An Altitudinal Transect of Drosophila robusta Sturtevant

Stalker, H; Carson, H

A morphological and cytological study of an altitudinal transect of Drosophila robusta Sturtevant was made near Gatlinburg, Tennessee. The transect included six collecting localities extending from 4000 to 1000 feet in altitude and covering a linear distance of eighteen miles. Eleven out of the fourteen gene arrangements found showed changes in frequency over the area studied. The changes in inversion frequencies with decrease in altitude were not simple recapitulations of changes observed from north to south in geographical studies. Some geographically northern inversions showed no change (2R), or an increase in frequency (3R), with decreasing altitude. In the case of the clearly northern 2L-3 and XL-1 a striking decrease in frequency with decreasing altitude was found. The disagreement between geographical and altitudinal clines in the case of 2R and 3R may be due to the fact that: (a) the environments in the north and at 4000 feet in Tennessee are similar but by no means identical; (b) the same gene arrangement may have different genetic values at different localities. Of the five morphological characteristics measured in F2 females, four: thoraxlength, femur-length, wing-length and wing-width, showed significant decrease with decreasing altitude. Of these four, femur-length and the two wing dimensions also decrease from north to south. Thorax-length is greatest in the south, and head-width which showed no change with altitude is also greatest in the south. A compound measurement Da (Fisher's Discriminant Function) so chosen as best to discriminate between the upper three and lower three localities indicated that the over-all morphological changes with altitude were greatest between 2000 and 1400 feet, with little change above or below this point. This is in agreement with the changes in the three most variable single characters, femur-length, wing-length and wing-width. The form of the changes indicated above is in contrast to the changes in inversion frequencies, which were pronounced above 2000 and below 1400 feet. The possible reasons for this discrepancy are discussed. Using the coefficients associated with the Discriminant D2, originally chosen so as best to discriminate strains from different climatic zones in the geographical study, a mean D2 was calculated for each mountain locality. Considering the values of D2 as measures of 'northernness' or 'southernness' of the morphology, it was concluded that the populations in the upper levels of the transect are significantly more northern in their morphology than those in the lower levels.

An Altitudinal Transect of Drosophila robusta Sturtevant

Series