Heterogeneity and density dependence in a field study of a tephritid-parasitoid interaction

Abstract.

• 1 The spatial distributions of two tephritid flies (Urophora stylata (Fabricius) and Terellia serratulae L.) attacking thistle flower heads and the levels of parasitism from their associated parasitoid guilds were studied over a 7-year period.
• 2 Using these data it is possible to seek both temporal, density dependent relationships between average levels of parasitism and host density per generation, and also any spatial patterns of parasitism contributing to stability that may be operating within the same field system.
• 3 Parasitism by the two most important generalist parasitoids of T.serratulae is a direct function of average T.serratulae density per year. There is little evidence of any stabilizing heterogeneity arising from the spatial distribution of parasitism within generations.
• 4 Temporal density dependence of Urophora stylata cannot be confirmed from the 7 years of study but there is evidence of spatial heterogeneity which may have an important effect on the dynamics of the host population.

     

Effects of light and temperature on symptom development and virus content of tobacco leaves inoculated with potato mop-top virus

Necrotic spots or small rings develop after 3–4 days in leaves of Nicotiana tabacum cv. Xanthi-nc inoculated with potato mop-top virus and kept at 14 °C in continuous light (4320 lux); a series of concentric necrotic rings of increasing diameter then form at 2- to 3-day intervals around each initial lesion. Successive rings take longer to appear when either the light intensity or the photoperiod is decreased. Virus accumulation is much decreased and lesions rarely develop either at 14· in darkness or at 22° in light. Virus accumulates rapidly when plants are transferred from these conditions to 14° in light (4320 lux), and necrotic spots or rings develop whose size depends on the interval between inoculation and transfer, and on the conditions during this period. In such plants, necrosis seems to occur only when conditions become favourable for virus synthesis, it is confined to recently infected cells and it does not prevent virus spread to further healthy cells. From the sizes of the necrotic rings, the virus is estimated to invade tissue in light (4320 lux) at c. 38 μm/h at 22° and c. 16 μm/h at 14°. Invasion in darkness at either temperature is very slow. Necrotic rings develop, and the rate of virus accumulation increases when inoculated plants are transferred from 22° in light (4320 lux) to 14° in darkness, but no lesions appear when the order of the treatments is reversed. The process of lesion formation thus includes an early phase requiring light and a later phase requiring low temperature. The light-requiring phase takes about a day at 14° but less at 22°. The later phase takes about 2 days in light (4320 lux) or 3 days in darkness.     

Immunological Potential of the in vitro Mixed Skin Cell-Leukocyte Reaction

THE mixed skin cell-leukocyte reaction (MSLR) is an in vitro reaction between dissociated skin cells and allogeneic peripheral blood leukocytes. It is similar in its immunological implications to the reaction between allogeneic leukocytes, commonly known as the mixed leukocyte reaction (MLR) or the mixed lymphocyte interaction (MLI). In the latter, immunological recognition between lymphocytes from genetically disparate donors can be detected and measured either by blastoid transformation, enhanced mitotic rate or induction of DNA synthesis^1–4. This system has been useful in the study of the transfer of transplantation immunity in vivo by lymphocytes sensitized in vitro⁵ and also in the identification of tissue incompatibility between donor and recipient^{6, 7}. In studies with inbred strains of rats the proliferative phase of the MLR has been shown to possess characteristics required for an immunological response; that is, selectivity and specificity^{8, 9}. Furthermore, mouse peripheral lymphocytes sensitized in mixed culture with allogeneic lymphocytes have been shown to engender immunospecific destruction of target cells¹⁰, an event which is directly associated with allograft rejection.     

THE INCUBATION PATCH OF BIRDS

1. The incubation patch of birds forms in the areas of the ventrum devoid of contour feathers (apteria) by processes involving defeathering of down, dermal and subdermal hypervascularization, oedema, and hyperplasia of the epidermis and dermal connective tissue. Its formation facilitates the transfer of heat to the eggs and hatched young. 2. In general, the sex that incubates develops a patch; this may be the female, both sexes, or the male, and the species within any one order tend to exhibit a common pattern. In some orders (e.g. Pelecaniformes) no patch develops. 3. The incubation patch begins to form before egg laying in passerine birds and during egg laying in Galliformes. Most of the patch responses in passerines are completed earlier in the reproductive cycle (late egg laying to middle incubation) than in Galliformes (middle incubation to early brooding). 4. In the passerines studied, in which usually only the female develops a patch, oestrogen and prolactin synergize to cause patch development; oestrogen given alone is effective because it synergizes with endogenous prolactin in intact birds. The role of progesterone is unclear, but it seems to mimic the effects of prolactin and also plays a role in the increase in skin sensitivity characteristic of patch development in the canary. 5. In the one galliform studied (California quail), in which both sexes develop a patch, either oestrogen or androgen synergizes with prolactin; oestrogen alone is ineffective in non-breeding quail because of insufficient endogenous prolactin levels. Prolactin alone causes epidermal hyperplasia and vascularization, and progesterone seems mainly to be involved in defeathering. In the starling also both sexes develop a patch, but testosterone plus prolactin is not effective as it is in the quail. 6. In the phalaropes, birds in which only the male develops a patch, androgen and prolactin are the effective synergists in patch development. Thus, there is a correlation between the sex which develops a patch in the wild and the steroid (oestrogen or androgen) which synergizes with prolactin. 7. In the brown-headed cowbird, a parasitic species which neither develops a patch nor incubates, there are no responses to exogenous hormones. The absence of patch development in one sex or in both can be due either to an absence of appropriate hormone levels or to a lack of sensitivity of the skin to hormones. 8. More study is needed of the transfer of heat from the patch area to the eggs or young, including the relationships between patch structure, surface area, surface temperature, type of nest, and the number and size of eggs. 9. Patch formation is affected by and influences behaviour. 10. More research is needed in regard to (I) the natural development and endocrine control of the incubation patch in various orders of birds: (2) the effects of patch formation on behaviour, and vice versa; (3) the mode of action of the hormones in patch formation, that is, whether it is direct or indirect, e.g. through the release of another hormone; (4) the specificity of the ventral skin to hormone administration; (5) the effects of stimulation of the patch by eggs in the nest on prolactin and gonado-trophin secretion; (6) the possible role of other, as yet untested, hormones on patch development; (7) the levels of endogenous hormones in relation to natural patch formation; and (8) possible correlations between the structure of the patch, the chronology of its formation, the clutch size and the condition of the young when hatched in various species.     

Effect of 1,1,1-Trichloro-2,2-bis(p-chlorophenyl)ethane on Cytochromes of the Photosynthetic Electron Transport System

It is shown that in susceptible barley DDT has a marked effecton cytochrome f responses, and on measurable levels of cytochromesb₅₅₉LP, b₅₅₉HP, and b₆. These effects, not shown by treatedresistant barley, are discussed in the light of known sitesof inhibition by DDT of photosynthetic electron transport.     

Predisposing effects of Eyespot (Pseudocercosporella herpotrichoides) on Septoria nodorum infection of winter wheat

All isolates of PeniciUium simplicissimum, P. verrucosum var. cyclopium, P. brevicompactum, P. multicolor, P. oxalicum, P. paxilli, Botrytis cinerea, and of a Gliocladium sp. obtained from necrotic virus-tested narcissus twin-scales previously dtoped in benomyl were tolerant to 1000 μg/ml of this fungicide in agar. Every necrotic twin-scale examined was infected with at least one of these species. The first two species were the most frequently isolated and a similar range of species infected different narcissus clones or different cultivars. It is argued that the use of benomyl during the twin-scaling programme should be discontinued as soon as a suitable alternative fungicide is found.     

Water Potential-Water Content Relationships In Apple Leaves

Three methods for determining the relationship between xylempressure potential as measured in a pressure chamber (an estimateof leaf water potential) and leaf relative water content werecompared for apple leaves. A range of leaf water contents wasobtained either by sampling leaves in the field at differenttimes of day and on days with differing evaporative demand,or by allowing evaporation from excised leaves in the laboratory,or by expressing sap by overpressurization in a pressure chamber.The first two methods gave very similar results, but the lasttended to give rather lower water potentials at any given watercontent. A possible explanation for these results and theirimplications for the estimation of osmotic potentials usingpressure-volume curves are discussed. Some osmotic adjustmentwas observed in trees droughted for 3 months, with estimatedosmotic potentials, both at full turgor and zero turgor, beingnearly 0.3 MPa lower than in irrigated controls.     

cAMP Control of HCN2 Channel Mg2+ Block Reveals Loose Coupling between the Cyclic Nucleotide-Gating Ring and the Pore

Hyperpolarization-activated cyclic nucleotide-regulated HCN channels underlie the Na⁺-K⁺ permeable I_H pacemaker current. As with other voltage-gated members of the 6-transmembrane K_V channel superfamily, opening of HCN channels involves dilation of a helical bundle formed by the intracellular ends of S6 albeit this is promoted by inward, not outward, displacement of S4. Direct agonist binding to a ring of cyclic nucleotide-binding sites, one of which lies immediately distal to each S6 helix, imparts cAMP sensitivity to HCN channel opening. At depolarized potentials, HCN channels are further modulated by intracellular Mg²⁺ which blocks the open channel pore and blunts the inhibitory effect of outward K⁺ flux. Here, we show that cAMP binding to the gating ring enhances not only channel opening but also the kinetics of Mg²⁺ block. A combination of experimental and simulation studies demonstrates that agonist acceleration of block is mediated via acceleration of the blocking reaction itself rather than as a secondary consequence of the cAMP enhancement of channel opening. These results suggest that the activation status of the gating ring and the open state of the pore are not coupled in an obligate manner (as required by the often invoked Monod-Wyman-Changeux allosteric model) but couple more loosely (as envisioned in a modular model of protein activation). Importantly, the emergence of second messenger sensitivity of open channel rectification suggests that loose coupling may have an unexpected consequence: it may endow these erstwhile “slow” channels with an ability to exert voltage and ligand-modulated control over cellular excitability on the fastest of physiologically relevant time scales.