COMPARISON OF LETHAL + SEMILETHAL FREQUENCIES IN SECOND AND THIRD CHROMOSOMES FROM A NATURAL POPULATION OFDROSOPHILA MELANOGASTER

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Abstract
Chromosomes were analyzed by two-class progeny tests. Matings in the test generation were CyXCy for the second and TM 1 X TM 1 for the third chromosomes, respectively. Chromosomes producing less than 16.7% +/+ progeny when homozygous were classified as lethals or semilethals. Collections were made from the natural populations in September and October, 1958 and July, 1959 The results indicate that the major autosomes in the South Amherst D. melanogaster population appear to carry similar amounts of recessive drastic genetic variability. Corresponding changes in the magnitude of this variability were also observed in both autosomes in collections during the Fall of 1958 and Summer of 1959 However, lethal frequency (0% +/+) in the two autosomes remained similar, approximately 20%. The low incidence of allelism among lethals in the respective autosomes in combined Fall collections and a similar low incidence of allelism among lethals between the Fall and following Summer suggest that many different lethals may be present in the large Fall population. Survival in the overwintering period may be random or perhaps is dependent on the genetic background in which a lethal is placed. The results also indicate that population size is now larger than during the previous 1947-1952 period of investigations.