Genetics of larval urea and ammonia tolerance and cross-tolerance in Drosophila melanogaster

Five laboratory populations of Drosophila melanogaster previously selected for over 60 generations for larval resistance to ammonium chloride (NH4Cl), and five populations selected for over 60 generations for larval resistanc e to urea, were investigated to determine the genetic mechanisms through wh ich such tolerance had evolved. To examine the genetics of tolerance to ure a and ammonia, egg-to-adult survivorship and developmental time were measur ed at two different NH4Cl levels and two different urea levels for each sel ection regime relative to the control lines, and among reciprocal crosses b etween each selection regime and the control lines. To examine tolerance to novel nitrogenous compounds (cross-tolerance), egg-to-adult survivorship a nd developmental time were measured at two different NH4Cl levels and two d ifferent urea levels for reciprocal crosses between the selection lines. Do minance is a major genetic factor in egg-to-adult survivorship in the prese nce of either urea or ammonia, while cross-tolerance to novel nitrogenous c ompounds also shows dominance as a major genetic mechanism controlling egg- to-adult survivorship. Dominance and X-linkage appear to be factors affecti ng developmental time in the presence of either urea or ammonia, although w e could not exclude cytoplasmic inheritance as influencing our results. Cro ss-tolerance to novel nitrogenous compounds shows dominance and X-linkage a s the main genetic factors controlling developmental time. We develop a sim ple hypothesis, in accordance with the results. that there may be two X-lin ked loci: one controlling urea tolerance and one controlling ammonia tolera nce, and one autosomal locus exerting a pleiotropic control of tolerance. H owever, many other possibilities exist.