Eight isofemale lines of Drosophila melanogaster were raised at four temperatures and at four yeast concentrations in their food. Temperature and food show a significant interaction in determining wing length and thorax length, affecting mean size per line and genetic variation between lines. The combination of low temperature and poor food conditions leads to a sharp increase in the genetic variation over lines of both body size characters. The increase in genetic variation in wing length under less favourable conditions is due to an increase in genetic variation of both cell size and cell number. Changes in wing area in response to both temperature and food level follow a common cell size/cell number trajectory. Changes in wing size are obtained by line-specific changes in the cellular composition of the wing, rather than by changes specific for the environmental factor.
Timing of puparium formation in Drosophila melanogaster is set by reaching a critical stage at which larvae attain the ability to pupariate. This critical stage is reached at a relatively constant size characterized by the mean critical weight, i.e. the weight at which 50% of surviving larvae pupate without further feeding. The mean critical weight might be affected by larval growth conditions. This hypothesis was tested by determining the mean critical weight in larvae raised at three temperatures and two food levels, for two isofemale lines from two populations. Pupariation probability is a function of larval weight. The two environmental variables affect pupariation probability and mean critical weight differently. Food level does not affect critical weight but affects weight‐independent mortality; higher temperatures lead to a reduction of mean critical weight. Mean critical weight shows substantial differences between lines; the differences are maintained over temperatures. Genetic variation in mean critical weights has ecological and evolutionary implications.
The reaction norms in Drosophila melanogaster of thorax length, wing length and cell size were determined for 28 isofemale lines from three populations to investigate the role of cell size in determining the response of body size to temperature during the preimaginal stages. Both overall level and plasticity of the reaction norms of thorax length and wing length are highly correlated, leading to a relatively constant wing-thorax ratio between lines. Genetic differences in overall level of wing size reaction norms are mainly caused by differences in cell number. The response of wing size to temperature consists of changes in cell size and, to a lesser extent, cell number. The cellular basis of genetic differences in plasticity shows a transition point at an intermediate level. In steeper reaction norms, genetic differences in plasticity result from differences in the plasticity of cell size, whereas less steep reaction norms only differ in the plasticity of cell number. A significant partial correlation between wing length plasticity and cell size plasticity, correcting for thorax length plasticity, indicates a role of cell size in determining the wing-thorax ratio.
A deterministic model was developed to simulate population growth of the agromyzid fly Liriomyza bryoniae and the parasitoid Dig/yphus isaea. The model has two driving variables, ambient temperature and leaf nitrogen content of the tomato plant. Results of a glasshouse experiment were used to validate the model. The timing of successive generations of leafminers was simulated accurately over four generations. Population growth of leafminers was correctly simulated during the first two generations, but overestimated in the third generation. Mortality of leafminers due to parasitism was overestimated in the first generation after introduction of parasitoids: 73o/o instead of the observed 30%. A nearly 100% mortality of leafminers was correctly simulated in the second generation after introduction of parasitoids. Sensitivity analysis was performed for three types of variables: (1) driving variables, temperature and leaf nitrogen content; (2) parasitoid traits, searching efficiency and allocation of attacks to host feeding and oviposition, and (3) introduction strategies for biological control, timing, number of releases and number of parasitoids per release. Population growth was sensitive to temperature, leaf nitrogen content, searching efficiency of parasitoids and numbers of parasitoids released.
The relation between oxygen consumption rate and larval growth rate in Drosophila melanogaster at different temperatures and food levels was analyzed to investigate whether different larval growth rates and adult body sizes are the consequences of different costs of growth. Four isofemale lines from two populations collected from France and Tanzania were compared. The rate of oxygen consumption per mg body weight increased with the relative rate of growth. Costs of growth (Cg), i.e., the increase in oxygen consumption due to growth, took 17.4% of the total energy invested in growth. No significant effect on Cg could be shown of either temperature or food level. Basal metabolic rate (Rb), i.e., the rate of oxygen consumption in the absence of growth, increased with temperature and food level at the intermediate temperature. The two French lines showed a higher rate of growth, independent of the environmental conditions, leading to a larger final body size. The two Tanzanian lines show a reduced Cg as compared to the French lines at 27.5°C, while showing no such difference at 17.5°C and 22.5°C. Yet, these differences in Cg do not lead to clear differences in growth efficiency between populations, indicating that differences in absorption rate are the main cause of both environmental and genetic differences in growth rate.
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