On the evolution of saline tolerance in the larvae of mosquitoes in the genus Ochlerotatus.

We examined physiological and morphological parameters associated with saline tolerance in the larvae of 11 species of mosquito in the genus Ochlerotatus that were collected from the wild in a variety of sites around North America. Saline tolerance was assayed, and all of the species were osmoregulators. Six of the species examined were found to be physiologically restricted to freshwater habitats, while the other five could successfully osmoregulate in both freshwater and saline water, including seawater. All larvae that were obligate freshwater forms had only one rectal segment, while all of the euryhaline osmoregulators had two. We were interested in the evolutionary pathway by which saline tolerance arose in this lineage. DNA sequence data were obtained by polymerase chain reaction amplification and sequencing of the D2 region of the 28s rDNA gene in all of the freshwater and saline-tolerant Ochlerotatus species we studied. When the morphological and physiological characters were mapped on the resultant cladogram, they revealed a complex pattern, with freshwater and saline-water forms being adjacent and interspersed through the tree. The data also demonstrate that saline tolerance has been gained and then lost at least once in this lineage. Two possible evolutionary scenarios are presented, but the one we favor is that saline tolerance arose one time in this lineage and repeated reversions to the freshwater condition have occurred.

Fecundity in Drosophila following desiccation is dependent on nutrition and selection regime.

Life-history trade-offs of populations of Drosophila melanogaster selected for enhanced desiccation resistance (D populations) include increased pupal mortality, retarded development, increased longevity, and decreased fecundity. We tested the effects of acute desiccation on fecundity and the effects of various nutritional regimes on fecundity following a bout of desiccation in the D populations and their control populations (C populations). Female flies were desiccated for 8 h (C populations) or 24 h (D populations) and then recovered on one of four treatments: distilled water, a saline solution, a saline and sucrose solution, or food. Each treatment was tested in the presence and absence of a yeast supplement. Fecundity was then measured on an oviposition medium over a 12-h period. Fecundity was also measured in flies that did not undergo a desiccation stress. Yeast increased fecundity under all conditions examined in the C populations but not in the D populations. There was an overall effect of recovery treatment on fecundity but no effect of desiccation stress.

Osmotic regulation in adult Drosophila melanogaster during dehydration and rehydration.

We have examined the osmoregulatory capacities of laboratory populations of the insect Drosophila melanogaster by measuring hemolymph osmotic concentration during desiccation and upon recovery from a bout of desiccation. Recovery treatments entailed allowing the flies access to distilled water, a saline solution or a saline+sucrose solution after a desiccation bout shown to reduce hemolymph volume by approximately 60%. Prior to desiccation, the hemolymph osmotic concentration was 353+/-11 mOsm. We found that Drosophila display strict osmotic regulation under prolonged conditions of dehydration. Osmotic regulation continued during recovery from desiccation, regardless of the fluid provided. This result is evidence that this insect does not require an external source of osmolytes or energy to regulate its hemolymph osmotic concentration or to restore hemolymph volume, which is reduced during desiccation. We also examined populations that have been selected for over 250 generations for enhanced desiccation resistance to identify physiological characters that have evolved in response to the selection regime. The selected lines displayed a reduced pre-desiccation hemolymph osmotic concentration (315+/-7 mOsm) and a marginally improved capacity for osmoregulation.