Body size reaction norms in Drosophila melanogaster: temporal stability and genetic architecture in a natural population

A natural population of Drosophila melanogaster was sampled twice over a 5- year interval from the same French locality in the same season. Reaction no rms of wing and thorax length and wing/thorax ratio, according to growth te mperature (12-31 degrees C) were analysed in tell isofemale lines for each sample. Reaction norms were very similar between years, showing not only a remarkable stability of the average size but also of the reactivity to temp erature. Wing and thorax length reaction norms were characterized by the co -ordinates of their maxima (MV = maximum value of character; TMV = temperat ure of maximum value). The wing/thorax ratio, which exhibited a decreasing sigmoid norm, was characterized by the co-ordinates of the inflexion point. Again, these characteristic values were found to be very similar for sampl es between years. The results were further analysed by pooling the 20 lines into a single data set. Heritability was significantly variable according to temperature, but in a fairly irregular way with lowest values at extreme temperatures. Genetic variance of the three traits exhibited more regular variation with a minimum at intermediate temperatures and maxima at extreme high or low temperatures. Such was also the case of evolvability, i.e. the genetic coefficient of variation. Heritability and evolvability were found to be slightly but negatively correlated, showing that they provide indepe ndent biological information. The temporal stability of a natural populatio n over the years suggests some stabilizing selection for both mean body siz e and plasticity. For laboratory evolution experiments, the natural origin population might be useful as a genetic control over time. (C) Inra/Elsevie r, Paris.