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) wer e analyzed in relation to growth temperature in a French natural popul ation of Drosophila simulans, using the isofemale lines method. The wi ng/thorax ratio was also studied. Data were compared to those of the s ibling species Drosophila melanogaster from the same locality. Flies w ere reared at seven constant temperatures, representing the whole ther mal range of the two species. Phenotypic and genetic variabilities wer e analyzed. For investigating the shape of the response curves (ie, re action norms) two methods were used: analysis of slope variations and polynomial adjustments. As expected from the relatedness of the two sp ecies, 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. Numer ous and sometimes great differences were also observed. At the phenoty pic level, D simulans was generally more variable, while at the geneti c level, it was less variable than D melanogaster. Isofemale line heri tabilities varied according to growth temperature, but with different patterns in the two species. In both species, sexual dimorphism increa sed with temperature, but the average values and the response curves w ere different. The reaction norms of wing and thorax lengths were main ly 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 inflexi on point). The possible adaptive significance of these variations rema ins unclear. Indeed, TMS variations suggest that D simulans could be m ore tolerant to cold than its sibling. On the other hand, the lower wi ng/thorax ratio of D simulans suggests a warm-adapted species.