Isoenzyme frequencies in long-term selection lines of Drosophila melanogaster I. Isoenzyme frequencies of the esterases (Est) and larval alkaline phosphatases (Aph) in temperature selected lines.

1) Lines of Drosophila melanogaster derived from a double cross of four laboratory stocks and selected for more than 125 generations were investigated by the isoenzyme method. The reference line RE was held constantly at 25°C, the selection lines 1a and 3a had a constant environment at 18°C, the selection lines 1b and 3b at 28°C. Additionally a permanent artificial migration was maintained between the selection lines 3a and 3b by exchange of food tubes. 2) The isoenzymes of the esterase controlled by the Est-6-locus and the larval alkaline phosphatase controlled by the Aph-D-locus were analysed. 3) Two of the laboratory stocks contain both alleles 6(F) and 6(S) of the Est-6-locus, the other two are homozygous for the allele 6(S). All three alleles D(1), D(2), and D(O) of the Aph-D-locus are present in each of the four laboratory stocks, except that stock 7 At lacks the allele D(2). 4) In the reference line RE both Est-6-alleles and all three Aph-D-alleles are present, but the alleles 6(F) and D(2) occur at very low frequencies. The alleles 6(S) and D(1) are the most frequent alleles, the silent allele D(O) occurs at a frequency of 0.197. 5) In the selection lines the alleles 6(S) and D(1) are the most frequent alleles, too. The allele 6(F) is lacking in all selection lines. In the lines 1a and 3b the allele D(2) is missing, but this allele was found in the lines 1b and 3a at very low frequency. The silent allele D(O) is common to all selection lines, with the nearly constant values of 0.141 to 0.177. These frequencies are smaller than in the reference line. 6) All selection lines have rather similar allele frequencies and can be compared with those of the line RE (except the allele 6(F)). But these allele frequencies differ from those of the four laboratory stocks. This is attributed to a change in the linkage groups by recombination and the organisation of new adaptive gene complexes during the first generations after the foundation of the double cross. 7) Selection for differing adaptation to temperature has only a small effect on the observed loci. 8) The relatively high frequency of the silent allele D(O) found in all lines is discussed with regard to selective neutrality, partial selectivity, and a mutation-selection balance.