From a laboratory stock of Drosophila melanogaster (Oregon), reared for more than 20 years at 18°C, two new populations were derived and maintained at 25° and 28°C for 8 years. The chromosomal and cytoplasmic contribution to genetic divergence between the two more extreme populations was estimated at 18°C and 28°C. Wing shape and two fitness components (fecundity and fertility) were taken into account. Fourier descriptors and the position of the centroid were taken as indicators either of wing shape variation, determined by a different response of the two wing compartments to temperature selection, or of wing shape variation determined by both compartments. The descriptors appear to be good characters: they show a variability which is genetically controlled and ascribable to genes located on specific chromosomes. The third chromosome is responsible for the adaptive difference to temperature. The genes which control wing shape are located on the second and third chromosome, although the contribution of each chromosome depends on the environment in which the flies develop. Cytoplasmic genes display an effect as large as that of chromosomes, and nucleus × cytoplasm interaction is present. The correlation between the genetic contributions to compartment‐dependent wing shape variation and the contributions to fitness is highly significant, especially at 28°C. Wing shape has, therefore, an adaptive significance in relation to temperature, which is particularly expressed in the environment where selection occurred.

From a laboratory stock of Drosophila melanogaster (Oregon), reared for more than 20 years at 18" C, a new population was derived and maintained at 28" C for 8 years. The chromosomal and cytoplasmic contribution to genetic divergence between the two populations was estimated. Six body traits and reproductive fitness were taken into account. The third chromosome is responsible for the adaptive difference for temperature between the two lines. Temperature-selected genes which control body size are located on the second and third chromosomes, although the contribution of each chromosome depends on the environment in which the flies develop. The correlation between the chromosomal and cytoplasmic contributions to different traits and fitness, changes with temperature. At 28" C the correlation between fitness and each body trait is proportional to the response to selection exhibited by each of them, but this is not true at 18" C. Body size has, therefore, an adaptive significance in relation to temperature, which is expressed only in the environment where selection occurs. Cytoplasmic genes affect almost all characters to an extent similar to that of chromosomal genes. Inter-chromosomal and nucleocytoplasmic interactions are present and also change with temperature. In general, genes selected in a given environment produce greater phenotypic changes in that environment than in another. The population that experienced both temperatures is fitter in both environments, suggesting that the capacity to adapt to warm temperatures depends on genes other than those which are involved in the adaptation to cold. 235 236

Long interspersed element-1s (LINE-1 or L1s) are abundant retrotransposons that occur in mammalian genomes and that can cause insertional mutagenesis and genomic instability. L1 activity is generally repressed in most cells and tissues but has been found in some embryonic cells and, in particular, in neural progenitors. Moreover, L1 retrotransposition can be induced by several DNA-damaging agents. We have carried out experiments to verify whether L1 retrotransposition is affected by oxidative DNA damage, which plays a role in a range of human diseases, including cancer and inflammatory and neurodegenerative disease. To this purpose, BE(2)C neuroblastoma cells, which are thought to represent embryonic precursors of sympathetic neurons, have been treated with hydrogen peroxide and subjected to an in vitro retrotransposition assay involving an episomal L1(RP) element tagged with enhanced green fluorescent protein. Our results indicate that hydrogen peroxide treatment induces an increase in the retrotransposition of transiently transfected L1(RP) and an increase in the expression of endogenous L1 transcripts. An increase of γ-H2AX foci and changes in the mRNA levels of MRE11, RAD50, NBN and ERCC1 (all involved in DNA repair) have also been found. Thus, oxidative stress can cause L1 dysregulation.

The body sizes and shapes of poikilothermic animals generally show clinal variation with latitude. Among the environmental factors responsible for the dine, temperature seems to be the most probable candidate. In the present work we analysed natural populations of Drosophila melanogaster collected at different geographical localities to determine whether the same selective forces acting on wing development in the laboratory are also at work in the wild. We show that the temperature selection acting on wing development in the laboratory is only one of the selective forces operating in the wild. The size differences between natural populations seem to depend exclusively on cell number whereas they depend on cell area in the laboratory. The two wing compartments behave as distinct units of selection subjected to different genetic control, confirming our previous observations on laboratory populations. In addition, subunits of development defined as regions of cell proliferation centres restricted within longitudinal veins can, in turn, be considered as subunits of selection. Their interaction during development and continuous natural selection around an optimum could explain the high wing shape stability generally found in natural populations.

The body sizes and shapes of poikilothermic animals generally show clinal variation with latitude. Among the environmental factors responsible for the dine, temperature seems to be the most probable candidate. In the present work we analysed natural populations of Drosophila melanogaster collected at different geographical localities to determine whether the same selective forces acting on wing development in the laboratory are also at work in the wild. We show that the temperature selection acting on wing development in the laboratory is only one of the selective forces operating in the wild. The size differences between natural populations seem to depend exclusively on cell number whereas they depend on cell area in the laboratory. The two wing compartments behave as distinct units of selection subjected to different genetic control, confirming our previous observations on laboratory populations. In addition, subunits of development defined as regions of cell proliferation centres restricted within longitudinal veins can, in turn, be considered as subunits of selection. Their interaction during development and continuous natural selection around an optimum could explain the high wing shape stability generally found in natural populations.