Effect of emigration on cannibalism and intraguild predation in aphidophagous ladybirds

Abstract.  1. The incidence and timing of emigration, cannibalism, and intraguild predation of larvae of three aphidophagous ladybirds (Coleoptera: Coccinellidae), Harmonia axyridis Pallas, Coccinella septempunctata brucki Mulsant, and Propylea japonica Mulsant, relative to the presence of prey was determined in the laboratory in single- and mixed-species populations.
2. In single-species populations, 80% of the larvae of C. s. brucki emigrated prior to the extinction of the aphid population and no larvae were lost due to cannibalism; however > 80% of the larvae of the other two species were still present when the aphid became extinct and the losses due to cannibalism for H. axyridis and P. japonica were 25% and 14% respectively. Finally, 28% of the P. japonica larvae completed their development, whereas no larvae of the other two species became adult.
3. In mixed-species populations, mortality of P. japonica attributable to cannibalism or intraguild predation increased greatly to 60%, whereas that of the other two species remained about the same. Consequently, survival of H. axyridis larvae improved and survival of P. japonica worsened; however the survival of C. s. brucki larvae was not affected by the other two species. Early emigration by C. s. brucki larvae may have enabled them to escape intraguild predation by H. axyridis in this system.     

Interaction between the light quality and flowering time pathways in Arabidopsis

Flowering in Arabidopsis is accelerated by a reduced ratio of red light to far-red light (R/FR), which indicates the proximity of competitive vegetation. By exploiting the natural genetic variation in flowering time responses to low R/FR, we obtained further insight into the complex pathways that fine-tune the transition to flowering in Arabidopsis. The Bla-6 ecotype does not flower significantly earlier in response to low R/FR, but is still able to display other features of shade avoidance, suggesting branching of low R/FR signalling. Here we show that the muted flowering response of Bla-6 is due to high levels of the floral repressor FLOWERING LOCUS C (FLC), conferred by a combination of functional FLC and FRIGIDA ( FRI ) alleles with a 'weak' FY allele. The Bla-6 FY allele encodes a protein with a corrupted WW binding domain, and we provide evidence that this locus plays a key role in the natural variation in light quality-induced flowering in Arabidopsis. In Bla-6, FLC blocks promotion to flowering by reduced R/FR by inhibiting expression of the floral integrator FLOWERING LOCUS T ( FT ) in a dose-dependent manner. Reduction of FLC removes this obstruction, and Bla6 plants then exhibit strong induction of FT and flower early in response to a low R/FR signal. This paper illustrates the intricate interaction of environmental signals and genetic factors to regulate flowering in Arabidopsis.     

Prey: predator ratio dependence in the functional response of a freshwater amphipod

1. First known for their shredding activity, freshwater amphipods also behave as active predators with consequences for prey population regulation and amphipod coexistence in the context of biological invasions. 2. A way to quantify predation is to determine the average consumption rate per predator, also known as its functional response (FR). 3. Although amphipods are gregarious and can display social interactions that can alter per capita consumption rates, previous studies using the FR approach to investigate amphipod predation ignored such potential mutual interference because they did not consider variations in predator density. 4. We investigated the FR of Echinogammarus berilloni feeding on dipteran larvae with joint variations in prey and predator densities. This bivariate experimental design allowed us to estimate interference and to compare the fits of the three main classes of theoretical FR models, in which the predation rate is a function of prey density alone (prey‐dependent models), of both prey and predator densities (predator‐dependent models) or of the prey‐to‐predator ratio (ratio‐dependent models). 5. The Arditi–Ginzburg ratio‐dependent FR model provided the best representation of the FR of E. berilloni, whose predation rate showed a decelerating rise to a horizontal asymptote as prey abundance increased. 6. Ratio dependence means that mutual interference between amphipods leads to prey sharing. Mutual interference is likely to vary between amphipod species, depending on their level of aggressiveness.     

Phenotypic variability in nesting success among Osmia lignaria propinqua females in a glasshouse environment: (Hymenoptera: Megachilidae)

Abstract. 1. The phenotypic variability of several estimates of fitness among seventeen nesting female O.lignaria Cresson was examined in a glasshouse provided with abundant resources.
2. Females exhibited wide variation in: (a) number of nests and cells made and in their rate of construction, (b) the incidence of mortality of offspring, and (c) the percentage of male offspring produced (52–100%).
3. In confirmation of earlier field studies: (a) more offspring of both sexes were produced during the first half of the nesting season, (b) most female offspring were produced early in the nesting season, (c) most parental investment during the last half of the flight season was made in male offspring, (d) there was a decrease in offspring size with season, with females decreasing more than males, and (e) the ratio of female/male body weight was = 1.8. These results appear to be due to ageing rather than to a reduction in resource availability.
4. No relation between female size and fecundity was evident.
5. There was no evidence that small females tended to produce a greater proportion of the smaller sex (males) than did large females.
6. Smaller offspring of both sexes had a much greater probability of dying overwinter than did large offspring. However, where the sexes overlapped in size (large males, small females), almost all females died and almost all males survived. We hypothesize that as size of offspring produced declines with season, a greater proportion of males are produced because they have a much greater probability of surviving at small body sizes.     

Reproductive ecology and growth of a population of brown trout (Salmo trutta L.) in an aquifer-fed stream of Old Castile (Spain)

In the River Lobos-Ucero and its tributary the River Avión-Milanos (Duero basin, Old Castile, Central Spain), two limestone streams fed by aquifers, the population of brown trout, as compared with the populations of other European streams, shows a high growth rate, high condition coefficients, short life-span and early age at first maturity. Gonad cycle was also studied. Size distributions of unshed eggs exhibit a dynamic activity with a bi-modal distribution from June onwards, spawning occurred in the last days of November. Fecundity (F) can be predicted from trout length (L, mm) according to the equation: F= –646.47+5.6167 · L. Numbers and standing crop of trout range from 18 to 3903 ind. ha^–1 and 3.6 to 452.9 Kg ha^–1, reaching higher values in the sites close to the aquifers. Egg production had values of 22.4 and 18.0 eggs m^–2 in the Rivers Ucero and Avión-Milanos respectively. Some factors suggested as regulators of these demographical characteristics are discussed in the light of recent literature.     

Cell density-dependent regulation of cell surface expression of two types of human tumor necrosis factor receptors and its effect on cellular response

Tumor necrosis factor (TNF) is a multipotential cytokine known to regulate the growth of a wide variety of normal and tumor cells. It has been shown that the density of cells in culture can modulate the growth regulatory activities of TNF, the mechanism of which, however, is not understood. In this report, we investigated the effect of cell density on the expression of TNF receptors. The receptors were examined on epithelial cells (e.g., HeLa), which primarily express the p60 form, and on myeloid cells (e.g., HL-60) known to express mainly the p80 form. We observed that binding of TNF to both cell lines decreased with increase in cell density. Scatchard analysis of binding on HeLa and HL-60 cells revealed a 4- to 5-fold reduction in the number of TNF receptors without any significant change in receptor affinity in both cell types at high density. The decrease in TNF receptor numbers at high cell density was also observed in several other epithelial and myeloid cell lines. The downmodulation at high cell density was unique to TNF receptors, since minimum change in other cell surface proteins was observed as revealed by fluorescent activated cell sorter analysis. Neutralization of binding with antibodies specific to each type of the receptors revealed that both the p60 and p80 forms of the TNF receptor were equally downmodulated. A decrease in leucine incorporation into proteins was observed with increase in cell density, suggesting a reduction in protein synthesis. Since inhibition of protein synthesis by cycloheximide also leads to a decrease in TNF receptors, it is possible that the density-dependent reduction in TNF receptor number is due to an overall decrease in protein synthesis. The density-dependent decrease in TNF receptors was accompanied by a decrease in intracellular reduced glutathione levels. A reduction in the number of receptors on TNF sensitive tumor cells induced by cell-density correlated with increase in resistance to the cytokine.     

Single-file diffusion through K+ channels in frog skin epithelium

Summary The ratio between the unidirectional fluxes of K⁺ across the frog skin with K-permeable outer membranes was determined in the absence of Na⁺ in the apical solutions. The experiments were performed under presteady-state conditions to be able to separate the flux ratio for K⁺ through the cells from contributions to the fluxes through extracellular leaks. The cellular flux ratio deviated strongly from the value calculated from the flux ratio for electrodiffusion. The experiments can be explained if the passive K transport through the epithelial cells proceeds through specific channels by single-file diffusion with a flux ratio exponent of about 2.5.