Alexandra G. Imasheva

Variation in wing length in Eurasian natural populations of Drosophila melanogaster

A study of 16 natural populations of Drosophila melanogaster from Eastern Europe, the Caucasus and Central Asia has revealed a cline 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 cline 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.

Variation in morphological traits of Drosophila melanogaster (fruit fly) under nutritional stress

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.

Stress temperatures and quantitative variation in Drosophila melanogaster

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.

SELECTION FOR KNOCKDOWN RESISTANCE TO HEAT IN DROSOPHILA MELANOGASTER AT HIGH AND LOW LARVAL DENSITIES

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.

Effect of stressful and nonstressful growth temperatures on variation of sternopleural bristle number in Drosophila melanogaster.

Abstract The effect of stressful (31°C) and nonstressful (25°C) growth temperatures on quantitative variation and developmental stability (fluctuating asymmetry) of Drosophila melanogaster was examined in a short-term selection experiment on sternopleural bristle number. Realized heritabilities based on 10 generations of selection in an upward direction did not differ between the two temperature regimes, which indicated that additive genetic variation was not affected by a high, stressful temperature. Phenotypic variability and fluctuating asymmetry of sternopleural bristles were significantly higher under stressful conditions when averaged over generations, although most pairwise comparisons in separate generations showed nonsignificant differences between temperatures. Corresponding Editor: L. Nunney

Geographic differentiation in wing shape in Drosophila melanogaster.

Genetic variation of a suite of 12 morphometric wing characters was examined in 16 natural populations ofDrosophila melanogaster from Eastern Europe and Central Asia using principal component analysis. The posterior wing compartment was found to differ in shape between the Eastern European and Central Asian populations. This result is in agreement with data on wing shape variation from exposure to high and low temperatures under laboratory conditions.

Genetic variation of morphological traits in Drosophila melanogaster under poor nutrition: isofemale lines and offspring–parent regression

Variation of three morphological traits (thorax length, wing length and sternopleural bristle number) was examined in Drosophila melanogaster reared on a medium with low yeast content and on a standard medium using the isofemale line analysis and offspring–parent regression. The aim was to test whether these experimental approaches give different patterns of changes in genetic variability estimates when stressful and nonstressful environments are compared. Heritabilities and genetic and phenotypic variances were generally higher in the isofemale line design than in the offspring–parent regression design under both standard and poor nutritional conditions. For each trait, the response of heritability to stress was similar in both designs: wing length exhibited lower heritability under poor nutrition, whereas heritabilities of thorax length and sternopleural bristle number did not differ between nutritional regimes. Statistically significant differences in the genetic variances and the environmental variances between stressful and nonstressful environments were recorded only in isofemale lines: the genetic variance of thorax length and the environmental variances of thorax length and wing length were higher under poor nutrition. The results are compared to literature data and possible reasons of increased genetic variability estimates in isofemale lines are briefly discussed.

Variation of allozyme loci in populations of Drosophila melanogaster from the former USSR.

Variation of eight allozyme loci, Acph, Adh, Est-6, Est-C, α-Gpdh, Idh (NADP-dependent), 6-Pgd and Pgm, was studied in 20 populations of Drosophila melanogaster from the territory of the former USSR, including Eastern Europe, the Caucasus and Central Asia. Adh exhibited weak regression on latitude, longitude, and also on the temperature of the hottest calendar month in these localities. For seven other loci, no regular geographical pattern was found. Neis coefficient of gene differentiation (GST) ranged from 0.010 (6-Pgd, statistically nonsignificant) to 0.178 (Idh) and was on average 0.058. The average Neis genetic distance (D) between the populations was 0.011. It is suggested that this fairly low level of interpopulation differentiation for the examined loci except Idh may be explained by extensive gene flow in combination with natural selection.

Effects of artificial stabilizing selection on Drosophila populations subjected to directional selection for another trait

Stabilizing selection for a set of morphometric wing traits was combined with directional selection for the increased expression of radius incompletus (ri) mutation of Drosophila melanogaster. Three experimental regimes were used: directional and stabilizing selection (stabilized lines); directional selection (unstabilized lines); no selection (controls). Response to selection for ri expression was similar in all selected lines but variation of this character was higher in the unstabilized lines compared to the stabilized ones. The competitive indices measured after termination of selection did not significantly differ under different treatments while fluctuating asymmetry was significantly lower in stabilized than in unstabilized lines. The possible causes of these differences are discussed.