P1 CLONES FROM DROSOPHILA-MELANOGASTER AS MARKERS TO STUDY THE CHROMOSOMAL EVOLUTION OF MULLERS-A-ELEMENT IN 2 SPECIES OF THE OBSCURA GROUPOF DROSOPHILA

Thirty P1 clones from the X chromosome (Muller's A element) of Drosoph ila melanogaster were cross-hybridized in situ to Drosophila subobscur a and Drosophila pseudoobscura polytene chromosomes. An additional rec ombinant phage lambda Dsuby was also used as a marker. Twenty-three (7 7%) of the P1 clones gave positive hybridization on D. pseudoobscura c hromosomes but only 16 (53%) did so with those of D. subobscura. Eight P1 clones gave more than one hybridization signal on D, pseudoobscura and/or D, subobscura chromosomes. All P1 clones and lambda Dsuby hybr idized on Muller's A element (X chromosome) of D. subobscura. In contr ast, only 18 P1 clones and lambda Dsuby hybridized on Muller's A eleme nt (XL chromosomal arm) of D. pseudoobscura; 4 additional P1 clones hy bridized on Muller's D element (XR chromosomal arm) of this species an d the remaining P1 clone gave one hybridization signal on each arm of the X chromosome. This latter clone may contain one breakpoint of a pe ricentric inversion that may account for the interchange of genetic ma terial between Muller's A and D elements in D. pseudoobscura. In contr ast to the rare interchange of genetic material between chromosomal el ements, profound differences in the order and spacing of markers were detected between D. melanogaster, D. pseudoobscura and D. subobscura. In fact, the number of chromosomal segments delimited by identical mar kers and conserved between pairwise comparisons is small. Therefore, e xtensive reorganization within Muller's A element has been produced du ring the divergence of the three species. Rough estimates of the numbe r of cytologically detectable inversions contributing to differentiati on of Muller's A element were obtained. The most reliable of these est imates is that obtained from the D, pseudoobscura and D. melanogaster comparison since a greater number of markers have been mapped in both species. Tentatively, one inversion breakpoint about every 200 kb has been produced and fixed during the divergence of D. pseudoobscura and D. melanogaster.