Body water content over a range of ambient salinities in the sheepshead minnow

Body water content was determined on groups of Cyprinodon variegatus after sequential acclimation to ambient salinities in the range from fresh water to 100%. A multiple linear regression model applied to the data included body mass ( M ) of individuals and acclimation salinity ( S ) as independent variables, and body water content ( W ) as the dependent variable. The model that described the relationship was W (g) = 0·059 + 0·723 M (g)–0001 S (%). Converting values of body water content to percent of wet body mass produced a range of values from 74·5% in fresh water to 72·0% at 100%, only a 2·5% difference in body water content over this wide range of ambient salinities.     

Feeding and reproductive biology of eleotrid fishes in a tropical estuary

Five species of eleotrid fishes ( Gobiomorus dormitor, Dormitator maculatus, Eleotris amblyopsis, Eleotris pisonis and Leptophilypnus fluviatilis ) are known from the Tortuguero estuary of Costa Rica. One, E. amblyopsis , appears tightly linked to the presence of mats of macrophytes and two others, E. pisonis and D. maculatus , are largely limited to this habitat in the lower estuary. All three of these species feed on hyacinth roots. Despite certain food and habitat specializations among the forms, there is evidence of competition and predation among four species. Individuals of breeding size have never been found in the system for D. maculatus and L. fluviatilis . Alternative means of establishing and maintaining eleotrid populations are suggested.     

Environmental influences on regulation of blood plasma/serum components in teleost fishes: a review

Concentrations of both inorganic and organic blood plasma/serum components of teleost fishes were reviewed in seven habitat/life-history categories. These were: freshwater; inland saline; estuarine and nearshore marine; pelagic and deep-sea; diadromous; southern cold-water; and northern cold-water. Plasma/serum osmolalities were compared among groups acclimated to/living in fresh and in salt water. Contributions of inorganic ions and colligative and non-colligative organic molecules were evaluated including with respect to melting and freezing points, and “antifreeze activity” of plasma/serum in species from cold marine waters. Possible roles of TMAO in deep-water fishes were reviewed. Discussion also included influences of ambient salinity and temperature on concentrations of plasma/serum components. Seasonal cycles of blood plasma/serum components were discussed, along with antifreeze concentrations in other body fluids and tissues of cold-water fishes. Regulatory patterns of plasma/serum osmolalities were compared among the most euryhaline of teleosts evaluated here. Highest mean values of plasma/serum osmolalities in sea water were seen in southern cold-water and in pelagic and deep-sea fishes. The southern cold-water group also had the lowest plasma/serum freezing points among these groups. Comparisons of mean plasma/serum Na⁺ and Cl⁻ concentrations among fishes from fresh waters did not differ significantly among groups, but species from cold marine waters showed higher levels than did other groups in marine waters. Plasma/serum osmotic, Na⁺ and Cl⁻ concentrations of these seven groups of teleosts were compared with those of other fish-like vertebrate groups. Possible impacts of global warming on regulatory responses of plasma/serum components were discussed.     

Dynamic subcellular localization of isoforms of the folate pathway enzyme serine hydroxymethyltransferase (SHMT) through the erythrocytic cycle of Plasmodium falciparum

Background

The folate pathway enzyme serine hydroxymethyltransferase (SHMT) converts serine to glycine and 5,10-methylenetetrahydrofolate and is essential for the acquisition of one-carbon units for subsequent transfer reactions. 5,10-methylenetetrahydrofolate is used by thymidylate synthase to convert dUMP to dTMP for DNA synthesis. In Plasmodium falciparum an enzymatically functional SHMT (PfSHMTc) and a related, apparently inactive isoform (PfSHMTm) are found, encoded by different genes. Here, patterns of localization of the two isoforms during the parasite erythrocytic cycle are investigated.

Methods

Polyclonal antibodies were raised to PfSHMTc and PfSHMTm, and, together with specific markers for the mitochondrion and apicoplast, were employed in quantitative confocal fluorescence microscopy of blood-stage parasites.

Results

As well as the expected cytoplasmic occupancy of PfSHMTc during all stages, localization into the mitochondrion and apicoplast occurred in a stage-specific manner. Although early trophozoites lacked visible organellar PfSHMTc, a significant percentage of parasites showed such fluorescence during the mid-to-late trophozoite and schizont stages. In the case of the mitochondrion, the majority of parasites in these stages at any given time showed no marked PfSHMTc fluorescence, suggesting that its occupancy of this organelle is of limited duration. PfSHMTm showed a distinctly more pronounced mitochondrial location through most of the erythrocytic cycle and GFP-tagging of its N-terminal region confirmed the predicted presence of a mitochondrial signal sequence. Within the apicoplast, a majority of mitotic schizonts showed a marked concentration of PfSHMTc, whose localization in this organelle was less restricted than for the mitochondrion and persisted from the late trophozoite to the post-mitotic stages. PfSHMTm showed a broadly similar distribution across the cycle, but with a distinctive punctate accumulation towards the ends of elongating apicoplasts. In very late post-mitotic schizonts, both PfSHMTc and PfSHMTm were concentrated in the central region of the parasite that becomes the residual body on erythrocyte lysis and merozoite release.

Conclusions

Both PfSHMTc and PfSHMTm show dynamic, stage-dependent localization among the different compartments of the parasite and sequence analysis suggests they may also reversibly associate with each other, a factor that may be critical to folate cofactor function, given the apparent lack of enzymic activity of PfSHMTm.