Founder Effects and Linkage Disequifibria in Experimental Populations of Drosophila ( allozymes / polymorphisms )

Six laboratory populations of Drosophila paulistorum were examined for changes in gene frequencies at an enzyme locus, tetrazolium oxidase (To). In some of the populations, the alleles were introduced on over 100 independently derived chromosomes. These populations showed considerable stability in gene frequencies although they were at widely different starting frequencies. Other populations were begun with only a few (about 6) independently derived chromosomes. These populations showed significant and somewhat erratic changes in To gene frequencies. The difference in behavior of the two sets of populations was almost certainly caused by linkage effects due to sample size. The implication of these studies in understanding the role of the founder effect in natural populations is briefly discussed. L'H6rtier and Tessier (1) were the first geneticists to use population cages to maintain large laboratory populations of Drosophila and to follow changes in gene frequencies due to selection. Since then this technique has been used to study selection on a variety of morphological markers, chromosomal polymorphisms and, most recently, biochemical polymorphisms. An important assumption in the studies with single gene markers is that the changes observed are due to selection at the locus being observed and not due to selection at a linked gene or block of genes. One way to partially avoid this complication is to introduce the marker into the laboratory populations on as many independently derived chromosomes as possible. This decreases the probability that the marker will be in linkage disequilibrium with other genes. Ohta and Kimura (2) have shown that the variance in linkage disequilibrium is equal to 1/(n 1) where n is the number of chromosomes extracted from a population in linkage equilibrium. The study presented in this paper illustrates the variable outcomes that occur in laboratory populations that have been started with different numbers of independently derived chromosomes. This study is similar to that of Dobzhansky and Pavlovsky (3) in which they showed the effect of drift on the outcome of changes in gene arrangement frequencies. In the Discussion we mention the implications of these results for understanding the genetics of natural populations, especially in relation to the founder effect. MATERIALS AND METHODS A collection of Drosophila paulistorum, Andean semispecies, from Mirasol near Sao Jos6 do Rio Preto, Brazil, was made Abbreviation: To, tetrazolium oxidase locus. by Dr. J. Gallo on 21 March 1970 and was sent to us in New York. One hundred six isofemale lines (lines started by one inseminated female from nature) were initiated and several enzyme polymorphisms were studied (4). One sex-linked locus, tetrazolium oxidase (To), was of particular interest because a substantial excess of heterozygotes over Hardy-Weinberg expectations was noted (5). The method used to study the allozymes of To is given in Richmond and Powell (5) and Richmond (4), and the biochemical nature of the protein coded by To has been studied by Brewer (6). Two electrophoretically detectable alleles exist at this locus in the Mirasol population; these are designated F (fast) and S (slow) to signify their relative electrophoretic mobilities in our procedure. Fis from the 106 isofemale lines, representing at least 318 X-chromosomes from nature, were combined into a composite population. Two isofemale lines homocygous for the alternative To alleles were maintained separately. After about 14 generations, single females were removed from the composite population, allowed to lay eggs, and subjected to electrophoresis. Thirty-six of these reisolated strains were started from mothers homozygous for the S allele; these strains were combined and used to start a population designated MS. Sixty of the reisolated strains were from F/F mothers; these were combined and used to start a population designated MF. Thus, these two populations, MF and MS, were started with more than 100 X-chromosomes each, from a population that had been started with over 300 chromosomes from nature. These populations were maintained at 250. Four other populations were begun with the twohomozygous isofemale lines from the original collection. Two populations with 80% F allele were started and maintained at 250 and 180; these are designated IF-25 and IF-18, respectively. Two other populations were begun with 80% of the S allele and maintained at 250 and 180; these are designated IS-25 and IS-18, respectively. Thus, these populations were begun with six X-chromosomes from nature, or possibly a few more if, as rarely occurs, the female was doubly inseminated. These populations were maintained on a medium of corn meal-molasses-agar in cages that allow an adult population size of well over 1000 flies. Periodically, samples of adult flies were taken from the cages and the frequencies of alleles at To were determined.