PHENOTYPIC PLASTICITY AND REACTION NORMS IN TEMPERATE AND TROPICAL POPULATIONS OF DROSOPHILA-MELANOGASTER - OVARIAN SIZE AND DEVELOPMENTAL TEMPERATURE

The plasticity of ovariole number relative to developmental temperatur e was studied in three populations of Drosophila melanogaster at both ends of the cline: a temperate French population and two equatorial Co ngolese. Ovary size was much greater in the French flies, in agreement with an already known latitudinal dine. Among isofemale lines, signif icant differences in genetic variability were observed between populat ions with a maximum variability at intermediate temperatures. Paramete rs of phenotypic variability (CV and FA) were not statistically differ ent among lines or populations, but a significant increase at low temp erature was demonstrated for both. The shapes of the response curves ( i.e., the norm of reaction) were analyzed by adjusting the data to a q uadratic equation. The parameters of the equation were highly variable among lines. On the other hand, the temperature for maximum value of ovarioles (TMV) was much less variable and exhibited only a slightly s ignificant difference between temperate and tropical flies (22.2 degre es C vs. 22.7 degrees C). During its geographic extension toward colde r places, D. melanogaster underwent a large, presumably adaptative, in crease in ovariole number but very little change in the norm of reacti on of that trait.