No abstract

In Drosophila the Abdominal‐B (Abd‐B) domain of the bithorax complex (BX‐C) spans over 100 kb and is responsible for specifying the identities of adult abdominal segments five (A5) to nine (A9), inclusive, and correspondingly, neuromeres 10–14 of the embryonic central nervous system. The domain consists of a region coding for two proteins, ABD‐BI (54 kd) and ABD‐BII (36 kd) and cis‐regulatory regions extending from infra‐abdominal‐5 (iab‐5) to iab‐9, inclusive. We have used a monoclonal anti‐ABD‐B antibody to infer that mutants in iab‐8 eliminate the expression of ABD‐BI in neuromeres 10–13, inclusive, and that mutants in iab‐9 eliminate expression of ABD‐BII in neuromere 14. ABD‐B expression is also analyzed in homozygotes for (i) loss‐of‐function mutants involving the iab‐5, iab‐6 and iab‐7 regions, (ii) gain‐of‐function mutants Miscadastral pigmentation (Mcp) and Superabdominal (Sab), and (iii) a trans‐regulator, Polycomb (Pc). ABD‐B expression along the antero‐posterior axis is colinear with the chromosomal order of the cis‐regulatory regions. The behavior of rearrangement‐associated iab‐6 and iab‐7 mutants suggests that the enhancer‐like region, iab‐5, and possibly also iab‐6, may be shared between the abd‐A and Abd‐B domains. Such sharing is proposed as a factor that tends to keep gene complexes intact during evolution.