Fig. 2. The expression of the BX-C genes in the fat body is related to cytologically visible changes of the 89DE BX-C region of the polytene chromosomes. (A) The 89DE region appears strongly condensed in polytene chromosomes of the salivary glands. (B) The 89DE region also appears condensed in polytene chromosomes in cells from the anterior parts of the fat body (big arrows in Fig. 1A). In situ hybridization with probes corresponding to the Ubx, abd-A and Abd-B genes clearly shows that these genes are tightly physically linked. (C) The 89DE region in polytene chromosomes of mid-posterior fat body cells appear strongly puffed and, by in situ hybridization with the same probe as in B, the three genes appear more physically distinct. (D-F) The BX-C genes that are active in fat body cells are also amplified relative to cells in which they are quiescent. (D) The structure of the Ubx and abd-A genes. The gray boxes indicate the exons. The black blocks above indicate the probes used to determine the degree of amplification of the corresponding genomic sequences. (E) The panel shows a Southern blot of genomic DNA from adult heads (a.h.), mid fat body (m.f.b.) and salivary glands (s.g.) probed with a DNA fragment corresponding to the rosy gene and two other fragments corresponding to the 5' and 3' regions of the Ubx gene. As has been shown (Moshkin et al., 2001), a comparison of the two Ubx signals with the rosy signal between adult heads and the salivary glands demonstrates that the 3' end of the Ubx gene is amplified while the 5' end is under replicated. We observed the same pattern in anterior fat body where the Ubx gene is repressed (not shown). However, in mid fat body, where the Ubx gene is active, both the 3' and 5' ends appear amplified. (F) The panel shows a Southern blot of genomic DNA from adult heads (a.h.), anterior fat body (a.f.b.), posterior fat body (p.f.b.) and salivary glands (s.g.) probed with a DNA fragment corresponding to rosy and a fragment corresponding to a region of abd-A. Again abd-A appears under replicated in salivary glands and anterior fat body where this gene is repressed, while it appears amplified in posterior fat body where abd-A is expressed. (G-L) Patterns of localization of the Polycomb (Pc) and Trithorax (Trx) proteins in the 89DE region of polytene chromosomes from salivary glands and mid-posterior fat body. (G,H) Staining of polytene chromosomes from salivary glands with Pc and Trx antibodies respectively. As has been shown, Pc is strongly accumulated on the BX-C locus (arrow). Trx is also located in the same region (arrow) although the fluorescence intensity suggests that is accumulated at much lower levels than Pc. (I,L) Staining of polytene chromosomes from mid posterior fat body with Pc (I) and Trx (L) antibodies. Pc is not detectable on strongly puffed 89DE region (large arrowhead) where the BX-C genes are active, while the same region shows a strong accumulation of Trx (arrow). Small arrowhead indicates a PC signal outside the 89DE region.