The effect of stressful and nonstressful rearing temperatures on phenotypic variation of four quantitative characters (thorax length, wing length, number of sternopleural chaetae, number of arista branches) and on developmental stability (fluctuating asymmetry) of the three latter characters was estimated in two Drosophila species: Drosophila melanogash and Drosophila buzzatii.In both species, a general trend for increasing of phenotypic variation and fluctuating asymmetry at stress temperatures was observed; in fluctuating asymmetry, this effect was more pronounced. An increase of phenotypic variation under stress was shown for all characters examined except sternopleural chaeta number in D. buzzatii. Comparison of species responses suggests that the increase of variation in D. melanogash was somewhat higher than in D. buzzatii.

A study of 16 natural populations of Drosophila melanogaster from Eastern Europe, the Caucasus and Central Asia has revealed a dine in wing length associated with geographical position of the populations. Wing length was shown to be positively correlated with temperature. The coefficient of variation in wing length was significantly different in town and orchard populations. The existence of a dine in wing length in the northern part of the species range and in the region where migration must be substantial suggests strong selection pressure acting in natural populations of D.melanogaster.

The effect of nutritional stress on phenotypical and genetic variation was examined for five morphological traits (thorax length, wing length, sternopleural chaeta number, abdominal chaeta number and arista branch number) in 30 isofemale lines of Drosophila melanogaster. Phenotypical variation of all traits except sternopleural chaeta number and fluctuating asymmetry of all bilateral traits were significantly higher in flies reared under poor feeding conditions. Estimates of isofemale line heritability (coefficients of intraclass correlation) did not show a consistent pattern among traits. However, additive genetic variance was generally higher in poor feeding conditions in all traits except sternopleural chaeta number, although these differences were not statistically significant. Similarly, estimates of evolvability were higher under nutritional stress for all traits except sternopleural chaeta number. These results suggest that nutritional stress increases the expression of genetic variation for some morphological traits in Drosophila and, in this respect, is similar to the effects of temperature stress studied previously.

Responses to short-term selection for knockdown resistance to heat (37°C) in Drosophila melanogaster reared under stressful (high larval density) and nonstressful (low larval density) conditions were compared. No difference in selection response between density treatments was found, A test of heat resistance (39°C) after pretreatment (37°C) did not reveal an increase in survival for selected lines as compared to controls. Flies reared at high density had higher knockdown resistance throughout the experiment. Resistance to heat was not associated with body size.

Using an isofemale line analysis, we analysed the consequences of extreme rearing temperatures for genetic variation in quantitative characters in Drosophila melanogaster. Three types of characters were used: life history (viability and developmental time), body size (thorax length and wing length) and meristic (number of sternopleural chaetae and number of arista branches). Phenotypic variation significantly increased under stress conditions in all morphological characters studied; for viability, it increased at the low stress temperature. Genetic variation, measured by the coefficient of intraclass correlation, was generally higher at both low and high stress temperatures for thorax length and sternopleural chaeta number. For wing length and viability, genetic variation was higher at the low extreme temperature. No consistent trend was found for genetic variation in arista branch number and developmental time. Our results agree with the hypothesis that genetic variation is increased in stressful environments. A possible mechanism underlying this phenomenon is briefly discussed.