Seasonal Variation in the Morphology of Drosophila robusta Sturtevant

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Abstract
A study of genetically controlled seasonal variation in morphology was made of a natural population of Drosophila robusta near St. Louis, Missouri. Five hundred F2 progenies from wild flies captured in the period from February to September 1946 were reared under uniform laboratory conditions, and five adult females were measured from each progeny. Of the five characters measured, thorax-length, head-width, femur-length, wing-length and wing-width, all showed significant seasonal heterogeneity. Using the compound measurement D2 (Fisher's Discriminant Function), as an indication of northernness or southernness of the morphology it was shown that the population underwent a regular, highly significant shift toward a southern morphology during the summer months June through August. This shift during the hot weather was in the direction expected if the southern morphology has high temperature adaptive value, and if the species undergoes a rapid selective response to seasonal temperature fluctuations. The seasonal changes in morphology are in marked contrast to the essential stability of the gene arrangement frequencies. Since such stability seems to indicate a relatively large breeding population it is felt that the observed morphological changes cannot be attributed to genetic drift, and are probably the result of natural selection. In order to determine the direct, nongenetic effect of the temperature of rearing on the morphology, five geographically diverse strains were used in which flies reared at 25.5⚬C and those reared at 17.6⚬ C. were compared morphologically. It was found that the low temperature group were more northern in their morphology than those reared at high temperatures. The northernness of the low temperature group was indicated both by their D2 values and by the typically northern shift in the proportion of head-width and wing-length. By comparing the F2 progenies coming from wild parents carrying different gene arrangements it was shown that the arrangements XR and XR-1 are associated with different morphological characteristics.

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