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First published online 5 November 2008
doi: 10.1242/dev.025700

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Boundary swapping in the Drosophila Bithorax complex

¹ NCCR, Frontiers in Genetics, University of Geneva, Department of Zoology and Animal Biology, 30 Quai Ernest Ansermet, 1211 Geneva 11, Switzerland.
² University of Geneva, Department of Zoology and Animal Biology, 30 Quai Ernest Ansermet, 1211 Geneva 11, Switzerland.
³ Institute of Genetics, Biological Research Centre, PO Box 521, H-6701 Szeged, Hungary.

View larger version (23K): [in this window] [in a new window]   Fig. 1. The cis-regulatory region of Abd-B. (A) The Abd-B cis-regulatory region. The arrows are a graphical illustration of the targeting of each cis-regulatory domain to the Abd-Bm promoter (Boulet et al., 1991). The iab-6, iab-7 and iab-8 cis-regulatory domains interact with the Abd-Bm promoter in PS11/A6, PS12/A7 and PS13/A8, respectively. (B) The wild-type expression pattern of Abd-B in the embryonic CNS is characterized by an anterior-to-posterior increasing step gradient in the level of Abd-B. (C,D) Molecular maps of the Fab-7 and Fab-8 region (upper line). The boundary deletions mimicked are shown on the middle line and the final conversion products are shown on the bottom line.

View larger version (50K): [in this window] [in a new window]   Fig. 2. Fab-7 is able to substitute for Fab-8. (A-D) Cuticles were prepared from adult homozygous females of the indicated genotypes [see Mihaly et al. (Mihaly et al., 1997) for a detailed description of cuticular phenotypes]. (A) Wild-type: A8 does not contribute to any visible tergite or sternite structure in the adult female. The A7 sternite (ventral) displays a characteristic shape with large bristles pointing towards the posterior (arrow). (B) Fab-8305: the A7 to A8 transformation leads to the absence or reduction of the tergite (dorsal) and sternite (ventral). (C) F8F7 homozygous females display a wild-type A7 cuticle. (D) Diagram of F8F7 substitution. (E-G) Abd-B expression in the embryonic CNS. (E) Wild-type: Abd-B is expressed in a step gradient pattern from PS10 to PS14. (F) Fab-8305: the Abd-B expression level in PS12 increases to the level normally observed in PS13. (G) F8F7 embryos show a restoration of the wild-type expression pattern of Abd-B.

View larger version (55K): [in this window] [in a new window]   Fig. 3. Abd-B misregulation in A7 of F8F7 hemizygous females. Although F8F7 homozygotes look essentially wild type, the hemiyzgous females F8F7/Df(3R)P9 harbor occasionally features reminiscent of Fab-8 homozygotes. (A) +/Df(3R)P9 hemizygous female. Note the narrow shape of the sternite on A7 with bristles pointing towards the posterior. (B,C) Two examples of F8F7/Df(3R)P9 females harboring opposite homeotic transformations. The cuticle in B has a wider seventh sternite with bristles oriented towards the lateral side similar to the sternite found on A6 (an iab-7 loss-of-function phenotype). The cuticle shown in C has spots of naked cuticle on the seventh tergite. Careful examination reveals the absence of trichomes in those spots, indicating a transformation towards A8. These opposing phenotypes are reminiscent of Fab-8 mutations in which A7 can be a mixture of A6 and A8 identity (Barges et al., 2000), suggesting that boundary function is slightly impaired in F8F7.

View larger version (55K): [in this window] [in a new window]   Fig. 4. Fab-8 is not able to substitute for Fab-7. (A-C) Cuticles were prepared from adult males (note that A7 does not contribute to any visible cuticle structures in the adult male). (A) Wild-type: the A6 sternite is recognizable by the absence of bristles (black arrow). (B) In Fab-72 homozygote males, A6 is partially transformed into A7 as revealed by the reduction of the A6 tergite (Mihaly et al., 1997). (C) In F7F8 males, the transformation of A6 to A5 is visible on the sixth sternite, which exhibits an A5 shape and has bristles (black arrow). (D) Diagram of the F7F8 substitution. (E-G) Abd-B expression in the embryonic CNS. (E) Wild-type. (F) Fab-72: the Abd-B expression level in PS11 increases to the level normally observed in PS12. (G) F7F8: the Abd-B expression level in PS11 decreases to a level similar to that seen in PS10.

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