Homologous association of the Bithorax-Complex during embryogenesis: consequences for transvection in Drosophila melanogaster

Transvection is the phenomenon by which the expression of a gene can be con trolled by its homologous counterpart in trans, presumably due to pairing o f alleles in diploid interphase cells. Transvection or trans-sensing phenom ena have been reported for several loci in Drosophila, the most thoroughly studied of which is the Bithorax-Complex (BX-C), It is not known how early trans-sensing occurs nor the extent or duration of the underlying physical interactions. We have investigated the physical proximity of homologous gen es of the BX-C during Drosophila melanogaster embryogenesis by applying flu orescent in situ hybridization techniques together with high-resolution con focal light microscopy and digital image processing. The association of hom ologous alleles of the BX-C starts in nuclear division cycle 13, reaches a plateau of 70% in postgastrulating embryos, and is not perturbed by the tra nscriptional state of the genes throughout embryogenesis. Pairing frequenci es never reach 100%, indicating that the are in equilibrium with a dissocia ted state, We determined the effects of translocations and a zeste protein null mutation, both of which strongly diminish transvection phenotypes, on the extent of diploid homologue pairing. Although translocating one allele of the BX-C from the right arm of chromosome 3 to the left arm of chromosom e 3 or to the X chromosome abolished trans-regulation of the Ultrabithorax gene, pairing of homologous alleles surprisingly was reduced only to 20-30% , A zeste protein null mutation neither delayed the onset of pairing nor le d to unpairing of the homologous alleles. These data are discussed in the l ight of different models for transregulation. We examined the onset of pair ing of the chromosome 4 as well as of loci near the centromere of chromosom e 3 and near the telomere of 3R in order to test models for the mechanism o f homologue pairing.