Measuring fitness by means of balancer chromosomes

We present the theoretical background to a new method for measuring genetic variation for total fitness in Drosophila. The method allows heterozygous effects on total fitness of whole wild-type chromosomes to be measured unde r normal demography with overlapping generations. The wild-type chromosomes are competed against two balancer chromosomes (B1, B2, say), providing a s tandard genotype B1/B2 against which variation in the fitness effects of th e wild-type chromosomes can be assessed. Fitness can be assessed in two way s: (i) at equilibrium of all three chromosomes under heterozygote advantage , and (ii) during displacement of one balancer by the other. Equilibrium wi th all three chromosomes present will be achieved only if the wild-type hom ozygote is not too fit, and if the fitnesses of the three heterozygotes are not too unequal. These conditions were not satisfied for any of a sample o f 12 lethal-bearing chromosomes isolated from a random-bred laboratory popu lation of Drosophila. At equilibrium, genotypic frequencies show low sensit ivity to changes in genotypic fitness. Furthermore, where all four genotype s are viable and fertile, supplementary information from cages with only tw o chromosomes present and from direct measurements of pre-adult viability a re required to estimate fitnesses from frequencies. The invasion method has the advantages of a greater sensitivity and of not requiring further data to estimate fitnesses if the wild-type homozygote is fertile. However, it r equires that multiple samples be taken as the invasion progresses. In a dis crete generation model, generation time influences fitness estimates from t his method and is difficult to estimate accurately from the data. A full ag e-structured model can also be applied to the data from both types of exper iment. For the invasion method, this gives fitness estimates close to those from the discrete generation model.