Abstract
A total of 1, 822 second chromosomes were isolated from eight samples of a natural population in Ichinomiya, Yamanashi prefecture over a two-year period beginning in October, 1964, and the preadult viability of these chromosomes in homozygous condition was measured. No significant seasonal differences in the frequency distribution of chromosomes with various viabilities were detected. The average frequency of lethal chromosomes was about 15%. There were also no significant seasonal changes in the lethal allelic rate. Sterile males increased in frequency toward winter, while the proportion of fertile, inseminated females remained constant at about 87%. Yearly changes in the frequency of lethal allelism and a visible mutant (bw) were observed. The relation of these various observations to proposed mechanisms for the maintenance of genetic loads in natural populations of D. melanogaster was discussed.
