Beneficial effect of fluorocarbon emulsion media on the function of neuromuscular preparations in vitro

The effects of liquid fluorocarbons as bathing media were determined by use of in vitro neuromuscular preparations. Rat hemidiaphragms were bathed in either oxygenated fluorocarbon (FC) emulsion or standard oxygenated Krebs solution. Contractile force in response to simple supramaximal nerve stimuli as well as to high frequency stimulation was greater, while twitch:tetanus ratio was smaller in FC emulsion. With such medium, post-tetanic potentiation of contraction was also more consistently observed. Indirectly stimulated diaphragms survived longer in FC emulsion. After cessation of oxygenation, oxygen tension (ρO(2)) of the medium declined more rapidly with Krebs than with FC emulsion; ρO(2) directly correlated with force of contraction. Similarly, in the chick biventer cervicis preparation, FC emulsion enhanced nerve-stimulated force of contraction; returning the preparation to standard Krebs solution reversed this phenomenon. Dose-resonse curves of muscle contraction in response to acetycholine and KCl administration were shifted upward during FC emulsion superfusion. Frequency of miniature endplate potentials was lower in FC emulsion than that observed in Krebs solution, measured from the same cell of the rat diaphragm. Resting membrane potentials were also greater in muscle cells sampled from FC emulsion-bathed preparations. These data suggest that FC emulsion is superior to standard Krebs solution as a bathing medium for in vitro neuromuscular preparations by virtue of the high solubility of oxygen in it.     

Episodic seedling growth in Allosyncarpia ternata,a lignotuberous,monsoon rainforest tree in northern Australia

On the western Arnhem Land Plateau, Northern Territory, Australia, seedlings of the canopy tree Allosyncarpia ternata S.T. Blake typically spend many years (perhaps decades) as small (<1 m), multistemmed plants on the forest floor. In this establishment phase, long periods of apparent inactivity are interrupted by episodes of rapid growth. This paper describes a 5‐year field‐monitoring program to examine the pattern of seedling growth and survival in allosyncarpia forest, and field and shadehouse measurements of lignotuber size. Individual seedlings may produce, each wet season, a number of fast‐growing stems, which then die back in the following dry season. As a result, mean annual above‐ground growth during this life stage is negligible. With each wet season, however, the seedling extends its below ground parts – a large lignotuber and a deep root system. After a number of years, when the lignotuber has grown large enough to sustain massive shoot growth, when a suitable light gap becomes available, and presumably when roots reach reliable dry‐season water supplies, the seedling grows rapidly. Thus, the shortage of saplings in allosyncarpia forest is due to the short time that individual plants spend at that particular growth‐stage, rather than to any dysfunction in recruitment.     

On the Unnecessary and Misleading Taxon “Cetartiodactyla”

Journal of Mammalian Evolution - The name “Cetartiodactyla” was proposed in 1997 to reflect the molecular data that suggested that Cetacea is closely related to Artiodactyla. Since...     

Extreme heterogeneity of population response to climatic variation and the limits of prediction

Certain general facets of biotic response to climate change, such as shifts in phenology and geographic distribution, are well characterized; however, it is not clear whether the observed similarity of responses across taxa will extend to variation in other population‐level processes. We examined population response to climatic variation using long‐term incidence data (collected over 42 years) encompassing 149 butterfly species and considerable habitat diversity (10 sites along an elevational gradient from sea level to over 2,700 m in California). Population responses were characterized by extreme heterogeneity that was not attributable to differences in species composition among sites. These results indicate that habitat heterogeneity might be a buffer against climate change and highlight important questions about mechanisms maintaining interpopulation differences in responses to weather. Despite overall heterogeneity of response, population dynamics were accurately predicted by our model for many species at each site. However, the overall correlation between observed and predicted incidence in a cross validation analysis was moderate (Pearson's r = 0.23, SE 0.01), and 97% of observed data fell within the predicted 95% credible intervals. Prediction was most successful for more abundant species as well as for sites with lower annual turnover. Population‐level heterogeneity in response to climate variation and the limits of our predictive power highlight the challenges for a future of increasing climatic variability.     

Reversing invasion in bistable systems

In this paper, we discuss a class of bistable reaction-diffusion systems used to model the competitive interaction of two species. The interactions are assumed to be of classic “Lotka–Volterra” type and we will consider a particular problem with relevance to applications in population dynamics: essentially, we study under what conditions the interplay of relative motility (diffusion) and competitive strength can cause waves of invasion to be halted and reversed. By establishing rigorous results concerning related degenerate and near-degenerate systems, we build a picture of the dependence of the wave speed on system parameters. Our results lead us to conjecture that this class of competition model has three “zones of response”. In the central zone, varying the motility can slow, halt and reverse invasion. However, in the two outer zones, the direction of invasion is independent of the relative motility and is entirely determined by the relative competitive strengths. Furthermore, we conjecture that for a large class of competition models of the type studied here, the wave speed is an increasing function of the relative motility.     

Temporal dynamics in non-additive responses of arthropods to host-plant genotypic diversity

Genotypic diversity within host‐plant populations has been linked to the diversity of associated arthropod communities, but the temporal dynamics of this relationship, along with the underlying mechanisms, are not well understood. In this study, we employed a common garden experiment that manipulated the number of genotypes within patches of Solidago altissima, tall goldenrod, to contain 1, 3, 6 or 12 genotypes m^?2 and measured both host‐plant and arthropod responses to genotypic diversity throughout an entire growing season. Despite substantial phenological changes in host plants and in the composition of the arthropod community, we detected consistent positive responses of arthropod diversity to host‐plant genotypic diversity throughout all but the end of the growing season. Arthropod richness and abundance increased with genotypic diversity by up to～65%. Furthermore, arthropod responses were non‐additive for most of the growing season, with up to 52% more species occurring in mixtures than the number predicted by summing the number of arthropods associated with component genotypes in monoculture. Non‐additive arthropod responses were likely driven by concurrent non‐additive increases in host‐plant aboveground biomass. Qualitative differences among host‐plant genotypes were also important early in the season, when specialist herbivores dominated the arthropod community. Neither arthropod diversity nor flower number was associated with genotypic diversity at the end of the growing season, when generalist floral‐associated herbivores dominated. Taken together, these results show that focusing on the temporal dynamics in the quantity and quality of co‐occurring host‐plant genotypes and associated community composition can help uncover the mechanisms that link intraspecific host‐plant diversity to the structure of arthropod communities. Furthermore, consistent non‐additive effects in genotypically diverse plots may limit the predictability of the arthropod community based solely on the genetic make‐up of a host‐plant patch.     

Colonization by nitrogen-fixing <Emphasis Type="Italic">Frankia</Emphasis> bacteria causes short-term increases in herbivore susceptibility in red alder (<Emphasis Type="Italic">Alnus rubra</Emphasis>) seedlings

Carbon allocation demands from root-nodulating nitrogen-fixing bacteria (NFB) can modulate the host plant’s chemical phenotype, with strong bottom–up effects on herbivores. In contrast to well-studied rhizobia, the effects of other important NFB on plant chemistry and herbivory are much less understood. Here, combining field surveys in the Oregon Coast Range, USA with laboratory experiments, we analyzed how N₂-fixing Frankia bacteria influenced plant growth, chemistry, and herbivory on Alnus rubra (red alder) seedlings. In the field, we quantified Frankia nodulation, herbivore damage, and plant size. In the laboratory, we grew seedlings with Frankia (F+), Frankia-free but nitrogen-fertilized (N+), or both uncolonized and unfertilized (F?N?) and assessed growth and leaf chemistry. We further conducted choice trials with black slugs, Arion rufus, a natural red alder herbivore. In the field, Frankia nodulation was significantly positively correlated with herbivory and negatively with seedling height. In contrast, in the lab, F+ as well as N+ seedlings were significantly taller than the F?N? controls. Seedlings from both treatments also had significantly increased leaf protein concentration compared to controls, whereas carbon-based nutritive compounds (carbohydrates) as well as leaf palatability-decreasing condensed tannins, lignin, and fiber were decreased in F+ but not in N+ treatments. In the choice assays, slugs preferred leaf material from F+ seedlings, but the effects were only significant in young leaves. Our study indicates that colonization by Frankia causes short-term ecological costs in terms of susceptibility to herbivory. However, the ubiquity of this symbiosis in natural settings suggests that these costs are outweighed by benefits beyond the seedling stage.     

Lack of evidence for reproductive isolation among ecologically specialised lycaenid butterflies

Abstract 1. The evolution of reproductive isolation between recently diverged or incipient species is a critical component of speciation and a major focus of speciation models. In phytophagous insects, host plant fidelity (the habit of mating and ovipositing on a single host species) can contribute to assortative mating and reproductive isolation between populations adapting to alternative hosts. The potential role of host plant fidelity in the evolution of reproductive isolation was examined in a pair of North American blue butterfly species, Lycaeides idas and L. melissa .
2. These species are morphologically distinct and populations of each species utilise different host plants; however they share 410 bp haplotypes of the mitochondrial cytochrome oxidase subunit I (COI) gene, indicating recent divergence.
3. Some populations using native hosts exhibited strong fidelity for their natal host plant over the hosts used by nearby populations. Because these butterflies mate on or near the host plant, the development of strong host fidelity may create reproductive isolation among populations on different hosts and restrict gene flow.
4. Tests of population differentiation using allozyme allele frequency data did not provide convincing evidence of restricted gene flow among populations. Based on morphological differences, observed ecological specialisation, and the sharing of genetic markers, these butterflies appear to be undergoing adaptive radiation driven at least partially by host shifts. Neutral genetic markers may fail to detect the effects of very recent host shifts in these populations due to gene flow and/or the recency of divergence and shared ancestral polymorphism.