Dynamics of chromosomal polymorphism and genetic load: an application of the two-locus multiplicative model with heterosis.

POPULATIONS of Drosophila pseudoobscura that are polymorphic for third chromosome gene arrangements and which carry an abundance of deleterious genes in heterozygous state provide an excellent opportunity to study the relationship of chromosomal polymorphism and genetic load. Utilizing such materials, EPLING and co-workers have proposed that different gene arrangements contain essentially different genetic complexes which tmd to be maintained throughout the distributional area of the species. Thus, they consider that the genetic load associated with a specific gene arrangement is simply one aspect of its inherent genetic composition (EPLING, TINDERHOLT and MATTONI 1961 ; MAYHEW et al. 1966). These suggestions were based largely on studies of TL (Tree Line) gene arrangements from widely different locations which were found to exhibit relatively high proportions of lethal plus semilethal genes and similar lethal mutation rates. Although there is little doubt that the genetic makeup of the same gene arrangement varies, depending on its geographical source (DOBZHANSKY 1950) , recent findings do suggest that each gene arrangement has retained some of its primordial genetic complex (PRAKASH and LEWONTIN 1968). DOBZHANSKY and co-workers have offered a different hypothesis to explain variation in genetic loads among different gene arrangements (DOBZHANSKY et al. 1963). They suggested that the genetic load exhibited by a particular gene arrangement depends on its frequency in the population and that rare gene arrangements will tend to carry more lethal genes than common ones. Their suggestion appears to be based on the proposition that two or more heterotic systems (in this case heterotic systems of karyotypes and of alleles) will impose a smaller segregation load on a population if they are linked in such a way that there is joint elimination of the less desirable karyotypes (i.e., the rarer ones) with the deleterious alleles. However, a consideration of the stable equilibrium populations associated with their models indicates that emphasis should be placed on the fact that association of rare gene arrangements and lethal alleles produces an optimum frequency of certain highly fit combinations of karyotypes and