Consequences of outbreeding on phenotypic plasticity in Drosophila mercatorum wings

A multivariate morphometric investigation was conducted on wings of two parthenogenetic Drosophila mercatorum strains and offspring (F1) of crosses between these parthenogenetic strains with highly inbred sexual individuals of the same species. The parental flies and F1 offspring were reared at three different temperatures: 20, 25, or 28°C. This design allows a comparison of completely homozygous individuals (parental generation) with identical heterozygote offspring (F1), which makes an analysis of phenotypic plasticity of morphometric traits possible, without a potentially confounding effect of genotype-environment interactions, which can increase the phenotypic variability. The same pattern of phenotypic plasticity of wing size between the homozygous parental strains and the heterozygous offspring was found in both strains with an apparent heterotic effect for wing size in the F1 at 25°C. At 20 and 28°C flies from the parental generation had the biggest wings. Phenotypic plasticity of shape was found to be strain dependent. A reduced level of developmental instability (DI) was found in the F1 as compared to the parental strain only in strain 1 reared at 20°C for the wing size and 25°C for the wing shape. For all the other treatments higher DI was found in the F1 when the difference was significant, which is suggestive of outbreeding depression. These findings are difficult to interpret since an apparent heterotic effect of size at 25°C is accompanied by higher DI (though not significant in strain 2) and complex changes in wing shape. Hence, we cannot conclude whether outbreeding lowers or increases the capacity to respond to environmental change via plastic responses and via changes of the level of DI. The degree of change of phenotypic plasticity and DI is trait specific, depending on the environment and on the genotypes which are hybridizing.