Proton-driven sodium secretion in a saline water animal

Aquatic animals residing in saline habitats either allow extracellular sodium concentration to conform to environmental values or regulate sodium to lower levels. The latter strategy requires an energy-driven process to move sodium against a large concentration gradient to eliminate excess sodium that diffuses into the animal. Previous studies of invertebrate and vertebrate species indicate a sodium pump, Na+/K+ ATPase, powers sodium secretion. We provide the first functional evidence of a saline-water animal, Aedes taeniorhynchus mosquito larva, utilizing a proton pump to power this process. Vacuolar-type H+ ATPase (VHA) protein is highly expressed on the apical membrane of the posterior rectal cells, and in situ sodium flux across this epithelium increases significantly in larvae held in higher salinity and is sensitive to Bafilomycin A1, an inhibitor of VHA. We also report the first evidence of splice variants of the sodium/proton exchanger, NHE3, with both high and low molecular weight variants highly expressed on the apical membrane of the posterior rectal cells. Evidence of NHE3 function was indicated with in situ sodium transport significantly inhibited by a NHE3 antagonist, S3226. We propose that the outward proton pumping by VHA establishes a favourable electromotive gradient to drive sodium secretion via NHE3 thus producing a hyperosmotic, sodium-rich urine. This H+- driven Na+ secretion process is the primary mechanism of ion regulation in salt-tolerant culicine mosquito species and was first investigated over 80 years ago.

Immature Insect Assemblages from the Early Cretaceous (Purbeck/Wealden) of Southern England

The record of immature insects from the non-marine Purbeck and Wealden groups (Lower Cretaceous) of southern England is reviewed and expanded. Fossils of adult terrestrial insects are locally common, but terrestrial immature remains are restricted to transported hemipterans, most of which are sessile nymphs or puparia resembling those of extant whiteflies (Aleyrodidae). Remains of immature aquatic insects are more diverse and comprise the extant orders Plecoptera, Ephemeroptera, Odonata, Trichoptera, Hemiptera and Diptera. The Trichoptera are represented by larval cases constructed from a variety of materials corresponding to several ichnogenera. The Wealden immature insects were preserved in predominantly freshwater fluvial settings, whereas the Purbeck ones occur in lagoonal palaeoenvironments, ranging in salinity from brackish to hypersaline. The composition of aquatic immature insect faunas in the latter offers potential for palaeosalinity analysis, although there are complicating factors relating to habitat stability. Uncommon trace fossils such as beetle borings in wood provide evidence of immature insects not represented by body fossils.

Dose and developmental responses of Anopheles merus larvae to salinity

Saltwater tolerance is a trait that carries both ecological and epidemiological significance for Anopheles mosquitoes that transmit human malaria, as it plays a key role in determining their habitat use and ecological distribution, and thus their local contribution to malaria transmission. Here, we lay the groundwork for genetic dissection of this trait by quantifying saltwater tolerance in three closely related cryptic species and malaria vectors from the Afrotropical Anopheles gambiae complex that are known to differ starkly in their tolerance to salinity: the obligate freshwater species A. gambiae and A. coluzzii, and the saltwater-tolerant species A. merus. We performed detailed comparisons of survivorship under varying salinities, using multiple strains of A. gambiae, A. coluzzii and A. merus, as well as F1 progeny from reciprocal crosses of A. merus and A. coluzzii. Additionally, using immunohistochemistry, we compared the location of three ion regulatory proteins (Na(+)/K(+)-ATPase, carbonic anhydrase and Na(+)/H(+)-antiporter) in the recta of A. coluzzii and A. merus reared in freshwater or saline water. As expected, we found that A. merus survives exposure to high salinities better than A. gambiae and A. coluzzii. Further, we found that exposure to a salinity level of 15.85 g NaCl l(-1) is a discriminating dose that kills all A. gambiae, A. coluzzii and A. coluzzii-A. merus F1 larvae, but does not negatively impact the survival of A. merus. Importantly, phenotypic expression of saltwater tolerance by A. merus is highly dependent upon the developmental time of exposure, and based on immunohistochemistry, salt tolerance appears to involve a major shift in Na(+)/K+-ATPase localization in the rectum, as observed previously for the distantly related saline-tolerant species A. albimanus.

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.

Cationic pathway of pH regulation in larvae of Anopheles gambiae

Anopheles gambiae larvae (Diptera: Culicidae) live in freshwater with low Na(+) concentrations yet they use Na(+) for alkalinization of the alimentary canal, for electrophoretic amino acid uptake and for nerve function. The metabolic pathway by which larvae accomplish these functions has anionic and cationic components that interact and allow the larva to conserve Na(+) while excreting H(+) and HCO(3)(-). The anionic pathway consists of a metabolic CO(2) diffusion process, carbonic anhydrase and Cl(-)/HCO(3)(-) exchangers; it provides weak HCO(3)(-) and weaker CO(3)(2-) anions to the lumen. The cationic pathway consists of H(+) V-ATPases and Na(+)/H(+) antiporters (NHAs), Na(+)/K(+) P-ATPases and Na(+)/H(+) exchangers (NHEs) along with several (Na(+) or K(+)):amino acid(+/-) symporters, a.k.a. nutrient amino acid transporters (NATs). This paper considers the cationic pathway, which provides the strong Na(+) or K(+) cations that alkalinize the lumen in anterior midgut then removes them and restores a lower pH in posterior midgut. A key member of the cationic pathway is a Na(+)/H(+) antiporter, which was cloned recently from Anopheles gambiae larvae, localized strategically in plasma membranes of the alimentary canal and named AgNHA1 based upon its phylogeny. A phylogenetic comparison of all cloned NHAs and NHEs revealed that AgNHA1 is the first metazoan NHA to be cloned and localized and that it is in the same clade as electrophoretic prokaryotic NHAs that are driven by the electrogenic H(+) F-ATPase. Like prokaryotic NHAs, AgNHA1 is thought to be electrophoretic and to be driven by the electrogenic H(+) V-ATPase. Both AgNHA1 and alkalophilic bacterial NHAs face highly alkaline environments; to alkalinize the larva mosquito midgut lumen, AgNHA1, like the bacterial NHAs, would have to move nH(+) inwardly and Na(+) outwardly. Perhaps the alkaline environment that led to the evolution of electrophoretic prokaryotic NHAs also led to the evolution of an electrophoretic AgNHA1 in mosquito larvae. In support of this hypothesis, antibodies to both AgNHA1 and H(+) V-ATPase label the same membranes in An. gambiae larvae. The localization of H(+) V-ATPase together with (Na(+) or K(+)):amino acid(+/-) symporter, AgNAT8, on the same apical membrane in posterior midgut cells constitutes the functional equivalent of an NHE that lowers the pH in the posterior midgut lumen. All NATs characterized to date are Na(+) or K(+) symporters so the deduction is likely to have wide application. The deduced colocalization of H(+) V-ATPase, AgNHA1 and AgNAT8, on this membrane forms a pathway for local cycling of H(+) and Na(+) in posterior midgut. The local H(+) cycle would prevent unchecked acidification of the lumen while the local Na(+) cycle would regulate pH and support Na(+):amino acid(+/-) symport. Meanwhile, a long-range Na(+) cycle first transfers Na(+) from the blood to gastric caeca and anterior midgut lumen where it initiates alkalinization and then returns Na(+) from the rectal lumen to the blood, where it prevents loss of Na(+) during H(+) and HCO(3)(-) excretion. The localization of H(+) V-ATPase and Na(+)/K(+)-ATPase in An. gambiae larvae parallels that reported for Aedes aegypti larvae. The deduced colocalization of the two ATPases along with NHA and NAT in the alimentary canal constitutes a cationic pathway for Na(+)-conserving midgut alkalinization and de-alkalinization which has never been reported before.

Shared salinity tolerance invalidates a test for the malaria vector Anopheles farauti s.s. on Guadalcanal, Solomon Islands [corrected]

Among the Punctulatus Group of Anopheles mosquitoes (Diptera: Culicidae), first-instar larvae of the medically unimportant freshwater Anopheles farauti species No. 7 survives a seawater tolerance test (STT) that was previously thought to be diagnostic for the saltwater-tolerant malaria vector species, An. farauti Laveran s.s. Salt tolerance in these two closely related isomorphic species appears to be a shared derived character within the Farauti Complex. Failure to differentiate An. farauti s.s. from An. farauti No.7 will overestimate potential malaria vector numbers and waste limited larval control resources. Use of the STT should therefore be discontinued on Guadalcanal and other techniques such as allozyme electrophoresis used instead [corrected].

Shared salinity tolerance invalidates a test for the malaria vector Anopheles farauti s.s. on Guadalcanal, Solomon Islands

Among the Punctulatus Group of Anopheles mosquitoes (Diptera: Culicidae), first-instar larvae of the medically unimportant freshwater Anopheles farauti species No. 7 survives a seawater tolerance test (SST) that was previously thought to be diagnostic for the saltwater-tolerant malaria vector species, An. farauti Laveran s.s. Salt tolerance in these two closely related isomorphic species appears to be a shared derived character within the Farauti Complex. Failure to differentiate An. farauti s.s. from An. farauti No. 7 will overestimate potential malaria vector numbers and waste limited larval control resources. Use of the SST should therefore be discontinued on Guadalcanal and other techniques such as allozyme electrophoresis used instead.