The genetic variability of third chromosomes in a local population of Drosophila melanogaster.

Five hundred and two third chromosomes were extracted from a large cage population of Drosophila melanogaster initiated two months after collection of the progenitors near Raleigh, North Carolina in 1970.---Salivary gland chromosomes of 489 chromosome lines were examined and 54 chromosomes were found to carry inversions. The inversions were classified into three polymorphic types [In (3L)P, In (3R)P, and In (3R)C] and two unique types. The polymorphic inversions were found in frequencies of 0.012, 0.88, and 0.010, respectively.--Viabilities of homozygotes and heterozygotes were examined. Chromosomes with lethals occurred with a frequency of 0.495: 0.537 in the group of inversion-carrying chromosomes and 0.490 in the group of inversion-free chromosomes. The average homozygote viability computed on the basis of an average heterozygote viability of 1.0000 was 0.3235 if lethal lines were included and 0.6290 if they were excluded. The detrimental load to lethal load ratio (D:L ratio) was 0.70 (=0.4636-0.6650). The average viability of lethal heterozygotes was significantly larger than that of lethal-free heterozygotes. It appears, however, that lethal genes in heterozygotes have deleterious effects on fitness as a whole.--The average degree of dominance for viability polygenes was estimated to be about 0.3-0.4 in lethal-free individuals and nearly zero in lethal heterozygotes. Overdominance or some form of balancing selection was suggested at some loci. The difference between the values obtained for average degree of dominance due to genetic backgrounds and superior vibaility of lethal heterozygotes (but not fitness as a whole) suggest that some epistasis or coadaptation occurs.--The results described above are similar to those obtained for the second chromosomes.