The aim of our work was to compare the mechanisms of resistance to aminopterin, inhibitor of the dihydrofolate reductase enzyme, between different Drosophila species and those described for cultured cells. Moreover we compared the systematic species divisions based on morphological traits and those based on a molecular approach. For this purpose, the effect of aminopterin on viability and wing phenotype was studied in different Drosophila species. Dihydrofolate reductase was measured in adult flies. We found an important dihydrofolate reductase activity in the melanogaster sub-group compared to the other species studies. Wing effect was observed only in this sub-group. The effects of aminopterin on the wing phenotype were very similar to the phenotype of rudimentary mutants. Both deplete the pyrimidine pool and it has been shown by the studies of the structural genes of the nucleotide pyrimidine pathway that the wing tissue is very sensitive to every pertubation of this metabolism.
The D. ananassae species was found to be fully resistant at the concentrations of the inhibitor tested. No or very little dihydrofolate reductase activity was detected. The binding of the enzyme to the inhibitor was comparable to that found in the Oregon strain of D. melanogaster. The purine and pyrimidine salvage pathways were investigated and the D. ananassae species displayed an important thymidine kinase activity. The D. ananassae flies were sensitive on Sang medium compared to the Oregon flies but were able to use exogenous bases or nucleosides more efficiently. Therefore the mechanism of resistance to aminopterin in Drosophila may be different from those described for methotrexate in mammalian cultured cells, as indicated by the results obtained for D. ananassae.