When originally isolated, homozygous adults displayed an ellipsoid body opened ventrally with a strong penetrance. This phenotype disappeared after a few generations, but outcrossing recovered an ellipsoid body defect with weak penetrance, indicating that the phenotype is very sensitive to genetic background. bstP/Df(2L)H20 or bstP/Df(2L)cact-255rv64 adults have a ventrally opened ellipsoid body phenotype that is stronger than that of bstP homozygotes. Heterozygotes have no adult central brain defect.
Excision of the P{GawB} element can revert the mutant phenotype.