Lerner's theory on the genetic relationship between heterozygosity, genomic co-adaptation, and developmental instability revisited

Question: What is the genetic relationship between heterozygosity and developmental instability? Methods: Using a new method for the quantification of developmental instability, we compare homozygous strains of Drosophila mereatorum (either parthenogenetic or highly inbred) and of their F1 progeny for the analysis of a symmetrical meristic trait (sternopleural bristle number). The method also implies the choice of samples in which the left-right correlation is close to zero. Conclusions: In the parthenogenetic strain, developmental instability was seven times greater than in the F1 progeny. In the highly inbred strain, developmental instability was only 2.6 times greater than in the F1 progeny. The greater developmental instability in the parthenogenetic strain may be accounted for by the immediate fixation of a large number of deleterious alleles. In the inbred strain, on the other hand, progressive inbreeding is likely to have purged the genome of some of its genetic load and its most deleterious variants. The greater developmental instability in the parthenogenetic and inbred strains implies a non-linear relationship between heritability (h(2)) and additive genetic variance (sigma(2)(a)) (which are both related to evolutionary potential), which is discussed in an evolutionary context. A linear relationship between the sigma(2)(a) of a trait and its h(2) is expected if developmental instability is constant and independent of the sigma(2)(a) of that trait. If the relationship between the sigma(2)(a) of a trait and its developmental instability is negative, an increased sigma(2)(a), which is in the numerator of the h equation, will imply a reduced phenotypic variability (sigma(2)(p)), which is in the denominator of the heritability equation. Consequently, the concomitant increase in sigma(2)(s) and reduction in sigma(2)(s) both contribute to an increase in sigma(2)(p) and therefore an increase in evolutionary potential.