Sucrose and proton cotransport in Ricinus cotyledons

In Ricinus cotyledons, evidence for proton extrusion came from observation of direct acidification of the medium in the presence of potassium salts. Increasing K⁺ influx with increasing pH suggested a link between K⁺ influx and H⁺ efflux by an H⁺ pump. The kinetics of K⁺ influx and H⁺ efflux were consistent with a 1:1 stoichiometry K⁺:H⁺, which may indicate either electrical coupling or carrier mediated exchange. The results were consistent with an H⁺ pump setting up an electrochemical potential gradient which provides the driving force for an H⁺-sucrose cotransport and the movement of K⁺. With reference to this, a model for phloem loading is suggested.     

Dispersal in a stream dwelling salmonid: Inferences from tagging and microsatellite studies

We used both direct (mark-recapture) andindirect (microsatellite analysis)methodologies to investigate dispersal betweentwo putative populations of brook charr (Salvelinus fontinalis) in Freshwater River,Cape Race, Newfoundland, Canada. Over a 5-yearstudy period, mark-recapture data revealed somemovement by fish, but the proportion ofrecaptured fish migrating from one populationarea to another was low (0–4.1%).Additionally, during sampling periods in thespawning seasons, no fish was found in thealternate population area to that of its firstcapture. Despite this pattern of limitedmovement, microsatellite analysis based onsixteen polymorphic loci provided no evidenceof genetic differentiation. Indirect estimatesof dispersal parameters varied greatly betweendifferent methods of analysis. While use of acoalescent-based model yielded estimatedmigration rates congruent with the results ofthe mark-recapture study, other methodsresulted in much higher estimates of migrationbetween the populations. In particular, thelack of genetic differentiation coupled withlikely violations of the assumed island modelprevented generation of meaningful estimates ofdispersal using F_st. The disparitiesbetween migration rates estimated from themark-recapture work and from the differentindirect methods highlight the difficulties ofusing indirect methods to estimate dispersal onan ecological timescale. However,mark-recapture methods can fail to detecthistorical or episodic movement that isimportant in an evolutionary context, and wetherefore argue that a combination of directand indirect methods can provide a morecomplete picture of dispersal than eitherapproach alone.     

Draft genome sequence of Streptomyces strain S4, a symbiont of the leaf-cutting ant Acromyrmex octospinosus

Streptomyces spp. are common symbionts of the leaf-cutting ant species Acromyrmex octospinosus, which feeds on basidiomycete fungus leaf matter and harvests the lipid- and carbohydrate-rich gongylidia as a food source. A. octospinosus and other ant genera use antifungal compounds produced by Streptomyces spp. and other actinomycetes in order to help defend their fungal gardens from parasitic fungi. Herein, we report the draft genome sequence of Streptomyces strain S4, an antifungal-producing symbiont of A. octospinosus.     

Performance of a clonal species in patchy environments: effects of environmental context on yield at local and whole-plant scales

Yield of the clonal plant Glechoma hederacea was compared at different spatial scales, in heterogeneous and homogeneous environments providing the same amount of nutrients. For the heterogeneous treatments, environments were created with different patch sizes and different degrees of contrast in nutrient concentration between patches of different quality. Total clone yield differed by almost 2.5-fold across treatments, being highest in environments with large patches and high contrast, lowest in environments with small patches and high contrast, and intermediate under homogeneous conditions. Compared with plants in homogeneous conditions, there were significant increases or decreases in yield at all scales of measurement in many of the heterogeneous treatments. These effects on yield reflected a combination of local responses to growing conditions and modification of these responses due to physiological integration with other parts of the plant growing in contrasting conditions, supporting the proposal of de Kroon et al. (2005 New Phytol 166:73–82). The results show that plant yield at all scales is strongly dependent on environmental context, and that maximum yield can only be realized under a limited range of heterogeneous conditions.     

Population size,habitat fragmentation,and the nature of adaptive variation in a stream fish

Whether and how habitat fragmentation and population size jointly affect adaptive genetic variation and adaptive population differentiation are largely unexplored. Owing to pronounced genetic drift, small, fragmented populations are thought to exhibit reduced adaptive genetic variation relative to large populations. Yet fragmentation is known to increase variability within and among habitats as population size decreases. Such variability might instead favour the maintenance of adaptive polymorphisms and/or generate more variability in adaptive differentiation at smaller population size. We investigated these alternative hypotheses by analysing coding-gene, single-nucleotide polymorphisms associated with different biological functions in fragmented brook trout populations of variable sizes. Putative adaptive differentiation was greater between small and large populations or among small populations than among large populations. These trends were stronger for genetic population size measures than demographic ones and were present despite pronounced drift in small populations. Our results suggest that fragmentation affects natural selection and that the changes elicited in the adaptive genetic composition and differentiation of fragmented populations vary with population size. By generating more variable evolutionary responses, the alteration of selective pressures during habitat fragmentation may affect future population persistence independently of, and perhaps long before, the effects of demographic and genetic stochasticity are manifest.     

Effects of physical connection and genetic identity of neighbouring ramets on root-placement patterns in two clonal species

Root-placement patterns were examined in the clonal species Glechoma hederacea and Fragaria vesca when grown with different types of neighbours. Three different patterns were predicted as consequences of different types of interactions between roots: the avoidance pattern if root growth decreases in the presence of neighbouring roots; the intrusive pattern if root growth increases towards neighbouring roots; and the unresponsive pattern if root growth is unaffected by neighbouring roots. Experiments were conducted in which physical connection between ramets, and the genetic identity of neighbouring ramets, were manipulated. The patterns of distribution of entire root systems and elongation rates of individual roots were measured. Root systems and individual roots of G. hederacea avoided contact with roots of neighbouring ramets, irrespective of connection to the neighbour and its genetic or specific identity. In contrast, F. vesca roots grew equally towards and away from intraspecific ramet neighbours and their elongation was stimulated by contact with roots of G. hederacea ramets. These results demonstrate that root-placement patterns of plants grown with different types of neighbours vary between species, and suggest that factors additional to resource depletion could be involved in their development.