Modification of Linkage Intensity by Natural Selection

ENETIC linkage or linkage of genes is one of the common genetic features G of all organisms from viruses to man.It may be of great concern to evolutionary geneticists how genetic linkage was developed and how the intensity of linkage has been adjusted in the evolutionary process.FISHER (1930) suggested that "the presence of pail-s of factors in the same chromosome, the selective advantage of each of which reverses that of the other, will always tend to diminish recombination, and therefore, to increase the intensity of linkage in the chromosomes of that species."He also recognized another agency which would increase recombination, namely, ithe constant spread of advantageous mutations which, unless they occur so seldom that each has become predominant before the next appears, can only come together in the same gamete by means of recombination.This problem was recently discussed at length but still semi-quantitatively by BODMER and PARSONS (1 962).There is a large amount of evidence that linkage intensity is under genetic control (cf.BODMER and PARSONS 1962).In Drosophila ananassue, for example, MORIWAKI (1940) identified a dominant gene, En-2, located on the right arm of the second chromosome, which enhances recombination between almost every pair of loci on the same chromosome in both males and females, while KIKKAWA (1937) found another gene (or genes) which induces crossing over in males in the third chromosome.CLARK and MARGULIES ( 1965) and HOWARD-FLANDERS and THERIOT ( 1966) also reported several recombination-deficient mutants in Escherichia coli K-12, which are probably due to single-gene mutation.Further, the effectiveness of artificial selection in reducing or increasing recombination values was reported by DETELFSEN and ROBERTS (1921), PARSONS (1958), andMUKHERJEE (1961) in Drosophila.The purpose of this paper is to present mathematical models for the development of genetic linkage and the modification of linkage intensity by natural selection.In the present paper we shall consider only those agencies which would increase the linkage intensity.KIMURA (1956) developed a model of a genetic system which leads to closer linkage by natural selection in diploid organisms, but he was concerned mainly with a crossover-reducing mechanism such as an inversion.