Frequency Distribution of Lethal Chromosomes in Small Populations of DROSOPHILA MELANOGASTER.

TUDYING the frequencies and allelic rates of lethal chromosomes in natural populations of Drosophila pseudoobscura, DOBZHANSKY and WRIGHT ( 1941 ) and WRIGHT, DOBZHANSKY and HOVANITZ (1942) showed that the frequency of lethals is much less than would be expected of completely recessive lethals under random mating and this lower frequency is due to either partial inbreeding within populations or selection against lethal heterozygotes. The same conclusion has been reached by CROW and TEMIN (1964) in a survey of Drosophila data obtained by various authors. In these studies the effective sizes of populations and the migration rates between populations were unknown parameters, so that they could not separate the effects of inbreeding from the effects of selection against heterozygotes. Thus, if these parameters are artificially determined in experimental populations, selection against lethal heterozygotes, which is one of the most controversial problems jn population genetics at present, would be studied more accurately, as pointed out by NEI (1 968). In recent years it has been shown with several organisms that the effective breeding size of natural populations is probably fairly small, though there is generally more or less migration between populations (e.g., KERSTER 1964; TINKLE 1965; NEI and IMAIZUMI 1966a; MERRELL 1968). In Drosophila meZanogaster, WALLACE (1966) obtained a high rate of allelism of lethal genes when flies were collected from small areas. This strongly suggests that the effective size in this species is also quite small. Thus, it is desirable to know the population dynamics of lethal genes in small populations. The theoretical distribution of lethal gene frequency in finite equilibrium populations was first obtained by WRIGHT (1937). Recently, extending WRIGHT'S theory, NEI (1968) studied the frequency distribution of lethal chromosomes rather than lethal gems. This study is important, because in most cases the frequencies of individual lethal genes cannot be observed but the frequency of lethalbearing chromosomes is determined. So far, however, no experimental studies have been, conducted on the frequency distribution of lethal genes or lethalbearing chromosomes. The present study is intended to determine experimentally the frequency distribution of lethal second chromosomes in small populations of Drosophila melanogaster and to investigate the type and degree of selection against lethal heterozygotes. The relation between the mean gene frequency of lethals and population size will also be examined.