Data are presented concerning the gene arrangements in both arms of the X-chromosome of Drosophila robusta in eight altitudinal transects. The major change appears to be the increase of gene arrangement XL-1 (and decrease in XL, sometimes also XL-2) with increasing altitude. In each transect only one combination of XL-1 with a right arm arrangement seems primarily responsible for the increases with altitudinal change, and only one combination of XL for the decreases. In four of the transects the increasing combination appears to be XL-1.XR, XL-1.XR-2 in the other four. A substantial decrease with elevation in XL-2.XR-2 occurs only in the four areas where XL-1.XR exhibits the major increase. These results indicate that interactions between arrangements play a significant role in the adaptation of this species to altitude. The interactions appear related to the X-chromosomal linkage equilibriums in these regions. The data also suggest that the species employs its inversions in different methods of adaptation in the Appalachian Mountains from those it uses in other parts of the range.
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