Osmoregulation in Drosophila melanogaster selected for urea tolerance

Animals may adapt to hyperosmolar environments by either osmoregulating or osmoconforming. Osmoconforming animals generally accumulate organic osmolyt es including sugars, amino acids or, in a few cases, urea, In the latter ca se, they also accumulate 'urea-counteracting' solutes to mitigate the toxic effects of urea, We examined the osmoregulatory adaptation of Drosophila m elanogaster larvae selected to live in 300 mmol l(-1) urea, Larvae are stro ng osmoregulators in environments with high NaCl or sucrose levels, but hav e increased hemolymph osmolarity on urea food. The increase in osmolarity o n urea food is smaller in the selected larvae relative to unselected contro l larvae, and their respective hemolymph urea concentrations can account fo r the observed increases in total osmolarity, No other hemolymph components appear to act as urea-counteractants. Urea is calculated to be in equilibr ium across body compartments in both selected and control larvae, indicatin g that the selected larvae are not sequestering it to lower their hemolymph osmolarity. The major physiological adaptation to urea does not appear to involve increased tolerance or improved osmoregulation per se, but rather m echanisms (e.g. metabolism, decreased uptake or increased excretion) that r educe overall urea levels and the consequent toxicity.