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HIS study was carried out in an attempt to assess the importance of epistasis in the deleterious effects of homozygosity. The experimental procedure utilized mating systems, which have now become standard, whereby chromosomes from natural populations of Drosophila melanogaster are extracted and made homozygous. Interaction between chromosomes was measured by comparison of the viability of flies simultaneously homozygous for the two major autosomes with flies homozygous for one or the other. At the same time, intrachromosome interactions were studied by comparison of three levels of inbreeding in a particular chromosome: 100% homozygosity due to identity by descent ( F = 1 ) ; 50% reduction in heterozygosity ( F = .5) ; and random combinations ( F = 0).The interaction of homozygous lethals and severe detrimentals is of minor consequence to the population because these mutants are of such low individual frequency that simultaneous homozygosity for two or more is a very unusual event in non-inbred populations. On the other hand, mildly deleterious genes are common enough as a group that their homozygous interactions may possibly have appreciable population consequences. Furthermore, their homozygous effects could be of the same general magnitude as their heterozygous effects and those of more drastic chromosomes, and it is as heterozygotes that most mutants probably have their major effect on the population. For these reasons, we have concentrated mainly on the interactions of mildly deleterious chromosomes.The results to be presented here show no statistically significant systematic interaction between mildly deleterious homozygotes on the two major autosomes. Within a chromosome there is a barely significant synergistic interaction. Our results will be compared with those from a similar experiment by SPASSKY, DOBZHANSKY, and . EXPERIMENTAL METHODSNatural populations of Drosophila melanogaster at two locations in Madison, Wisconsin provided the material for the study. Flies for Experiments I, 111, and IV were collected during the early fall seasons of 1962 I1 were collected in August and September of 1963 at another location approximately a mile distant. Males and females from the populations were mated in single pairs to generate wild-type * Paper number 1198 from the Laboratory of Genetics T h s work was aided by grants from the National Institutes of Health (GM 15422 and GM07666).

The major components of the SD system have been examined in two natural populations of D. melanogaster to investigate how SD behaves and is maintained in nature and to estimate its impact and efficiency. A twofold approach was used: (1) direct measurements of segregation distortion in wild males and (2) measurement of sensitivity of wild SD  + chromosomes to SD action. Characterization of newly isolated SD chromosomes and of a large number of SD  + chromosomes from nature demonstrated that (1) SD can operate efficiently in the wild genome: 45% of SD/SD  + males collected from nature had k values larger than 0.70. (2) Forty-three of 44 newly recovered SD chromosomes are of the SD-72 type, having a small pericentric inversion that maintains tight linkage among the Sd, E(SD) and Rsp loci in the SD complex. In 1956, most SD chromosomes in Madison lacked this inversion. (3) Only 12 of the 44 SD chromosomes carried a recessive lethal (compared with five of six in 1956), and many of the viable SD chromosomes were fertile as homozygotes, indicating that SD homozygotes need not have obvious reductions in fitness. (4) Among more than 500 wild chromosomes assayed for response to distortion by a strong SD, at least 40-50% were sensitive, about 33% were partially sensitive and 17% were insensitive. This frequency of sensitives is higher than in reports from some other populations. An estimated 12% of the wild chromosomes were classified as true Rspi by their constellation of effects, including a special test of ability to cause self-distortion of a "suicide" chromosome, R(cn)-10. In a direct assay with R(cn)-10, an independent sample of 99 chromosomes from nature gave 30% putative Rspi. Thus, these populations contain in the range of 12-30% Rspi. (5) Chromosomes supersensitive to SD, previously described for certain laboratory stocks, were also found to coexist in nature with SD. (6) Profiles of wild chromosomes with a panel of three or four different SD testers suggest a series of allelic alternatives at the Rsp locus including supersensitive, sensitive, semisensitive and insensitive, and that loci other than Rsp may also be important in determining the effect of SD in nature.