Abstract
Two hundred and eighty second chromosomes were extracted in the native cytoplasm from the Akayu population, one of the northern populations of D. melanogaster in Japan, and various population-genetic parameters such as effective size of the population (Ne≅5000), frequency of lethal-carrying second chromosomes (Q=0.23), allelism rate of lethol second chromosomes (Ic=0.0137), homozygous detrimental and lethal loads (D=0.235; L=0.263), average degree of dominance of mildly deleterious genes (hE≅O.27), and components of genetic variance for viability-additive (σA2) and dominance (σD2)- (σA2=0.0023; σD2=0.0009) were estimated. The analysis of these data strongly indicates that, in this population, mutation-selection balance is the mechanism for the maintenance of genetic variability of viability. Furthermore, these estimates were compared with those of the Aomori population, whose second chromosomes were extracted in the M type cytoplasm. Since the estimates of the above parameters of both populations were very similar to each other, it was concluded that mutations were not induced due to the P-M type hybrid dysgenesis in the process of extracting the chromosomes. Thus, both the Aomori and Akayu populations can be considered to be a typical northern population in Japan where genetic variabilities of viability are maintained by the balance between mutation and selection pressures.
