The genetics of Drosophila subobscura populations. XVII. Further genic heterogeneity within electromorphs by urea denaturation and the effect of the increased genic variability on linkage disequilibrium studies.

Urea denaturation of allozymes was used to provide finer resolution of allelic states within classes of different electrophoretic mobility. This method gives perfectly repeatable results. About 170 isogenic strains for the O chromosome of Drosophila subobscura, derived from two natural populations, were constructed. Their gene arrangements were studied, as well as eight polymorphic genes located on the O chromosome (Est-5, Odh, Ao, ME, Xdh, Lap, Pept-1 and Acph). Crosses performed indicate that differences in urea sensitivity are genetically controlled by the same genes that control electrophoretic mobility. Twice as many alleles have been detected in comparison to the usual electrophoretic method. However, the effective number of alleles did not increase considerably. Studies of linkage disequilibria, by taking into account the finer resolution of allelic states, gave results nearly identical with those obtained in studies where the usual electrophoretic method was used. Although the power of the test is diminished, the absence of genic associations seems to indicate that there are no hidden linkage disequilibria in electrophoretic studies (because of consolidation effects of real alleles into few electromorph classes). The paucity of linkage disequilibria would indicate that there are no epistatic interactions such as those suggested in the model of FRANKLIN and LEWONTIN (1970).