Growth temperature and genetic variability of wing dimensions in Drosophila: opposite trends in two sibling species

Thirteen linear wing dimensions were measured in 10 isofemale lines of Dros ophila melanogaster and D. simulans grown at seven constant temperatures fr om 12 to 31 degreesC. Within-line (environmental) variability, estimated by the within-line coefficient of variation (CVw), exhibited similar variatio n patterns in the two species, that is higher values at extreme (low or hig h) temperatures. The magnitude of variation was, however, greater in D. sim ulans which appears to be more responsive to thermal change. A clear hyperb olic relationship between trait mean value and CVw was also observed in bot h species, arising from measurement errors which are relatively more pronou nced on shorter traits. Genetic variability was analysed bq considering bot h the genetic CV (CVg, evolvability) and isofemale line heritability (intra class correlation). Both parameters provided independent information, as sh own by a lack of correlation between them. Moreover, CVg was negatively cor related M with trait mean value, while heritability showed a positive corre lation. With respect to thermal environment, both parameters exhibited simi lar reaction patterns which contrasted the two species. Genetic variability in D. melanogaster followed a convex reaction norm, with higher values at extreme (high or low) temperatures, and this observation agrees with previo us independent investigations. Surprisingly, D. simulans revealed an opposi te pattern, with a maximum genetic variability in the middle of the range. Such data point to the danger of drawing general conclusions from the analy sis of a single species.