TRANSVECTION IN THE IAB-5,6,7 REGION OF THE BITHORAX COMPLEX OF DROSOPHILA - HOMOLOGY INDEPENDENT INTERACTIONS IN TRANS

The Abdominal-B (Abd-B) gene of the bithorax complex (BX-C) of Drosoph ila controls the identities of the fifth through seventh abdominal seg ments and segments in the genitalia (more precisely, parasegments 10-1 4). Here we focus on iab-5, iab-6 and iab-7, regulatory regions of Abd -B that control expression in the fifth, sixth and seventh abdominal s egments (parasegments 10-12). By analysis of partial BX-C deficiencies , we show that these regions are able to promote fifth and sixth abdom inal segment identities in the absence of an Abd-B gene in cis. We est ablish that this ability does not result from cis-regulation of the ad jacent abd-A or Ubx genes of the BX-C but rather occurs because the ia b-5 6,7 region is able to interact with Abd-B in traits. We demonstrat e that this interaction is proximity dependent and is, therefore, a ca se of what E. B. LEWIS has called transvection. Interactions of this t ype are presumably facilitated by the synapsis of homologues that occu rs in somatic cells of Dipterans. Although transvection has been detec ted in a number of Drosophila genes, transvection of the iab-5,6,7 reg ion is exceptional in two ways. First, interaction in trans with Abd-B does not require that homologues share homologous sequences within, o r for some distance to either side of, the BX-C. This is the first cas e of transvection shown to be independent of local synapsis. A second unusual feature of iab-5,6,7 transvection is that it is remarkably dif ficult to disrupt by heterozygosity for chromosome rearrangements. The lack of requirement for local synapsis and the tenacity of trans-inte raction argue that the iab-5,6,7 region can locate and interact with A bd-B over considerable distance. This is consistent with the normal ro le of iab-5,6,7, which must act over some 20-60 kb to influence its re gulatory target in cis at the Abd-B promoter. Evidence is presented th at trans-action of iab-5,6 7 requires, and may be mediated by, the reg ion between distal iab-7 and Abd-B. Also, we show that iab-5,6,7 trans vection is independent of the allelic state of zeste, a gene that infl uences several other cases of transvection. The long-range nature of i nteractions in trans between iab-5,6,7 and Abd-B suggests that similar interactions could operate effectively in organisms lacking extensive somatic pairing. Transvection may, therefore, be of more general sign ificance than previously suspected.