Patrick, M.L.*; Bradley, T.J.: Salt-Lovin’ Skeeters: The Osmoconforming Strategy of Mosquito Larvae

Culex is a genus of mosquito that possesses both freshwater restricted (e.g. C. quinquefasciatus) and euryhaline larvae (e.g. C. tarsalis), the latter being able to tolerate up to 70% full strength seawater. While held in freshwater conditions, both the euryhaline and the freshwater-restricted species are hyperosmotic regulators and maintain similar hemolymph ion concentrations. However, when environmental osmolality is raised using salts, C. quinquefasciatus cannot survive whereas C. tarsalis employs the osmoconforming strategy, in which hemolymph osmolality conforms to the environment thereby eliminating the gradient for water loss. Following an acute increase in environmental salinity, the larva of C. tarsalis, regulate body volume by increasing their drinking rate of the external medium. Increases in hemolymph Na and Cl concentrations are attenuated by the differential stimulation of Na efflux and reduction of Cl influx. The rise in hemolymph osmolality during acclimation to higher salinity is attributed to the accumulation of two organic osmolytes: the amino acid proline and the disaccharide trehalose. Hemolymph proline levels increase almost 50 fold and trehalose accumulates two fold from freshwater values. Additionally, proline is accumulated intracellularly to levels equivalent to that found in the extracellular compartment (hemolymph), the first such case reported in salt-tolerant animals. By manipulating environmental osmolality using salts, compatible solutes (sorbitol), or a combination thereof, we determined that proline accumulation is cued by changes in hemolymph NaCl whereas trehalose follows environmental osmolality. The source for the proline accumulation during the osmoconforming response is not from hemolymph proteins but rather the stimulation of proline synthesis from glutamate. Funded by NSF grant IBN 9723404.