Osmoregulation in Drosophila melanogaster selected for urea tolerance.

Valerie A. Pierce,Allen G. Gibbs,Laurence D. Mueller

doi:10.1242/jeb.202.17.2349

Valerie A. Pierce, Allen G. Gibbs + Show 1 more

Open Access

https://doi.org/10.1242/jeb.202.17.2349

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Abstract

Animals may adapt to hyperosmolar environments by either osmoregulating or osmoconforming. Osmoconforming animals generally accumulate organic osmolytes including sugars, amino acids or, in a few cases, urea. In the latter case, they also accumulate 'urea-counteracting' solutes to mitigate the toxic effects of urea. We examined the osmoregulatory adaptation of Drosophila melanogaster larvae selected to live in 300 mmol l(-)(1) urea. Larvae are strong osmoregulators in environments with high NaCl or sucrose levels, but have increased hemolymph osmolarity on urea food. The increase in osmolarity on urea food is smaller in the selected larvae relative to unselected control larvae, and their respective hemolymph urea concentrations can account for the observed increases in total osmolarity. No other hemolymph components appear to act as urea-counteractants. Urea is calculated to be in equilibrium across body compartments in both selected and control larvae, indicating 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 mechanisms (e.g. metabolism, decreased uptake or increased excretion) that reduce overall urea levels and the consequent toxicity.

Highlights

Hyperosmotic environments create osmotic pressure, favoring the movement of water out of the animal
We examined whether any of the normal components of D. melanogaster hemolymph demonstrated urea-counteraction by assessing whether their concentrations changed in the presence of urea and whether the magnitude of the change differed between control and selected populations
In 1992, five outbred baseline (B) populations of Drosophila melanogaster, derived from an ancestral population (Ives), were each split into two populations, one of which was exposed to urea during the larval period (‘selected’, MX), while the other was fed standard culture food (‘control’, UU; Joshi et al, 1996)

Summary

Introduction

Hyperosmotic environments create osmotic pressure, favoring the movement of water out of the animal. Sea water is the best known hyperosmotic environment, containing high levels of sodium chloride, but other environments can be hyperosmotic. Some lakes and ponds contain high levels of other inorganic salts. Microbes may encounter high osmolarities in the tissues of their host, and plant nectaries can have significantly higher osmolarities than the hemolymph of the insects that live in them (Nicolson, 1994, 1998). Some animals osmoregulate, maintaining relatively constant cellular and blood (or hemolymph) concentrations, even against large osmotic gradients. Hyporegulating animals, including marine teleosts, brine shrimp Artemia salina and saline-water mosquitoes, compensate for the osmotic loss of water by drinking the external medium. Excess salts are excreted through the gills in marine organisms or secreted into the urine in the insect rectum (Bradley, 1987; Holliday et al, 1990; Kirschner, 1993)

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