Consequences of the colonisation of leaves by fungi and oomycetes for leaf consumption by a gammarid shredder

1. Leaf litter breakdown by shredders in the field is affected by leaf toughness, nutritional value and the presence of secondary compounds such as polyphenols. However, experiments involving the use of single fungal strains have not supported the assumption that leaf parameters determine food selection by shredders perhaps because of a failure to test for high consumption prior to isolation of fungal strains, overrepresentation of hyphomycetes or the potential effects of accompanying bacteria. In this study, we used bacteria‐free, actively growing fungi and oomycetes isolated from conditioned leaf litter for which a shredder had already shown high consumption rates. 2. Black alder (Alnus glutinosa) leaf litter was exposed to the littoral zone of Lake Constance in autumn, and subsamples were analysed for leaf parameters and consumption by Gammarus roeselii under standard conditions at regular intervals. On dates with a high consumption rate of the exposed leaves, 14 single strains of fungi and oomycetes were isolated, freed of bacteria and grown on autoclaved leaves. 3. Six of eight measured leaf parameters of exposed leaves were significantly correlated with Gammarus consumption rates, with high colinearity among leaf parameters hampering the identification of causal relations between leaf parameters and feeding activity. 4. When single strains of fungi and oomycetes were grown on autoclaved leaf litter, toughness of colonised leaves was always lower than in the control and the content of protein, N and P were increased. There were pronounced strain‐specific effects on leaf parameters. Consumption rates also differed significantly, with nine of fourteen isolates consumed at higher rates than controls and none proving to be a deterrent. Protein and polyphenol content were significantly correlated with consumption rates. Oomycete‐colonised leaves were consumed at similar rates but were of lower food quality than fungi‐colonised leaves. 5. We argue that direct strain‐specific attractant or repellent effects of fungi and oomycetes on consumption by G. roeselii are not important. However, we found indirect strain‐specific role operating via effects on leaf parameters.     

In vitro spore germination and infection of cultivars of Rubus and Rosa by downy mildews from both hosts

The cardinal temperatures for in vitro germination of conidia of imported and indigenous isolates of downy mildew from hosts in the genera Rubus and Rosa were similar. A high percentage of conidia germinated above 2°C and germination remained between 80% and 90% up to 15°C or 20°C, depending on the isolate. The highest incidence of disease on leaf disks of Tummelberry (blackberry × red raspberry) inoculated with an isolate of Peronospora rubi occurred at c. 15°C, with infection over a range from 2°C to 28°C. Tests on leaf disks in vitro, and leaflets of primocane and lateral shoots in plastic tunnels, with three hybrid berry (blackberry x red raspberry), six blackberry and nine red raspberry cultivars showed the hybrid berries to be most susceptible. In a plastic tunnel infected drupelets of red raspberry fruits developed more slowly and failed to ripen evenly compared with uninfected drupelets. Similar malformation of infected fruits occurred in a plantation of Tummelberry. An isolate of P. rubi attacked severely both Tummelberry and rose cv. Can Can. Fluorescence microscopy after staining with aniline blue showed that leaf disks of Tummelberry were extensively colonised by intercellular mycelium of P. sparsa isolated from rose, even though sporulation was sparse or absent. This supports the view that P. rubi and P. sparsa may be conspecific. Oospores of P. rubi were found routinely within leaf disks of Rubus cultivars inoculated in vitro and once in naturally infected leaflets of Tummelberry.     

Are leaves ‘freewheelin'? Testing for a Wheeler‐type effect in leaf xylem hydraulic decline

A recent study found that cutting shoots under water while xylem was under tension (which has been the standard protocol for the past few decades) could produce artefactual embolisms inside the xylem, overestimating hydraulic vulnerability relative to shoots cut under water after relaxing xylem tension (Wheeler et al. 2013). That study also raised the possibility that such a ‘Wheeler effect’ might occur in studies of leaf hydraulic vulnerability. We tested for such an effect for four species by applying a modified vacuum pump method to leaves with minor veins severed, to construct leaf xylem hydraulic vulnerability curves. We tested for an impact on leaf xylem hydraulic conductance (K_x) of cutting the petiole and minor veins under water for dehydrated leaves with xylem under tension compared with dehydrated leaves after previously relaxing xylem tension. Our results showed no significant ‘cutting artefact’ for leaf xylem. The lack of an effect for leaves could not be explained by narrower or shorter xylem conduits, and may be due to lesser mechanical stress imposed when cutting leaf petioles, and/or to rapid refilling of emboli in petioles. These findings provide the first validation of previous measurements of leaf hydraulic vulnerability against this potential artefact.     

COMPARISON OF THE EFFECT OF DIFFERENT SHORT CHAIN FATTY ACIDS ON THE GROWTH AND DIFFERENTIATION OF HUMAN COLONIC CARCINOMA CELL LINESIN VITRO

The aim of this study was to compare the effects of acetate, propionate, butyrate, iso-butyrate, valerate, iso-valerate and caproate on cell growth and on the activities of alkaline phosphatase (AP) and dipeptidyl aminopeptidase IV (DPP IV) by three human colonic adenocarcinoma cell lines. In addition to butyrate, propionate and valerate inhibited cell proliferation of the three cell lines. The other SCFAs did not influence cell proliferation. AP and DPP IV activities were strongly stimulated by butyrate on two of the three cell lines. On HT-29, AP was strongly stimulated, however DPPIV expression remained undetectable. Propionate and valerate exhibited a weaker stimulation, the other SCFAs being ineffective. The effect of SCFAs on cell proliferation and differentiation clearly depends on the number of carbons and on the configuration of the basic structure of the molecule.     

Population genetics of insecticide resistance in the mosquito Culex pipiens

Thirty years of control of the mosquito Culex pipiens using organophosphate insecticides (OP) has selected for OP-resistance alleles on a world-wide scale. As reviewed here, studies at the levels of gene and population allow identification of the main forces driving this process of adaptation. Three loci are involved in OP-resistance in C. pipiens. For two of these, adaptive mutations were found to be rare events, such that the ubiquitous distribution of certain resistance alleles could only be explained as deriving from a single origin by mutation followed by extensive migration. Population structure analyses confirmed that long-distance migration is frequent. Thus, different resistance alleles could accumulate and compete within populations soon after their origin by mutation. The different selection pressures acting on these alleles, i.e. their selective advantage in the presence of OP and their disadvantage (resistance cost) in absence of OP, were also analysed. Substantial differences in resistance cost among alleles present within the Mediterranean area were discovered. Long-term surveys of Mediterranean populations confirmed the pivotal importance of resistance cost in shaping the evolution of this adaptive polymorphism. Some hypotheses on the functional links between the nature of the initial mutation events and the subsequent evolution of polymorphism are discussed.