抄録
Frequency of the second chromosomes bearing lethal gene (or genes) in natural populations was examined in five stations, in summer and autumn seasons and in successive years in Hiroshima Prefecture. The combined frequencies of lethal and semilethal chromosomes varied with the stations, seasons or years from 2.2 per cent to 20.7 per cent. The allelic rates within a population ranged from zero to 60 per cent. These results suggest that these populations were small in size. From the results of allelic rates, the narrowest territory of a breeding unit was estimated. The frequency of lethal chromosomes was revealed to be increased by 2.53 per cent and that of semilethal ones by 0.99 per cent from summer to autumn, and about seven generations may elapse in natural populations during this period. The combined increment of lethal and semilethal chromosomes was statistically significant. The allelic rate was observed to be decreased by 2.26 per cent during the two seasons. In spite of such an accumulation of lethal genes or decrease of allelic rate in the breeding season, any remarkable changes in the frequency or rate were not observed in the same season of successive years. Therefore, the seasonal changes in lethal frequency and the allelic rate seem to be repeated from year to year with the same fluctuation.
Based on these results, the breeding mode of natural populations of D. melanogaster distributed in Hiroshima area was discussed.
