory (measured at 0-3h post-training) is strongly reduced at earlier time-points and completely absent at 3h as is the persistent memory measured 24h post-training. The mutants flies however do not show any deficits in odor avoidance or shock reactivity compared to wild-type.

Mushroom body calyces in mbm^N337 mutants are reduced to approximately 20-50% volume of wild-type flies. Gross brain morphology appears relatively normal.

smu¹ females are semi-lethal.

smu¹ heterozygous flies demonstrate comparable patterns of landmark orientation, indicating similar responses to visual stimulation in Buridan's paradigm as control flies.

smu¹ heterozygous males exhibit reduced activity levels and walk slower than control flies.

The central complex appears to be unaffected in mutant adult brains. The mushroom body is small, and the calyx in particular is severely reduced in size and the peduncle is barely visible. Long and short latency responses are indistinguishable from wild-type flies in both the DLM (flight) and TTM (jump) muscles. EC₅₀ values (the concentration at which half of the long latency responses are expected to fail) for halothane and enflurane for mutant flies are indistinguishable from the EC₅₀ values of Canton-S controls.

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, smu¹ produces the following phenotype; the calyx is 1/4 normal size, the peduncle and lobes are thin, the ellipsoid body is open ventrally and the noduli are usually misshapen. When placed in a Canton S background, the mushroom body defects are similar to the phenotype in the original genetic background, and the central complex and other brain structures are normal.