Lack of Fitness Cost

Studies of the potential costs associated with xenobiotic resistance in the absence of the selective agent can suffer from several confounding experimental factors. First, fitness costs associated with strains in which resistance has been repeatedly selected for in the laboratory are unlikely to represent fitness costs associated with resistance mechanisms found in the field. Second, the resistant and susceptible strains compared are also often genetically unrelated and any observed costs may therefore be independent of the resistance trait itself Third, when insects are used they are often not checked for the presence of microbial pathogens, such as Wolhachia, which can influence the outcome of crosses between infected and uninfected strains. 

Traits commonly used to quantify fitness, fecundity, viability of eggs, larvae and pupae, lifespan and developmental rate were recorded for homozygous resistant (RR) and homozygous susceptible (SS) flies. We also compared heterozygous flies where resistance was inherited from the female (RS) and where resistance was inherited from the male [SR] [13]. 

In a trend similar to that seen for viability, the larval and pupal development of the RS genotype is accelerated in comparison with the SR genotype at 20 °C. This advantage is, however, obscured when development is faster at 25 °C. 

The fitness of each genotype is plotted relative to the most fit which is given a value of 1.0. An asterisk indicates significant differences at the 5% level (ANOVA with Tukey post-hoc pairwise comparisons) and individual P values are given above each histogram. 

---