Genetic decay of balancer chromosomes in Drosophila melanogaster.

Theoretical considerations predict that balancer chromosomes in Drosophila melanogaster should accumulate numerous deleterious mutations with time. We counted the number of recessive lethal mutations on two balancer chromosomes from the In(2LR)SM1/In(2LR)Pm strain maintained in our lab, after making the balancers heterozygous with deficiencies from second-chromosome Kyoto Deficiency kit strains. We detected 10 recessive lethal mutations in the balancer In(2LR)Pm, which is consistent with the mutation rate estimated previously. However, we detected only three mutations, a significantly smaller number, in the balancer In(2LR)SM1, although this may be an artifact. In conclusion, we observed genetic decay over an estimable timescale by using balancers with historical records. Thus, balancers of any strain may have accumulated many unidentified recessive lethal mutations.