The mutations previously designated as flightless-I3, flightless-O2 and standby are members of the W-2 lethal complementation group at the base of the X-chromosome of Drosophila melanogaster.

By using a well defined panel of chromosomal deficiencies, duplications and lethals, we have mapped three mutations causing flightlessness, flightless-I3, flightless-O2 and standby, to a single lethal complementation group (termed W-2) at the base of the X-chromosome of D. melanogaster. We also show that a fourth flightless mutation, termed grounded, previously mapped near to the base of the X-chromosome, is distal to the cytogenetic interval 18F to 20F. Mutants homozygous for the flightless-I3, flightless-O2 and standby mutations exhibit abnormalities of myofibrillar arrangements in the indirect flight muscles. They have distorted Z-bands and the myofibrils are often displaced from their normal parallel arrangement. These viable flightless mutations are all hypomorphs since the homozygous deficiency of the W-2 X-chromosomal region is lethal to the organism.