Studies on Natural Populations of Drosophila. I. Heat Resistance and Geographical Variation in Drosophila melanogaster and D. simulans

Abstract

1. Natural populations of Drosophila melanogaster and Drosophila simulans were captured from different geographical regions in Lebanon, Egypt, and Uganda. 2. Offspring of such populations were reared under different temperatures ranging from 10⚬ to 31.5⚬ C in Drosophila melanogaster and to 30.5⚬ C in Drosophila simulans. 3. Wing and thorax length are greater at lower temperatures, decreasing gradually with increasing temperature, while the percentage of emergence is lower at the two extremes of temperature and significantly higher at optimal conditions. 4. Wing/thorax ratio is about 2 under optimal conditions, increasing at lower temperatures and decreasing at higher ones. 5. Phenotypic variance of wing length is higher at both extremes, and much higher at warmer temperatures. 6. Complete sterility, i.e., eggs failing to produce adults, occurs at 31.5⚬ C in all populations of Drosophila melanogaster and at 30.5⚬ C in Drosophila simulans. 7. Lebanon populations, i.e., northern populations, are more vigorous, less variable and more viable than all other populations studied at almost every temperature, while populations from Uganda show superiority to all other populations only at the higher temperatures, i.e., their original climatic conditions. 8. Extremes of temperature adversely affect sex-ratio. At the two extremes of temperature females appear to be the stronger sex and are significantly more numerous than males. At milder temperature the sexes occur in equal numbers.