Phenological shifts in North American red squirrels: disentangling the roles of phenotypic plasticity and microevolution

Phenological shifts are the most widely reported ecological responses to climate change, but the requirements to distinguish their causes (i.e. phenotypic plasticity vs. microevolution) are rarely met. To do so, we analysed almost two decades of parturition data from a wild population of North American red squirrels (Tamiasciurus hudsonicus). Although an observed advance in parturition date during the first decade provided putative support for climate change‐driven microevolution, a closer look revealed a more complex pattern. Parturition date was heritable [h² = 0.14 (0.07–0.21 (HPD interval)] and under phenotypic selection [β = ?0.14 ± 0.06 (SE)] across the full study duration. However, the early advance reversed in the second decade. Further, selection did not act on the genetic contribution to variation in parturition date, and observed changes in predicted breeding values did not exceed those expected due to genetic drift. Instead, individuals responded plastically to environmental variation, and high food [white spruce (Picea glauca) seed] production in the first decade appears to have produced a plastic advance. In addition, there was little evidence of climate change affecting the advance, as there was neither a significant influence of spring temperature on parturition date or evidence of a change in spring temperatures across the study duration. Heritable traits not responding to selection in accordance with quantitative genetic predictions have long presented a puzzle to evolutionary ecologists. Our results on red squirrels provide empirical support for one potential solution: phenotypic selection arising from an environmental, as opposed to genetic, covariance between the phenotypic trait and annual fitness.     

Cation Penetration through Isolated Leaf Cuticles

The rates of penetration of various cations through isolated apricot Prunus armeniaca L. leaf cuticles were determined. Steady state rates were measured by using a specially constructed flow-through diffusion cell. The penetration rates of the monovalent cations in group IA followed a normal lyotropic series, i.e., CS⁺ ≥ Rb⁺ > K⁺ > Na⁺ > Li⁺. The divalent cations all penetrated through the cuticle more slowly than the monovalent cations. Comparison of the relative values of k (permeability coefficient) and D (diffusion coefficient) indicates that the penetration of ions through isolated cuticles took place by diffusion and was impeded by charge interactions between the solute and charge sites in the penetration pathway. Cuticular penetration rates of K⁺ and H₂O at pH above 9 were of similar magnitude. At pH 5.5 H₂O penetration was not affected but that of K⁺ was greatly reduced. From this observation and from data on cuticle titration and ion adsorption studies, we hypothesize that cuticular pores are lined with a substance (perhaps a protein) which has exposed positively charged sites.     

Visualization and modelling of the thermal inactivation of bacteria in a model food.

A large number of incidents of food poisoning have been linked to undercooked meat products. The use of mathematical modelling to describe heat transfer within foods, combined with data describing bacterial thermal inactivation, may prove useful in developing safer food products while minimizing thermal overprocessing. To examine this approach, cylindrical agar blocks containing immobilized bacteria (Salmonella typhimurium and Brochothrix thermosphacta) were used as a model system in this study. The agar cylinders were subjected to external conduction heating by immersion in a water bath. They were then incubated, sliced open, and examined by image analysis techniques for regions of no bacterial growth. A finite-difference scheme was used to model thermal conduction and the consequent bacterial inactivation. Bacterial inactivation rates were modelled with values for the time required to reduce bacterial number by 90% (D) and the temperature increase required to reduce D by 90% taken from the literature. Model simulation results agreed well with experimental results for both bacteria, demonstrating the utility of the technique.     

Fibroblast growth factor receptor signaling in Xenopus retinal axon extension

Fibroblast growth factor receptors (FGFRs) and N-cadherin both regulate axon extension in developing Xenopus retinal ganglion cells (RGCs). Cultured cerebellar neurons have been shown to require FGFR activity for N-cadherin–stimulated neurite outgrowth, raising the possibility that N-cadherin is a FGFR ligand. To investigate this possibility in the developing visual system, retinal neurons were transfected with a dominant-negative FGFR (XFD) and plated on purified N-cadherin substrates. XFD-expressing neurons extended markedly shorter processes than control GFP-expressing neurons, implicating a role for FGFRs in N-cadherin–stimulated neurite outgrowth. To examine whether N-cadherin and FGFRs share the same pathway or use distinct second messenger pathways, specific inhibitors of implicated signaling molecules were added to neurons stimulated by N-cadherin, basic fibroblast growth factor (bFGF), or brain-derived nerve factor (BDNF) (which stimulates RGC outgrowth by a FGFR-independent mechanism). Diacylglycerol (DAG) lipase and Ca²⁺/calmodulin kinase II inhibitors both significantly reduced outgrowth stimulated by N-cadherin or bFGF but not by BDNF. Furthermore, we show that inhibiting DAG lipase activity in RGC axons extending in vivo toward the optic tectum reversibly slows axon extension without collapsing their growth cones. Thus, a common second-messenger signaling pathway mediating both N-cadherin– and bFGF-stimulated neurite extension is consistent with a model in which N-cadherin directly modulates the FGFR or a model whereby both FGFR and N-cadherin regulate the same second-messenger system. © 1998 John Wiley & Sons, Inc. J Neurobiol 37: 633–641, 1998     

Association with pedogenic iron and aluminum: effects on soil organic carbon storage and stability in four temperate forest soils

Soil organic carbon (SOC) can be stabilized via association with iron (Fe) and aluminum (Al) minerals. Fe and Al can be strong predictors of SOC storage and turnover in soils with relatively high extractable metals content and moderately acidic to circumneutral pH. Here we test whether pedogenic Fe and Al influence SOC content and turnover in soils with low Fe and Al content and acidic pH. In soils from four sites spanning three soil orders, we quantified the amount of Fe and Al in operationally-defined poorly crystalline and organically-complexed phases using selective chemical dissolution applied to the soil fraction containing mineral-associated carbon. We evaluated the correlations of Fe and Al concentrations, mean annual precipitation (MAP), mean annual temperature (MAT), and pH with SOC content and ¹⁴C-based turnover times. We found that poorly crystalline Fe and Al content predicted SOC turnover times (p < 0.0001) consistent with findings of previous studies, while organically-complexed Fe and Al content was a better predictor of SOC concentration (p < 0.0001). Greater site-level MAP (p < 0.0001) and colder site-level MAT (p < 0.0001) were correlated with longer SOC turnover times but were not correlated with SOC content. Our results suggest that poorly crystalline Fe and Al effectively slow the turnover of SOC in these acidic soils, even when their combined content in the soil is less than 2% by mass. However, in the strongly acidic Spodosol, organo-metal complexes tended to be less stable resulting in a more actively cycling mineral-associated SOC pool.