Selection against homozygotes in the ADH polymorphic system of Drosophila melanogaster associated with the lethal factor l(2) Stm.

In the natrual populations +Tüb, +Prov, and +Rov, similar AdhF allele frequencies occur (qF = 0.11, 0.18, AND 0.08, respectively). However, there is a discrepancy in that the AdhF allele in +Tüb is closely linked to the lethal factor l(2)Stm, which reduces relative fitness of the F phenotype to zero. In spite of this, polymorphism is maintained also in +Tüb, because the heterozygotes are superior to the homozygous S type (relative fitness = 0.88). Under laboratory culture conditions, in +Tüb the relative fitness of the S genotype further decreases to 0.6. After outcrossing the lethal factor, relative fitnesses for S, FS, and F become 0.6, 1, and 0.48, respectively, implying that fitness for S remains the same. Relative values for S, FS, and F in +Prov, not affected by the lethal factor, are calculated by the maximum average fitness method to be 1, 1.2, and 0.2 under the assumption that heterozyglous FS are similarly superior to S as in the natural +Tüb population and all allele frequencies found are stable equilibrium values.