Osmotic Homeostasis in Dictyostelium Discoideum: Excretion of Amino Acids and Ingested Solutes

ABSTRACT. The response to osmotic stress in axenically cultured Dictyostelium discoideum was examined. Hypoosmotic buffers elicited two changes in the large (-50 mM) cytosolic pool of amino acids: a) the total size of the pool diminished, while b) about half of the initial pool was excreted. Hyperosmotic stress had the opposite effect. Among the predominant amino acids in the pool were glycine, alanine and proline. Putrescine, the major diamine, was neither excreted nor modulated. Recently ingested radioactive amino acids were excreded in preference to those in the cytoplasm, suggesting that the endocytic pathway might be involved in water excretion. Furthermore, hypoosmotic stresss strimulated the selective exretion of small, membrane-impermeable fluorescent dyes which had been ingested into endocytic vacuoles. Caffeine inhibited the excretion of the fluorophores but not the amino acids. We conclude that the response of Dictyostelium to osmotic stress is complex and includes both modulation of the cytoplasmic amino acid pool and the excretion of amino acids and other small solutes from the endocytic pathway.     

Coupling of the Deformation of a Model Endothelial Cell and a Steady Laminar Flow

Abstract

Mechanical forces influence endothelial cell's (EC) morphology and functions. In this work it was proposed a numerical analysis of steady laminar flows near a modelled monolayer of elastic ECs in order to determine the local distributions of mechanical forces on the surface and inside the cell.

Numerical results showed that the flow induced non uniform mechanical stresses on cell surface and led to a cell deformation. These numerical results were compared with experimental measurements of the deformation of cultured human aortic endothelial cells under flow. It will be interesting to study eventual correlations between the distributions of biological receptors (cytoskeleton, adhesion molecules, etc.) and that of the non-uniform mechanical forces.     

Sex‐biased dispersal and high levels of gene flow among local populations in the argentine boa constrictor,Boa constrictor occidentalis

Abstract The knowledge of dispersal is essential to understand the ecology of any species, since population dynamics, spatial distribution and genetic structure are closely tied to patterns of movement. In this paper we estimate dispersal patterns in natural populations of the endangered snake Boa constrictor occidentalis (Serpentes, Boidae), using allozymes as genetic markers. Blood samples were obtained from a total of 120 adult individuals of nine localities from two areas, Sobremonte and Pocho, in Argentina. Only four out of a total of 24 loci were polymorphic: 6‐Pgdh‐1, Cat‐1, Ldh and Hp. The values of expected and observed mean heterozygosities, the percentage of polymorphic loci and mean number of alleles per locus for each population confirm the low levels of genetic variability in this snake. F_ST and N_em mean values were 0.0089 and 46.1 for Sobremonte and 0.0379 and 7.46 for Pocho, indicating important levels of gene flow. A comparison of F_ST between genders in both areas suggests a male‐biased dispersal, which could be explained by the characteristics of the mating system: males carry out an active mate search of receptive females, and philopatry could be selected in females due to the benefits of the familiarity with the natal area in the use of local resources.     

Establishing species' environmental requirements to understand how the southernmost species of South American pitvipers (Bothrops,Viperidae) are distributed: A niche‐based modelling approach

Three Bothrops species are known to be present along an extensive and generally xeric band of the south‐eastern portion of South America. Yet, the environmental factors responsible for the maintenance of the structure of this community have remained undetermined. To have a better understanding of snakes' geographic range limits in Argentina, we modelled the ecological niche of Bothrops alternatus, B. ammodytoides and B. diporus to identify areas of their occurrence and sympatry. We used snakes presence records and pseudo‐absence data with topographic (altitude and slope), vegetation and 19 bioclimatic variables as predictors. These variables were evaluated for each species, using Multiple Logistic Regression Analysis. The distribution of B. alternatus was correlated with temperature, precipitation and vegetation; that of B. ammodytoides with altitude, slope, precipitation, temperature and vegetation; and that of B. diporus related only to precipitation and temperature variables. Five variables appeared in all three models (i.e. annual precipitation, precipitation seasonality, precipitation of the driest month, mean diurnal temperature range and temperature range), although with different effects in different species. Probable sympatry areas were identified, for B. alternatus – B. ammodytoides in a narrow area, for B. diporus – B. alternatus in a wide area and B. ammodytoides – B. diporus in a small area. While some factors constrain the distribution of the southernmost pitvipers community in South America to spatially non‐overlapping ranges, others facilitate sympatry by allowing spatial coexistence. We found high sympatry between phylogenetically distantly related pitvipers. Niche‐based Geographic Information Systems modelling allowed us to identify the environmental factors correlated with each pitviper range in Argentina and in its contact zones, and to understand species distributional limits.