Adaptation to marginal habitats by evolution of increased phenotypic plasticity

In an island population receiving immigrants from a larger continental population, gene flow causes maladaptation, decreasing mean fitness and producing continued directional selection to restore the local mean phenotype to its optimum. We show that this causes higher plasticity to evolve on the island than on the continent at migration-selection equilibrium, assuming genetic variation of reaction norms is such that phenotypic variance is higher on the island, where phenotypes are not canalized. For a species distributed continuously in space along an environmental gradient, higher plasticity evolves at the edges of the geographic range, and in environments where phenotypes are not canalized. Constant or evolving partially adaptive plasticity also alleviates maladaptation owing to gene flow in a heterogeneous environment and produces higher mean fitness and larger population size in marginal populations, preventing them from becoming sinks and facilitating invasion of new habitats. Our results shed light on the widely observed involvement of partially adaptive plasticity in phenotypic clines, and on the mechanisms causing geographic variation in plasticity.     

Integrated assessment of ecosystem integrity of large northern rivers: the Northern Rivers Basins Study example

The Northern River Basins Study (NRBS) assessed the ecologicalstate of three complex northern river basins for which we had a limitedunderstanding of the physical, chemical and biological environment. Amajor challenge of this study was the choice of ecosystem components tomonitor. This process was guided by determining the indicators thatwould best assess environmental state relative to ecosystem objectivesarticulated by people living within the basin. This charge lead the NRBSto use a cumulative effects assessment framework that measured stressorexposure and their effects along with the development of plausiblecause-effect mechanisms. Thus, the NRBS developed and applied anintegrated assessment framework that included public stakeholders,various levels of government and scientists. A key feature of thisframework was the ability to allow feedback among these groups toproduce the syntheses of science results, and the resultant managementresponses that ultimately modified environmental regulations. Papers inthis series present the results of the NRBS and emphasize theinterdisciplinary nature of this project. Investigations evaluated theroles of flow regulation, nutrients, contaminants, and their interactiveeffects, on the integrity of the Athabasca, Peace and Slave rivers,Canada. Novel contributions of the NRBS model included the cooperationamong governments, aboriginal peoples, non-government organizations,industry and other stakeholders, and the two-way communications flowamong the scientific community and these groups.     

Oligoryzomys flavescens (Rodentia, Muridae): gene flow among populations from central-eastern Argentina

In species acting as hosts of infectious agents, the extent of gene flow between populations is of particular interest because the expansion of different infectious diseases is usually related to the dispersal of the host. We have estimated levels of gene flow among populations of the sigmodontine rodent Oligoryzomys flavescens, in which high titers of antibodies have been detected for a Hantavirus in Argentina that produces a severe pulmonary syndrome. Enzyme polymorphism was studied by means of starch gel electrophoresis in 10 populations from the area where human cases of Hantavirus have occurred. Genetic differentiation between populations was calculated from F_ST values with the equation Nm = [(1/F_ST−1]/4. To assess the relative importance of current gene flow and historical associations between populations, the relationship of population pairwise log Nm and log geographic distance was examined. Low F_ST (mean = 0.038) and high Nm (15.27) values suggest high levels of gene flow among populations. The lack of an isolation by distance pattern would indicate that this species has recently colonized the area. The northernmost population, located on the margin of a great river, shows very high levels of gene flow with the downstream populations despite the large geographic distances. Passive transport of animals down the river by floating plants would promote unidirectional gene flow. This fact and the highest mean heterozygosity of that northernmost population suggest it is a center of dispersal within the species' range. This revised version was published online in July 2006 with corrections to the Cover Date.     

Golgi apparatus activity and membrane flow during scale biogenesis in the green flagellateScherffelia dubia (Prasinophyceae). II: Cell wall secretion and assembly

Summary Secretion of the cell wall (theca) in the scaly green flagellateScherffelia dubia (Prasinophyceae) has been examined by electron microscopy during cytokinesis. The bi-laminate wall forms by the extracellular amalgamation of two layers of scales produced in the Golgi apparatus (GA). Each mature GA cisterna contains ca. 12,000 scales of two distinct varieties arranged in two layers on the cisternal membrane. GA cisternae undergo turnover and one scale containing cisterna matures from thetransface of each dictyosome every 3–4 minutes. Cisternae then fuse with the plasma membrane at the anterior end of the cell releasing the scales onto the cell surface. The two layers of wall scales integrate on the cell surface in a time-dependent self-assembly process. The first scales deposited commence assembly at the cell posterior and the wall develops anteriorly by edge growth. The daughter cell wall is composed of ca. 1.2 million scales deposited in about 3 hours. Calculations of net membrane flow strongly indicate extensive endocytosis during wall deposition.     

Diffusive transport in Stokeslet flow and its application to plankton ecology

In this paper we consider the advective/diffusive transport of a solute near a hovering zooplankter. We approximate the fluid flow with that of a Stokeslet, corresponding to the plankter exerting a point force on the water, and assume that the plankter acts as a point source for the transported solute, located at the same point as the force. We find an analytical expression in closed form for the steady-state concentration of the solute. We also discuss the situation where the plankter performs Brownian motion. Finally we apply the results to the courtship of the marine copepod Pseudocalanus elongatus, where the male performs a mating dance below the hovering female. For this situation, our model supports the hypothesis that the mating dance is guided by the plume of a signalling pheromone.