COMPARISON OF LETHAL MUTATIONS OF DROSOPHILA-MELANOGASTER WITH DROSOPHILA-SIMULANS WHEN DETECTED BY THE ATTACHED-X METHOD

The purpose of this study was to evaluate the attached-X method compared with the standard Basc method, and, using this method, to find out whether the observed differences in genetic polymorphisms are related to differences in lethal mutation rates in D. melanogaster and D. simulans. When EMS-treated Drosophila melanogaster males are mated to untreated attached-X females, a decrease in the progeny sex ratio (male/female + male) is observed due to the induced lethal mutations on the X chromosome. The decrease in the frequency of male progeny were shown as the attached-X index. The expected male number is calculated from the control sex ratio. The difference between the expected and the observed male numbers, expressed as the ratio to the expected male number, defines the attached-X index. The index values for various EMS concentrations were compared to the lethal frequencies obtained by the standard Basc method for the same EMS treatments, and gave a highly positive correlation (gamma = 0.993, p < 0.01, d.f. = 2), thus providing an alternative method for evaluation of possible mutagens. The attached-X method was applied to D. simulans, of which natural populations are known to have relatively low genetic variation, and frequencies of the EMS-induced X chromosome lethal mutations were estimated and compared with those in D. melanogaster. The results indicate that D. melanogaster is slightly more sensitive in the sperm and spermatogonial stages, but less susceptible in the spermatid stage when compared with D. simulans. Since the spermatid stage occupies a relatively short period in spermatogenesis, a higher mutability of D. simulans during this stage probably does not make a significant contribution to the genetic variability of this species.