P1 clones from Drosophila melanogaster as markers to study the chromosomal evolution of Muller's A element in two species of the obscura group of Drosophila

Abstract

Thirty P1 clones from the X chromosome (Muller's A element) of Drosophila melanogaster were cross-hybridized in situ to Drosophila subobscura and Drosophila pseudoobscura polytene chromosomes. An additional recombinant phage lambda Dsuby was also used as a marker. Twenty-three (77%) of the P1 clones gave positive hybridization on D. pseudoobscura chromosomes 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 hybridized on Muller's A element (X chromosome) of D. subobscura. In contrast, only 18 P1 clones and lambda Dsuby hybridized on Muller's A element (XL chromosomal arm) of D. pseudoobscura; 4 additional P1 clones hybridized on Muller's D element (XR chromosomal arm) of this species and the remaining P1 clone gave one hybridization signal on each arm of the X chromosome. This latter clone may contain one breakpoint of a pericentric inversion that may account for the interchange of genetic material between Muller's A and D elements in D. pseudoobscura. In contrast to the rare interchange of genetic material between chromosomal elements, 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 markers and conserved between pairwise comparisons is small. Therefore, extensive reorganization within Muller's A element has been produced during the divergence of the three species. Rough estimates of the number of cytologically detectable inversions contributing to differentiation of Muller's A element were obtained. The most reliable of these estimates 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.

Selection, Recombination and Demographic History in Drosophila miranda

“…If this inversion was asymmetric, it could have caused more DNA to be translocated to XR from XL than from XR to XL. This is consistent with the in situ hybridization results of Segarra et al (1995), who found that genes move only from element A to D, but not the reverse, and could explain the smaller size of chromosomal arm XL in D. miranda. If each chromosomal arm has on average one crossing over event per meiosis, as suggested on the basis of cytological data in Drosophila (Ashburner 1989), this would imply that genes on chromosome XL undergo twice as much recombination per unit length than genes on other chromosomal arms.…”

“…If there is a reasonably good correspondence between size of the polytene chromosomes and sequence physical map, chromosome XL may contain only about half as much DNA as do the other chromosomal arms. In fact, some genes that are located on Muller's element A in D. melanogaster (which corresponds to XL in the pseudoobscura group) map to element D (chromosome XR) in D. pseudoobscura (Segarra et al 1995). Segarra et al (1995) concluded that this is probably the result of a pericentric inversion.…”

“…In fact, some genes that are located on Muller's element A in D. melanogaster (which corresponds to XL in the pseudoobscura group) map to element D (chromosome XR) in D. pseudoobscura (Segarra et al 1995). Segarra et al (1995) concluded that this is probably the result of a pericentric inversion. If this inversion was asymmetric, it could have caused more DNA to be translocated to XR from XL than from XR to XL.…”

Chromosomal Rearrangement Inferred From Comparisons of 12 Drosophila Genomes

“…Overview of the rearrangement process: Figure 3 (Segarra et al 1995). Muller F of D. willistoni has fused to Muller E (Papaceit and Juan 1998).…”

“…Muller (1940) overcame this problem when he developed a standard nomenclature that assigned a letter to each of the chromosomal arms or elements on the basis of the Drosophila melanogaster genome (chromosomal arm equals Muller element: X ¼ A, 2L ¼ B, 2R ¼ C, 3L ¼ D, 3R ¼ E, 4 ¼ F). Sturtevant and Novitski (1941) showed that the conservation of chromosomal elements extended to species across the entire genus of Drosophila.The conservation of the gene content within Muller elements across the genus Drosophila has been well supported as more species have been examined and as molecular genetic markers have been used to develop more detailed genetic and physical maps (Spassky and Dobzhansky 1950;Loukas et al 1979;Steinemann et al 1984;Whiting et al 1989;Segarra et al 1995Segarra et al , 1996Ranz et al 1997Ranz et al , 2001Ranz et al , 2003. As the density of genetic and physical markers on the maps of Drosophila species has increased, it has become clear that gene order within Muller elements is not conserved among species (Ranz et al 2001).…”