Duelling timescales of host movement and disease recovery determine invasion of disease in structured populations

The epidemic potential of a disease is traditionally assessed using the basic reproductive number, R ₀. However, in populations with social or spatial structure a chronic disease is more likely to invade than an acute disease with the same R ₀, because it persists longer within each group and allows for more host movement between groups. Acute diseases 'perceive' a more structured host population, and it is more important to consider host population structure in analyses of these diseases. The probability of a pandemic does not arise independently from characteristics of either the host or disease, but rather from the interaction of host movement and disease recovery timescales. The R _* statistic, a group-level equivalent of R ₀, is a better indicator of disease invasion in structured populations than the individual-level R ₀.     

Paramecium Has Two Regulatory Subunits of Cyclic AMP-Dependent Protein Kinase, One Unique to Cilia

ABSTRACT. The subunit composition and intracellular location of the two forms of cAMP-dependent protein kinase of Paramecium cilia were determined using antibodies against the 40-kDa catalytic (C) and 44-kDa regulatory (R₄₄) subunits of the 70-kDa cAMP-dependent protein kinase purified from deciliated cell bodies. Both C and R₄₄ were present in soluble and particulate fractions of cilia and deciliated cells. Crude cilia and a soluble ciliary extract contained a 48-kDa protein (R₄₈) weakly recognized by one of several monoclonal antibodies against R₄₄, but not recognized by an anti-R₄₄ polyclonal serum. Gel-filtration chromatography of a soluble ciliary extract resolved a 220-kDa form containing C and R₄₈ and a 70-kDa form containing C and R₄₄. In the large enzyme, R₄₈ was the only protein to be autophosphorylated under conditions that allow autophosphorylation of R₄₄ The subunits of the large enzyme subsequently were purified to homogeneity by cAMP-agarose chromatography. Both C and R₄₈ were retained by the column and eluted with 1 M NaCl; no other proteins were purified in this step. These results confirm that the ciliary cAMP-dependent protein kinases have indistinguishable C subunits, but different R subunits. The small ciliary enzyme, like the cell-body enzyme, contains R₄₄, whereas R₄₈ is the R subunit of the large enzyme.     

Transmission assumptions generate conflicting predictions in host-vector disease models: a case study in West Nile virus

This review synthesizes the conflicting outbreak predictions generated by different biological assumptions in host–vector disease models. It is motivated by the North American outbreak of West Nile virus, an emerging infectious disease that has prompted at least five dynamical modelling studies. Mathematical models have long proven successful in investigating the dynamics and control of infectious disease systems. The underlying assumptions in these epidemiological models determine their mathematical structure, and therefore influence their predictions. A crucial assumption is the host–vector interaction encapsulated in the disease-transmission term, and a key prediction is the basic reproduction number, R ₀. We connect these two model elements by demonstrating how the choice of transmission term qualitatively and quantitatively alters R ₀ and therefore alters predicted disease dynamics and control implications. Whereas some transmission terms predict that reducing the host population will reduce disease outbreaks, others predict that this will exacerbate infection risk. These conflicting predictions are reconciled by understanding that different transmission terms apply biologically only at certain population densities, outside which they can generate erroneous predictions. For West Nile virus, R ₀ estimates for six common North American bird species indicate that all would be effective outbreak hosts.     

Somaclonal variation in rice – Submergence tolerance and other agronomic characters

The ability to withstand complete submergence was studied in R₂ seedlings raised by self-pollination from 158 R₁ plants regenerated from callus of mature rice seeds (cultivars FR13A and Calrose). Compared to parental controls, significant improvement in submergence tolerance as assessed by percentage survival, number of new leaves and chlorophyll content of the third leaf, was found in 5 of the 115 cv. FR13A somaclones and 3 of the 43 cv. Calrose somaclones tested using an aquarium system in a controlled-temperature glasshouse. With some exceptions these improvements were also observed in the R₃ generation when tested under field conditions in Thailand. Variation in other agronomically important characters was observed in the R₂ plants grown in a temperature glasshouse under 2 contrasting environments.     

Relationship between photosynthetic radiation-use efficiency of barley canopies and the photochemical reflectance index (PRI)

Some processes of excess radiation dissipation have been associated with changes in leaf reflectance near 531 nm. We aimed to study the relations between the photochemical reflectance index (PRI) derived from this signal, and photosynthetic radiation-use efficiency (defined as net CO₂ assimilation rate/incident photon flux density) in a cereal canopy. Measurements of reflectance, fluorescence, gas exchange and xanthophyll cycle pigments were made in the morning, midday and afternoon in barley canopies with two levels of nitrogen fertilization. The photosynthetic radiation-use efficiency decreased at midday, mainly in the third leaf, in both treatments, with lower values for the nitrogen deficient leaves. The zeaxanthin content showed the inverse pattern, increasing at midday and in the nitrogen deficient treatment. The photosynthetic radiation-use efficiency was well correlated with the epoxidation state, EPS (violaxanthin + 0.5 antheraxanthin)/(violaxanthin + antheraxanthin + zeaxanthin). The PRI [here defined as (R₅₃₉ - R₅₇₀)/(R₅₃₉+ R₅₇₀)] was significantly correlated with epoxidation state and zeaxanthin and with photosynthetic radiation-use efficiency. These results validate the utility of PRI in the assessment of radiation-use efficiency at canopy level.     

Herbicide multiple-resistance in a Lolium rigidum biotype is endowed by multiple mechanisms: isolation of a subset with resistant acetyl-CoA carboxylase

The development of herbicide multiple-resistance in weed species represents a major threat to current agricultural practices. The mechanistic basis for herbicide multiple-resistance has been investigated in a population of the annual grass weed Lolium rigidum Gaud. (annual ryegrass) resistant to herbicides affecting 6 target sites. A subset of the resistant population (R₂ subset) has been isolated by germination on a medium containing the acetyl-CoA carboxylase (ACCase, EC 6.4.1.2) inhibiting herbicide, sethoxydim ((2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one)). This 12% R₂ subset of the population is 600 times more resistant to sethoxydim and between 30 to 200 times more resistant to other ACCase inhibitors than the bulk of the R population. The subset has a form of ACCase which is 6 to 55 times less sensitive to inhibition by these herbicides than the enzyme present in the bulk of the resistant or in the susceptible population. There was no difference in the uptake and metabolic degradation of [4-¹⁴C]sethoxydim between the R₂ subset and the unselected R population. These results show the accumulation of different resistance mechanisms in that single population. Furthermore we propose that this accumulation of multiple resistance mechanisms is the basis for herbicide multiple-resistance in this biotype.     

Parasite strain coexistence in a heterogeneous host population

Heterogeneity in host susceptibility and transmissibility to parasite attack allows a lower transmission rate to sustain an epidemic than is required in homogeneous host populations. However, this heterogeneity can leave some hosts with little susceptibility to disease, and at high transmission rates, epidemic size can be smaller than for diseases where the host population is homogeneous. In a heterogeneous host population, we model natural selection in a parasite population where host heterogeneity is exploited by different strains to varying degrees. This partitioning of the host population allows coexistence of competing parasite strains, with the heterogeneity-exploiting strains infecting the more susceptible hosts, in the absence of physiological tradeoffs and spatial heterogeneity, and even for markedly different transmission rates. In our model, intermediate-strategy parasites were selected against: should coexistence occur, an equilibrium is reached where strains occupied only the extreme ends of trait space, under appropriate conditions selecting for lower R₀.     

EFFECTS OF TEMPERATURE ON CERTAIN POPULATION PARAMETERS OF LIRIOMYZA SATIVAE BLAN-CHARD (DIPTERA: AGROMYZIDAE)

Abstract  Effects of temperature on population parameters and the intrinsic rate of natural increase of the leafminer, Liriomyza sativae Blanchard, were studied at constant temperatures, 15, 20, 25, 30 and 3593 80% RH and a photoperiod of 12 :12 (L:D) in the laboratory with Phaseolus vulgaris as the host plant. Developmental time of the immature stage decreased from 38 d at 15C to lld at 35⁹3 Regression equations relating temperature ( t ) to development rates ( y ) for egg, larval and pupal stages, were y = 1.7862t - 13.841, y = 1.162 t - 4.946 and y = 0.634 t - 5.146, respectively. Longevity of female adult decreased from 20 d at 15°C to 9 d with temperature up to 35C The most favorable temperature range for reproduction was 20°C - 30^oC in which the fecundity ranged from 158 to 282 eggs per female. The lowest total mortality was 9% at 25^oC and the highest was 49% at 35 93 High intrinsic rate of natural increase ( r_m ) was 0.27 and high net reproductive rate ( R₀ ) was 116.8 at temperature range between 25 t and 30 t indicating that this range was optimal for population growth and that population density might increase 117 times per generation under this temperature condition. Mean generation time (T) and time for population to double (t) decreased as temperature increased, showing a negative linear trend with temperature. The relationship between finite rate of increase (A) and temperature, however, was a positive linear regression.     

The 44-kDa Regulatory Subunit of the Paramecium cAMP-Dependent Protein Kinase Lacks a Dimerization Domain and May Have a Unique Autophosphorylation Site Sequence

ABSTRACT. The 44-kDa regulatory subunit (R₄₄) of one form of cAMP-dependent protein kinase of Paramecium was purified, and two partial internal amino acid sequences from it were used to clone the corresponding cDNA. This R₄₄ cDNA clone was 1022-bp long, including 978 bp of coding sequence and 7 bp and 37 bp of 5' and 3' untranslated sequences, respectively. A 1.1-kb mRNA was labeled on a Northern blot. The deduced R₄₄ amino acid sequence had 31%–38% positional identity to the sequences of other cloned cAMP-dependent protein kinase regulatory subunits. R₄₄ sequence showed equal sequence similarity to mammalian types I and II regulatory subunits. The N -terminal sequence encoding the regulatory subunit dimerization domain found in most regulatory subunits is not present in the R₄₄ clone, confirming the lack of regulatory subunit dimer formation previously reported for the Paramecium cAMP-dependent protein kinase. The putative autophosphorylation site of R₄₄ contains the amino acid sequence TRTS, distinct from the consensus sequence RRXS, where X is any residue, found in other autophosphorylated cAMP-dependent protein kinase regulatory subunits and many cAMP-dependent protein kinase substrates.     

Elasticity analysis in epidemiology: an application to tick-borne infections

The application of projection matrices in population biology to plant and animal populations has a parallel in infectious disease ecology when next-generation matrices (NGMs) are used to characterize growth in numbers of infected hosts ( R ₀). The NGM is appropriate for multi-host pathogens, where each matrix element represents the number of cases of one type of host arising from a single infected individual of another type. For projection matrices, calculations of the sensitivity and elasticity of the population growth rate to changes in the matrix elements has generated insight into plant and animal populations. These same perturbation analyses can be used for infectious disease systems. To illustrate this in detail we parameterized an NGM for seven tick-borne zoonoses and compared them in terms of the contributions to R ₀ from three different routes of transmission between ticks, and between ticks and vertebrate hosts. The definition of host type may be the species of the host or the route of infection, or, as was the case for the set of tick-borne pathogens, a combination of species and the life stage at infection. This freedom means that there is a broad range of disease systems and questions for which the methodology is appropriate.     

Constraints on parasite fecundity and transmission in an insect-STD system

Parasites that are sexually transmitted (causing sexually transmitted diseases, or STDs) can have important effects on host population dynamics, but we know almost nothing for such parasites about constraints on fecundity and transmission. In this study, we have examined the effect of two potentially important constraints in one of the few empirically well-studied animal-STD systems, the ladybird, Adalia bipunctata , and its sexually transmitted mite, Coccipolipus hippodamiae . Using a factorial design, we manipulated: (i) within-host competition, by varying infection intensity; and (ii) host condition, by introducing dietary stress. Infection with C . hippodamiae significantly reduced ladybird survival whether or not diet was restricted, and restricting diet led to reduced survival regardless of infection status. Increased infection intensity and reduced host condition (dietary stress) both independently constrained per capita rates of parasite egg production and the development of infective larvae. Furthermore, when host condition was compromised, significantly fewer larvae were transmitted per adult mite during copulation. The effect of infection intensity on per mite transmission was more complex: there was no significant effect when hosts were fed normally, but when the ladybirds were nutritionally stressed higher infection intensity was associated with a slight increase in the numbers that were transmitted (despite the fact that mite fecundity was reduced under these conditions). These results indicate that host condition and within-host competition may both play an important role in shaping the epidemiology of the A . bipunctata – C . hippodamiae system, by influencing the parasite's basic reproductive ratio (R₀) and the rate of epidemic spread. Our data also extend general insights into STD ecology, by highlighting the importance of constraints on disease dynamics that are likely to be widespread.     

Transgenic rice plants: Characterization of two generations of seed progeny

Transgenic rice plants have been regenerated from kanamycin-resistant callus of Oryza sativa (cv. Taipei 309) derived from protoplasts electroporated with pCaMVNEO carrying the neomycin phosphotransferase II ( nptII ) gene. Of 6 randomly selected plants, all contained the nptll gene, but only 2 plants expressed NPTII activity. The transgenic plants were significantly shorter, produced fewer tillers, took longer to flower and had reduced fertility compared to non-transformed protoplastderived plants. Fifty-six seeds collected from one transgenic plant expressing NPTII activity germinated on medium containing kanamycin sulphate to give 16 green, first seed generation (R₁) plants. The latter could be divided into 3 groups: (i) Plants which set seed, had normal floret morphology and produced a total of 76 seeds; (ii) Plants which flowered, but which failed to set seed; (iii) Plants which failed to flower, were shorter and had significantly fewer tillers than plants of groups (i) and (ii). The nptII gene was present in all transgenic R₁ plants, but only 8 plants expressed the gene. Phenotypic characteristics, observed in transgenic R₁ plants were also seen in the transforned R₂ plants. These included reduced stature, a longer vegetative phase and reduced fertility compared to non-transformed plants.     

Sibling cannibalism among juvenile vundu under controlled conditions. II. Effect of body weight and environmental variables on the periodicity and intensity of type II cannibalism

Cannibalism among starved groups of juvenile (19–48 days old) vundu catfish Heterobranchus longifilis was 66·5% nocturnal, and its impact under modified day length was proportional to the duration of the dark phase. Shallow depth and high population density decreased the intensity of cannibalism, whereas low density and deeper environments had an opposite effect. The presence of refuges had no significant effect on cannibalism. The maintenance ( R _maint) and maximum ( R _max) daily food rations (% day⁻¹) of cannibals feeding on live prey were modelled as R _maint=3·899 W _C^0·327 ( r ²=0·684; d.f.=31), and R _max=49.545 W _C^0·321 ( r ²=0·999; d.f.=5), where W _C was the body weight of the cannibal (g). The latter model indicated that the impact of a cannibal on a population decreased by a 20% margin each time the cannibal doubled its body weight, and suggested that cannibalism among vundu would become insignificant for cannibals heavier than 30 g. The significance of these findings is discussed within the contexts of vundu aquaculture and of general, conceptual models of the dynamics of cannibalism among fishes.     

The intrinsic mean time to extinction: a unifying approach to analysing persistence and viability of populations

Analysing the persistence and viability of small populations is a key issue in extinction theory and population viability analysis. However, there is still no consensus on how to quantify persistence and viability. We present an approach to evaluate any simulation model concerned with extinction. The approach is devised from general Markov models of stochastic population dynamics. From these models, we distil insights into the general mathematical structure of the risk of extinction by time t, P₀(t). From this mathematical structure, we devise a simple but effective protocol – the ln(1−P₀)-plot – which is applicable for situations including environmental noise or catastrophes. This plot delivers two quantities which are fundamental to the assessment of persistence and viability: the intrinsic mean time to extinction, T_m, and the probability c₁ of the population reaching the established phase. The established phase is characterized by typical fluctuations of the population's state variable which can be described by quasi-stationary probability distributions. The risk of extinction in the established phase is constant and given by 1/T_m. We show that T_m is the basic currency for the assessment of persistence and viability because T_m is independent of initial conditions and allows the risk of extinction to be calculated for any time horizon. For situations where initial conditions are important, additionally c₁ has to be considered.     

Threshold conditions for infection persistence in complex host-vectors interactions

As classically defined by Macdonald in the early 1950s, for the case of diseases with one vector and one host, the Basic Reproduction Number, R0, is defined as the number of secondary infections caused by a single infective of the same type (vector or host) during its infectiousness period in an entirely susceptible population. In the case of a disease which has one vector and one host, it is easy to show that R0 coincides with the threshold for the establishment of an endemic state: if R0 > 1 (< 1), the disease can invade (cannot invade) the host population. In this paper we examine various epidemic situations in which there are more than one vector and/or host. We show that in those more complex systems it is not possible to deduce a single R0 but rather a threshold for infection persistence which is a composite of several quantities closely related to the classical expression of R0. Another definition of R0 given by Diekmann, Heesterbeek and Metz, and denoted in this paper R0NGO is discussed and applied as an alternative to calculate the thresholds for infection establishment.     

Salinity-related fitness differences amongst GPI genotypes in the mosquitofish Gambusia holbrooki (Poeciliidae: Teleostei)

Salinity-induced mortality was examined in the mosquitofish Gambusia holbrooki to determine if natural selection associated with the tidal inundation of lower reach populations could potentially maintain glucose-6-phosphate isomerase ( GPI -2; E.C. 5.3.1.9) allele frequency variation observed at field sites. In vitro experiments established if survival was independent of GPI -2 genotype, and/or standard body length (S.L.). Bioassay results at 17%₀ were consistent with lower survival of GPI -2*88/88 homozygote genotypes, GPI -2*100/100 genotypes in juvenile size classes, and greater survival of GPI -2*109/100 genotypes; 100% mortality was observed at salinities of 30%₀. In the early stages of the 30%₀ bioassay, juvenile fish carrying each GPI -2 allele suffered similar, high rates of mortality. Following 19 weeks exposure to 4–8%₀ NaCl the frequency of GPI -2*88 alleles decreased in adult G. holbrooki (> 17 mm SL). Salinity-induced mortality was found to be a selective influence capable of creating and maintaining GPI -2 allele frequency differences among G. holbrooki subpopulations, and/or between sex/size cohorts within subpopulations, but only at intermediate salinities. Whether saltwater inundation at field sites maintains inter-population diversity and produces local adaptation will be determined by the age structure of the population during periods of exposure, fluctuations in salinity as determined by the relationship between rainfall and tidal regime, and the level of immigration from non-affected sites.     

Pressure-volume analyses on shoots of Picea abies and leaves of Coffea liberica at various temperatures

Shoots from two ecologically different evergreen tree species, Picea abies (L.) Karst and Coffea liberica Hiern, were used to carry out pressure volume (PV) measurements at 5–35°C. For this purpose a pressure chamber was equipped with thermoelectric temperature regulation. The non-linear sections of the resultant PV curves were sigmoidal for both species, with recognizable points of inflexion. These points, at around ψ= 1.12 MPa and relative water content (RWC) = 88.5% for Picea and at ψ= 0.92 MPa and RWC = 95.5% for Coffea , were characterised by a temporary increase in the resistance to water flow of the entire shoot (R₅).
The maximum value of the bulk modulus of elasticity (ɛ max) was also in the region of the point of inflexion. This value was considerably higher in Coffea than in Picea . The osmotic pressures at full water saturation (π₀) and at turgor loss point (π_p) showed a clear temperature dependence between 15 and 35°C differing only slightly from the theoretically expected situation. At 25°C these values were 1.72 and 2.48 MPa. respectively, for Picea and 1.58 and 1.87 MPa. respectively, for Coffea . The turgor loss point occurred at 76–77% RWC in Picea and at 86% RWC in Coffea , the proportion of apoplastic water varied between 22 and 25% in Picea but was only 9–10% in Coffea . The ecological differences between the two species are reflected in their temperature dependence for R₅, which was much steeper for Coffea than for Picea . The energy of activation for the water conductance of the whole shoots was 13.0–14.4 kJ mol⁻¹ in Picea and about 23 kJ mol⁻¹ in Coffea .     

Population variability among three small mammal species in the semiarid Neotropics: the role of density-dependent and density-independent factors

We addressed the role of density-dependent (direct and delayed) and density-independent (precipitation) factors in shaping the dynamics of fluctuating populations of three small mammal species. Using a stepwise regression procedure, we tested the effects of nonlagged population density (log₁₀ N_t-1), lagged population density (log₁₀ N_t-2), and annual precipitation on the per capita rate of population change of Phyllotis darwini, Akodon olivaceus , and Thylamys elegans in two habitat types of a semiarid region of Chile. The most irruptive species ( P. darwini ) showed direct and delayed density-dependent effects in equatorial subpopulation, and only direct density-dependence in polar subpopulation. The per capita rates of population change of A. olivaceus showed direct density-dependent and precipitation effects in both habitats types, while T. elegans showed direct density-dependence and precipitation effects in the equatorial subpopulation but only a marginal effect of direct density-dependence in the polar subpopulation. The presence of delayed density-dependent strongly suggests the importance of biological interactions in shaping the dramatic irruptions exhibited by P. darwini.     

Assessing mortality changes from size‐frequency curves

The relationship between the instantaneous mortality rate ( Z ) and the instantaneous change in length‐frequency distribution of organisms per unit of animal size (μ _L ) takes the following form: Z  = μ _L k ( L _∞ −  L ) +  k , where k and L _∞ are coefficients of the von Bertalanffy equation, and L is organism size (length). Z and μ_L change coherently when they are measured for a specific size or age class. Therefore, observations of μ _L can provide information on the relative changes in mortality. This is useful when no precise information about animal growth is available and growth curve is assumed to be invariable. This method was tested on a heavily exploited population of St Peter's fish (mango tilapia) Sarotherodon galilaeus in Lake Kinneret, Israel, where large fluctuations in the size structure of the catch have occurred over the past few years. Analysis of the changes in the length‐frequency distributions showed that the changes in μ _L over multiple years estimated for fully exploited fish reflected the respective changes in Z .