Isofemale lines in Drosophila: an empirical approach to quantitative trait analysis in natural populations.

Founding isofemale lines from wild collected females is a basic tool for investigating the genetic architecture of Drosophila natural populations. The method permits the analysis of quantitative traits under laboratory conditions, with a much broader scope than the mere evidence of a significant genetic heterogeneity among lines. Genetic variability is generally demonstrated by a significant coefficient of intraclass correlation, but several experimental precautions are needed and explained here. The relationship between classical (additive) heritability and intraclass correlation is not straightforward, presumably because the genetic bottlenecks due to the initiation of the lines unravel a significant, nonadditive genetic variance due to dominance and epistatic effects. It is thus suggested to consider intraclass correlation as a specific genetic parameter that enables comparisons between different traits, different populations or different environments. The use of isofemale lines is, however, not restricted to the calculation of an intraclass correlation. It can be used to estimate genetic correlations among traits or environments. The method is also convenient for the analysis of phenotypic plasticity in relation to an environmental gradient. A precise description of the response curves (the reaction norms) is possible, distinguishing trait parameters and plasticity parameters. A fairly general conclusion is that, for a given trait, mean value and plasticity are genetically independent. It is also possible to analyze traits, which, like sexual dimorphism, must be measured on different individuals, and even to demonstrate their genetic variability. In many cases, further empirical and theoretical analyses are possible and needed. It is argued that, in the future, isofemale lines will have an increasing significance among the various techniques appropriate to the analysis of quantitative evolutionary genetics in a diversity of species.