Genetic change of recobination value in Drosophila melanogaster. II. Simulated natural selection

Selection of Gl-Sb coupling heterozygotes was carried out for more than one hundred generations commencing with six independent lines drawn from a common base population. Population sizes were eight, sixteen and forty-eight parents per generation. The effect of natural selection on recombination value was measured by sampling and testing females at varying intervals of time. There was a significant reduction in percentage recombination between Gl and Sb from fifteen to a level between five and ten in four out of six of the original lines. In most cases this reduction occurred rather rapidly after the initiation of the experiment. In the remaining two lines there was no significant decrease in recombination value; there was, however, a significant increase in at least one subline of this group. The rapid rate of change of recombination value is most readily explained by the presence of a recombination modifying gene which is linked to the modified region. Genetic random drift was again shown to have an important effect on changes in recombination value in small populations. High recombination was almost completely recessive to low recombination in the one case examined. Lethal genes were fixed in sheltered regions of unmarked third chromosomes in five lines or sublines. These results are discussed in relation to the mode of development of permanent heterozygosity in some species of plants.