Differences in chromosome A arrangement betweenDrosophila madeirensis andDrosophila subobscura

Summary The proximal half of the A (=X) chromosome ofD. madeirensis has a gene arrangement very similar to the A1 or A6 inversions found inD. subobscura. Polytene chromosome analysis of hybrids betweenD. madeirensis and strains ofD. subobscura homozygous for such inversions shows, however thatD. madeirensis has a gene arrangement different from any known forD. subobscura. These results provide evidence for a greater differentiation of the X chromosome in these species than has previously been described; it seems that the X chromosome is the only one that has undergone structural variation during the speciation process.