Body size and developmental temperature in Drosophila simulans: Comparison of reaction norms with sympatric Drosophila melanogaster

Reaction norms of two size-related traits (wing and thorax length) were analyzed in relation to growth temperature in a French natural population of Drosophila simulans, using the isofemale lines method. The wing/thorax ratio was also studied. Data were compared to those of the sibling species Drosophila melanogaster from the same locality. Flies were reared at seven constant temperatures, representing the whole thermal range of the two species. Phenotypic and genetic variabilities were analyzed. For investigating the shape of the response curves (ie, reaction norms) two methods were used: analysis of slope variations and polynomial adjustments. As expected from the relatedness of the two species, many similarities were observed. Notably, the reaction norms of wing and thorax lengths exhibited a maximum at low temperature, while the wing/thorax ratio was a regularly decreasing sigmoid curve. Numerous and sometimes great differences were also observed. At the phenotypic level, D simulans was generally more variable, while at the genetic level, it was less variable than D melanogaster. Isofemale line heritabilities varied according to growth temperature, but with different patterns in the two species. In both species, sexual dimorphism increased with temperature, but the average values and the response curves were different. The reaction norms of wing and thorax lengths were mainly characterized by different TMSs (temperatures of maximum size) with lower values in D simulans. This species was also characterized by a much lower wing/thorax ratio with a higher TIP (temperature of inflexion point). The possible adaptive significance of these variations remains unclear. Indeed, TMS variations suggest that D simulans could be more tolerant to cold than its sibling. On the other hand, the lower wing/thorax ratio of D simulans suggests a warm-adapted species.