Internal deletion (Rowe, EMBO J. 7: 1107--1114). UbxC1 chromosome makes hybrid RNA, 5' exons of the abd-A transcription unit joined to the 3' exons of the Ubx transcription unit.
Deletion of approximately 100kb DNA within the Ubx and abd-A regions of the bithorax complex. The proximal breakpoint is within the Ubx transcription unit at -51kb within an intron (coordinates as in FBrf0040194 and FBrf0042053). This deletes the Ubx promoter, the major 5' coding region and one of the two microexons, but leaves the second microexon and the 3', homeobox-containing exon intact. The distal breakpoint is within a 20kb intron of abd-A, 2-2.5kb from the last exon of the 5' cluster. This deletes the 3' cluster of abd-A exons which encode the homeobox and carboxy-terminal regions of the abd-A protein.
Homozygous larvae have transformed abdominal segments; A1 shows a thoracic pattern (which is probably metathoracic but is difficult to discriminate from mesothoracic) as indicated by the denticle size, the presence of ventral pits and fully developed Keilin's organs. A2-A8 show a mixture of thoracic and abdominal pattern elements, with a gradient from A2 (mostly thoracic) to A8 (mostly abdominal). Hemizygous larvae have a more extreme phenotype; A1 and A2 develop a complete thoracic pattern, and A3-A8 have more thoracic features than in homozygotes. Ventral pits and defective Keilin's organs are present down to A7 in both homo- and hemizygous larvae. Heterozygotes have slightly enlarged halteres and differentiate some bristles in the anterior margin, and frequently have a partial transformation of wing territory to haltere and partial transformation of A1 to a more posterior phenotype.
In homozygous embryos A1 is thoracic with Keilin's organs and ventral pits (resembling T2 more closely than T3 - contrasts with the results in FBrf0063361). A2 to A4 show clear abdominal characteristics, with the denticle morphology becoming progressively more like that of the normal A2/A4 type more posteriorly. Ventral pits are present on all abdominal segments from A1 to A7. A2 to A7 are more 'thoracic' in hemizygotes than in homozygotes.
In homozygotes the first and second abdominal segments show thoracic characteristics, and the ventral denticles of the more posterior abdominal segments resemble those of the first abdominal segment.
Ubx provides the carboxyl end of protein and homeobox. The product is now under the control of the Ubx regulatory elements.
Complementation and phenotypic analysis indicates that UbxC1 retains partial Ubx activity. The developmental properties of the UbxC1 appear to be a mixture of those of Ubx and abd-A. UbxC1 is derepressed in esc- embryos.