Comparative ecophysiological measurements on the light responses,water relations and desiccation tolerance of the filmy ferns Hymenophyllum wilsonii Hook. and H. tunbrigense (L.) Smith

Chlorophyll-fluorescence and infrared gas analyser measurements show saturation of photosynthetic electron flow and CO(2) uptake at generally lower irradiances in Hymenophyllum tunbrigense than in H. wilsonii, but with wide variation in both species (63-189 micromol m(-2) s(-1) PPFD in H. tunbrigense, 129-552 micro mol m(-2) s(-1) PPFD in H. wilsonii), probably related to both site and season. Non-photochemical quenching (at 400 micromol m(-2) s(-1) PPFD) ranged from 2.1 to 8.1, with no significant difference between the species. Pressure-volume curves from thermocouple-psychrometer measurements give full-turgor osmotic potentials of approx. -1.4 MPa in both species, and indicate low apoplast fractions and high cell-wall elastic moduli. Leaves of H. tunbrigense recovered within 24 h from up to 7 d desiccation at water potentials ranging from -40 MPa (74 % relative humidity, RH) to -220 MPa (20 % RH); after 15 or 30 d, desiccation recovery was slower and less complete, and leaves were severely damaged at the highest and lowest humidities. Hymenophyllum wilsonii recovered well from up to 30 d desiccation at -114 and -220 MPa, but at -40 MPa it showed signs of damage after 15 d, and was severely damaged or killed after 30 d. Results are discussed in relation to the ecological and geographical distributions of the two species, and to the adaptive strategies of filmy ferns in general.     

Selection for high diapause incidence in blow flies (Calliphora vicina) maintained under long days increases the maternal critical daylength: some consequences for the photoperiodic clock

Using a population of Calliphora vicina from southern Scotland (55 degrees N), showing a critical day length for maternal induction of diapause of about 14.5 h per day, strains of flies were selected for a high incidence of larval diapause under long day length (LD 16:8 h). Diapause incidence was raised from under 10% to almost 100% within five or six generations. The selected flies showed an increase in their critical day length to over 16 h per day, and a high incidence of larval diapause under very long photophases. Selected flies, however, showed mean circadian periods for locomotor activity little different from the original stock, or from flies selected for high diapause under LD 12:12 h, and a Nanda-Hamner profile lacking peaks and troughs of diapause incidence at about 24 h intervals. These results are interpreted to show that the photoperiodic regulation of diapause and the control of overt behavioural rhythmicity are 'separate' physiological systems.     

Multiple steps of phosphorylation of activated rhodopsin can account for the reproducibility of vertebrate rod single-photon responses

Single-photon responses (SPRs) in vertebrate rods are considerably less variable than expected if isomerized rhodopsin (R*) inactivated in a single, memoryless step, and no other variability-reducing mechanisms were available. We present a new stochastic model, the core of which is the successive ratcheting down of R* activity, and a concomitant increase in the probability of quenching of R* by arrestin (Arr), with each phosphorylation of R* (Gibson, S.K., J.H. Parkes, and P.A. Liebman. 2000. Biochemistry. 39:5738-5749.). We evaluated the model by means of Monte-Carlo simulations of dim-flash responses, and compared the response statistics derived from them with those obtained from empirical dim-flash data (Whitlock, G.G., and T.D. Lamb. 1999. Neuron. 23:337-351.). The model accounts for four quantitative measures of SPR reproducibility. It also reproduces qualitative features of rod responses obtained with altered nucleotide levels, and thus contradicts the conclusion that such responses imply that phosphorylation cannot dominate R* inactivation (Rieke, F., and D.A. Baylor. 1998a. Biophys. J. 75:1836-1857; Field, G.D., and F. Rieke. 2002. Neuron. 35:733-747.). Moreover, the model is able to reproduce the salient qualitative features of SPRs obtained from mouse rods that had been genetically modified with specific pathways of R* inactivation or Ca2+ feedback disabled. We present a theoretical analysis showing that the variability of the area under the SPR estimates the variability of integrated R* activity, and can provide a valid gauge of the number of R* inactivation steps. We show that there is a heretofore unappreciated tradeoff between variability of SPR amplitude and SPR duration that depends critically on the kinetics of inactivation of R* relative to the net kinetics of the downstream reactions in the cascade. Because of this dependence, neither the variability of SPR amplitude nor duration provides a reliable estimate of the underlying variability of integrated R* activity, and cannot be used to estimate the minimum number of R* inactivation steps. We conclude that multiple phosphorylation-dependent decrements in R* activity (with Arr-quench) can confer the observed reproducibility of rod SPRs; there is no compelling need to invoke a long series of non-phosphorylation dependent state changes in R* (as in Rieke, F., and D.A. Baylor. 1998a. Biophys. J. 75:1836-1857; Field, G.D., and F. Rieke. 2002. Neuron. 35:733-747.). Our analyses, plus data and modeling of others (Rieke, F., and D.A. Baylor. 1998a. Biophys. J. 75:1836-1857; Field, G.D., and F. Rieke. 2002. Neuron. 35:733-747.), also argue strongly against either feedback (including Ca2+-feedback) or depletion of any molecular species downstream to R* as the dominant cause of SPR reproducibility.     

Testing the correlation for clustered categorical and censored discrete time-to-event data when covariates are measured without/with errors

In the analysis of clustered categorical data, it is of common interest to test for the correlation within clusters, and the heterogeneity across different clusters. We address this problem by proposing a class of score tests for the null hypothesis that the variance components are zero in random effects models, for clustered nominal and ordinal categorical responses. We extend the results to accommodate clustered censored discrete time-to-event data. We next consider such tests in the situation where covariates are measured with errors. We propose using the SIMEX method to construct the score tests for the null hypothesis that the variance components are zero. Key advantages of the proposed score tests are that they can be easily implemented by fitting standard polytomous regression models and discrete failure time models, and that they are robust in the sense that no assumptions need to be made regarding the distributions of the random effects and the unobserved covariates. The asymptotic properties of the proposed tests are studied. We illustrate these tests by analyzing two data sets and evaluate their performance with simulations.     

Modulation of host immune responses by nematode cystatins

Parasitic nematodes, living in the intestinal tract or within tissues of theirs hosts, are constantly exposed to an array of immune effector mechanisms. One strategy to cope with the immune response is the release of immunomodulatory components that block effector mechanisms or interact with the cytokine network. Among the secreted nematode immunomodulators, cysteine protease inhibitors (cystatins) are shown to be of major importance. Nematode cystatins inhibit, among others, proteases involved in antigen processing and presentation, which leads to a reduction of T cell responses. At the same time nematode cystatins modulate cytokine responses, the most prominent trait being the upregulation of IL-10, a Th2 cytokine, by macrophages. In this situation, IL-10 leads among others to downregulation of costimulatory surface molecules of macrophages. These properties contribute to induction of an anti-inflammatory environment, concomitant with a strong inhibition of cellular proliferation. This setting is believed to favour the survival of worms. An opposite activity of nematode cystatins is the upregulation of production of inducible nitric oxide by IFN-gamma activated macrophages, an intrinsic property of natural cysteine protease inhibitors. This shows that these proteins can act as proinflammatory molecules under certain circumstances. A comparison of the immunomodulatory effects of cystatins of filarial nematodes with homologous proteins of the free-living nematode Caenorhabditis elegans revealed distinct differences. Caenorhabditis elegans cystatins induce the production of the Th1 cytokine IL-12, in contrast to filarial cystatins that upregulate IL-10. Caenorhabditis elegans cystatins hardly inhibit cellular proliferation. These data suggest that cystatins of parasitic nematodes have multiple, specific capacities for immunomodulation, acting in parallel on different immune effector mechanisms. Elucidation of the mechanisms involved might be useful in the development of immunotherapeutic reagents in the future.     

Water deficits and hydraulic limits to leaf water supply

Many aspects of plant water use -- particularly in response to soil drought -- may have as their basis the alteration of hydraulic conductance from soil to canopy. The regulation of plant water potential (Psi) by stomatal control and leaf area adjustment may be necessary to maximize water uptake on the one hand, while avoiding loss of hydraulic contact with the soil water on the other. Modelling the changes in hydraulic conductance with pressure gradients in the continuum allows the prediction of water use as a function of soil environment and plant architectural and xylem traits. Large differences in water use between species can be attributed in part to differences in their 'hydraulic equipment' that is presumably optimized for drawing water from a particular temporal and spatial niche in the soil environment. A number of studies have identified hydraulic limits as the cause of partial or complete foliar dieback in response to drought. The interactions between root:shoot ratio, rooting depth, xylem properties, and soil properties in influencing the limits to canopy water supply can be used to predict which combinations should optimize water use in a given circumstance. The hydraulic approach can improve our understanding of the coupling of canopy processes to soil environment, and the adaptive significance of stomatal behaviour.     

Fixed behaviours and migration in parasitic flatworms

This paper considers how fixed behaviours may play a role in post-larval migrations of Entobdella soleae. A general argument is that a shift away from the paradigm of orientation is required to elucidate the mechanisms that parasites use to navigate on the surface of their hosts. Some migrations may rely on fixed behaviours (genetically programmed stereotyped behaviours) that often evolve under predictable environmental conditions with reliable signals. In turbulent and stochastic free-living environments, homeostatic hosts present very predictable topological substrates and physico-chemical characteristics to their parasites. Over the course of evolution on these predictable host substrates, adaptive behaviours in the parasites can become fixed. Examples of endoparasite migration behaviour, particularly that of the common liver fluke, Fasciola hepatica, will be used to develop an approach based on the perceptual worlds of migrating parasites. An important conclusion is that multi-disciplinary approaches, firmly rooted in an understanding of each parasite's natural history, are requisite to successful interpretation of migration behaviours on the host.     

Variation in the life history pattern of Tetranychus urticae (Acari: Tetranychidae) after selection for dispersal

The spider mite Tetranychus urticae shows variation in its dispersal capacity (i.e., the leaf quality at which a female decides to disperse). We were able to artificially select mites that had either a high or a low dispersal capacity, indicating that this trait was genetically controlled. We then compared correlated responses to this selection. Mites with a genetically high dispersal capacity (‘HD’ strains) had a higher diapause incidence and a lower performance compared to mites with a low dispersal capacity (‘LD’ strains). A possible effect of random genetic drift during the selection was negligible. Our results suggest that differential dispersal capacity is associated with contrasting life history patterns as a result of natural selection. This revised version was published online in July 2006 with corrections to the Cover Date.     

Brassinosteroid-Mediated Stress Responses

Brassinosteroids (BRs) are a group of naturally occurring plant steroidal compounds with wide-ranging biological activity that offer the unique possibility of increasing crop yields through both changing plant metabolism and protecting plants from environmental stresses. In recent years, genetic and biochemical studies have established an essential role for BRs in plant development, and on this basis BRs have been given the stature of a phytohormone. A remarkable feature of BRs is their potential to increase resistance in plants to a wide spectrum of stresses, such as low and high temperatures, drought, high salt, and pathogen attack. Despite this, only a few studies aimed at understanding the mechanism by which BRs promote stress resistance have been undertaken. Studies of the BR signaling pathway and BR gene-regulating properties indicate that there is cross-talk between BRs and other hormones, including those with established roles in plant defense responses such as abscisic acid, jasmonic acid, and ethylene. Recent studies aimed at understanding how BRs modulate stress responses suggest that complex molecular changes underlie BR-induced stress tolerance in plants. Analyses of these changes should generate exciting results in the future and clarify whether the ability of BRs to increase plant resistance to a range of stresses lies in the complex interactions of BRs with other hormones. Future studies should also elucidate if BRI1, an essential component of the BR receptor, directly participates in stress response signaling through interactions with ligands and proteins involved in plant defense responses.