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 (NH 4 Cl), and five populations selected for over 60 generations for larval resistance to urea, were investigated to determine the genetic mechanisms through which such tolerance had evolved. To examine the genetics of tolerance to urea and ammonia, egg-to-adult survivorship and developmental time were measured at two different NH 4 Cl levels and two different urea levels for each selection regime relative to the control lines, and among reciprocal crosses between each selection regime and the control lines. To examine tolerance to novel nitrogenous compounds (cross-tolerance), egg-to-adult survivorship and developmental time were measured at two different NH 4 Cl levels and two different urea levels for reciprocal crosses between the selection lines. Dominance is a major genetic factor in egg-to-adult survivorship in the presence of either urea or ammonia, while cross-tolerance to novel nitrogenous compounds also shows dominance as a major genetic mechanism controlling egg-to-adult survivorship. Dominance and X-linkage appear to be factors affecting developmental time in the presence of either urea or ammonia, although we could not exclude cytoplasmic inheritance as influencing our results. Cross-tolerance to novel nitrogenous compounds shows dominance and X-linkage as the main genetic factors controlling developmental time. We develop a simple hypothesis, in accordance with the results, that there may be two X-linked loci: one controlling urea tolerance and one controlling ammonia tolerance, and one autosomal locus exerting a pleiotropic control of tolerance. However, many other possibilities exist.