Waning of maternal immunity and the impact of diseases: the example of myxomatosis in natural rabbit populations

Myxomatosis is a leporipoxvirus that infects the european rabbit, inducing a high mortality rate. Observations lead us to hypothesize that a rabbit carrying maternal antibodies (or having recovered) can be infected (or re-infected) upon being exposed (or re-exposed) to the virus. Infection will lead to mild disease, boosting host immune protection. Using a modelling approach we show that this phenomenon may lead to a difference of impact of myxomatosis according to its transmission rate. Young are exposed when they still carry maternal antibodies and develop a mild disease in high transmission populations. Our results show that the impact of myxomatosis is generally higher in epidemic situations compared to populations where the virus circulates all the year. As a consequence, waning of acquired immunity and the continuous supply of newborn along the year may reduce the impact of the disease.     

Evolution of microparasites in spatially and genetically structured host populations: The example of RHDV infecting rabbits

Several studies have shown that classical results of microparasite evolution could not extend to the case where the host species shows an important spatial structure. Rabbit haemorrhagic disease virus (RHDV), responsible for rabbit haemorrhagic disease (RHD), which recently emerged in rabbits, has strains within a wide range of virulence, thus providing an interesting example of competition between strains infecting a host species with a metapopulation structure. In addition, rabbits may show a genetic diversity regarding RHDV susceptibility. In the present paper we use the example of the rabbit-RHDV interaction to study the competition between strains of a same microparasite in a host population that is both spatially and genetically structured. Using metapopulation models we show that the evolution of the microparasite is guided by a trade-off between its capacity to invade subpopulations potentially infected by other strains and its capacity to persist within the subpopulation. In such a context, host genetic diversity acts by reducing the number of hosts susceptible to each strain, often favouring more persistent—and generally less virulent—strains. We also show that even in a stochastic context where host genes regularly go locally extinct, the microparasite pressure helps maintain the genetic diversity in the long term while reinforcing gene loss risk in the short term. Finally, we study how different demographic and epidemiologic parameters affect the coevolution between the rabbit and RHDV.     

Female fecundity traits in wild populations of African annual fish: the role of the aridity gradient

The evolution of life history is shaped by life expectancy. Life‐history traits coevolve, and optimal states for particular traits are constrained by trade‐offs with other life‐history traits. Life histories contrast among species, but may also diverge intraspecifically, at the level of populations. We studied the evolution of female reproductive allocation strategy, using natural populations of two sympatric species of African annual fishes, Nothobranchius furzeri and Nothobranchius orthonotus. These species inhabit pools in the Mozambican savanna that are formed in the rainy season and persist for only 2–10 months. Using 207 female N. furzeri from 11 populations and 243 female N. orthonotus from 14 populations, we tested the effects of genetic background (intraspecific lineage) and life expectancy (position on the aridity gradient determining maximum duration of their temporary habitat) on female fecundity traits. First, we found that variation in female body mass was small within populations, but varied considerably among populations. Second, we found that fecundity was largely defined by female body mass and that females spawned most of their eggs in the morning. Third, we found that the trade‐off between egg size and egg number varied among lineages of N. furzeri and this outcome has been confirmed by data from two separate years. Overall, we demonstrate that local conditions were important determinants for Nothobranchius growth and fecundity and that eggs size in arid region was less limited by female fecundity than in humid region.     

Estimating the immunity coverage required to prevent epidemics in a community of households

An estimation of the immunity coverage needed to prevent future outbreaks of an infectious disease is considered for a community of households. Data on outbreak size in a sample of households from one epidemic are used to derive maximum likelihood estimates and confidence bounds for parameters of a stochastic model for disease transmission in a community of households. These parameter estimates induce estimates and confidence bounds for the basic reproduction number and the critical immunity coverage, which are the parameters of main interest when aiming at preventing major outbreaks in the future. The case when individuals are homogeneous, apart from the size of their household, is considered in detail. The generalization to the case with variable infectivity, susceptibility and/or mixing behaviour is discussed more briefly. The methods are illustrated with an application to data on influenza in Tecumseh, Michigan.     

Stochastic epidemics in dynamic populations: quasi-stationarity and extinction

Empirical evidence shows that childhood diseases persist in large communities whereas in smaller communities the epidemic goes extinct (and is later reintroduced by immigration). The present paper treats a stochastic model describing the spread of an infectious disease giving life-long immunity, in a community where individuals die and new individuals are born. The time to extinction of the disease starting in quasi-stationarity (conditional on non-extinction) is exponentially distributed. As the population size grows the epidemic process converges to a diffusion process. Properties of the limiting diffusion are used to obtain an approximate expression for τ, the mean-parameter in the exponential distribution of the time to extinction for the finite population. The expression is used to study how τ depends on the community size but also on certain properties of the disease/community: the basic reproduction number and the means and variances of the latency period, infectious period and life-length. Effects of introducing a vaccination program are also discussed as is the notion of the critical community size, defined as the size which distinguishes between the two qualitatively different behaviours. Received: 14 February 2000 / Revised version: 5 June 2000 / Published online: 24 November 2000     

Modeling the growth of individuals in plant populations: local density variation in a strand population of Xanthium strumarium (Asteraceae)

We studied the growth of individual Xanthium strumarium plants growing at four naturally occurring local densities on a beach in Maine: (1) isolated plants, (2) pairs of plants ≤1 cm apart, (3) four plants within 4 cm of each other, and (4) discrete dense clumps of 10-39 plants. A combination of nondestructive measurements every 2 wk and parallel calibration harvests provided very good estimates of the growth in aboveground biomass of over 400 individual plants over 8 wk and afforded the opportunity to fit explicit growth models to 293 of them. There was large individual variation in growth and resultant size within the population and within all densities. Local crowding played a role in determining plant size within the population: there were significant differences in final size between all densities except pairs and quadruples, which were almost identical. Overall, plants growing at higher densities were more variable in growth and final size than plants growing at lower densities, but this was due to increased variation among groups (greater variation in local density and/or greater environmental heterogeneity), not to increased variation within groups. Thus, there was no evidence of size asymmetric competition in this population. The growth of most plants was close to exponential over the study period, but half the plants were slightly better fit by a sigmoidal (logistic) model. The proportion of plants better fit by the logistic model increased with density and with initial plant size. The use of explicit growth models over several growth intervals to describe stand development can provide more biological content and more statistical power than "growth-size" methods that analyze growth intervals separately.     

From filaments to function: The role of the plant actin cytoskeleton in pathogen perception,signaling and immunity

The eukaryotic actin cytoskeleton is required for numerous cellular processes, including cell shape, development and movement, gene expression and signal transduction, and response to biotic and abiotic stress. In recent years,research in both plants and animal systems have described a function for actin as the ideal surveillance platform, linking the function and activity of primary physiological processes to the immune system. In this review, we will highlight recent advances that have defined the regulation and breadth of function of the actin cytoskeleton as a network required for defense signaling following pathogen infection. Coupled with an overview of recent work demonstrating specific targeting of the plant actin cytoskeleton by a diversity of pathogens,including bacteria, fungi and viruses, we will highlight the importance of actin as a key signaling hub in plants, one that mediates surveillance of cellular homeostasis and the activation of specific signaling responses following pathogen perception. B4 ased on the studies highlighted herein, we propose a working model that posits changes in actin filament organization is in and of itself a highly specific signal, which induces, regulates and physically directs stimulus-specific signaling processes, most importantly, those associated with response to pathogens.     

Determinants of reproductive success in voles: space use in relation to food and litter size manipulation

Spacing behaviour of female mammals is suggested to depend on the distribution and abundance of food. In addition, food limitation has been found to constrain the reproductive success of females. However, whether females maximize their reproductive success by adjusting space use in relation to current food availability and reproductive effort (e.g. litter size) has not been experimentally studied. We examined these questions by manipulating simultaneously food resources (control vs. food supplementation) and litter sizes (control vs. plus two pups) of territorial female bank voles (Clethrionomys glareolus) in large outdoor enclosures. Females with supplementary food had smaller home ranges (foraging area) and home range overlaps than control females, whereas litter size manipulation had no effect on space use. In contrast, the size of territory (exclusive area) was not affected by food supplementation or litter size manipulation. As we have previously shown elsewhere, extra food increases the reproductive success of bank vole females in terms of size and proportion of weaned offspring. According to the present data, greater overlap of female home ranges had a negative effect on reproductive success of females, particularly on survival of offspring. We conclude that higher food availability increases the reproductive success of bank vole females, and this effect may be mediated through lower vulnerability of offspring to direct killing and/or detrimental effects from other females in the population. Moreover, it seems that when density of conspecifics is controlled for, home range sizes of females, but not territoriality, is related to food resources in Clethrionomys voles.     

Coexistence of Banksia species in southwestern Australia: the role of regional and local processes

Abstract. 60 of the 75 Banksia species are confined to southwestern Australia where five or six species often coexist. We explored the role of regional species richness, niche differentiation, and habitat specialization in structuring banksia assemblages. The diversity of growth forms and categories of seed production and response to fire were assessed in actual assemblages at 40 sites throughout southwestern Australia. Diversity indices at each site were compared with those from null communities assembled on the basis of the abundance and sociability of taxa in regional species pools. The relationship between local and regional species richness suggests that processes at the scale of 100-m² quadrats limit local richness and therefore coexistence. However, there was no consistent evidence that taxa are differentiated by growth form or regeneration strategy. No particular biological profile makes a banksia adept at coexisting with a wide range of other taxa. Habitat specialization is an important factor contributing to lower local richness than would be predicted from niche differentiation of taxa in regional pools. There is recent empirical evidence of several mechanisms whereby the number of coexisting banksias is increased beyond the limits suggested by simple niche theories. Variability in the fire regime also provides a mechanism for maintaining local species richness because different fires favour recruitment of different taxa.     

Diversity and infection prevalence of endosymbionts in natural populations of the chestnut weevil: relevance of local climate and host plants

Many insects are ubiquitously associated with multiple endosymbionts, whose infection patterns often exhibit spatial and temporal variations. How such endosymbiont variations are relevant to local adaptation of the host organisms is of ecological interest. Here, we report a comprehensive survey of endosymbionts in natural populations of the chestnut weevil Curculio sikkimensis, whose larvae are notorious pests of cultivated chestnuts and also infest acorns of various wild oaks. From 968 insects representing 55 localities across the Japanese Archipelago and originating from 10 host plant species, we identified six distinct endosymbiont lineages, namely Curculioniphilus, Sodalis, Serratia, Wolbachia, Rickettsia and Spiroplasma, at different infection frequencies (96.7%, 12.8%, 82.3%, 82.5%, 28.2% and 6.8%, respectively) and with different geographical distribution patterns. Multiple endosymbiont infections were very common; 3.18±0.61 (ranging from 1.74 to 5.50) endosymbionts per insect on average in each of the local populations. Five pairs of endosymbionts (Curculioniphilus-Serratia, Curculioniphilus-Wolbachia, Sodalis-Rickettsia, Wolbachia-Rickettsia and Rickettsia-Spiroplasma) co-infected the same host individuals more frequently than expected, while infections with Serratia and Wolbachia were negatively correlated to each other. Infection frequencies of the endosymbionts were significantly correlated with climatic and ecological factors: for example, higher Sodalis, Wolbachia and Rickettsia infections at localities of higher temperature; lower Wolbachia and Rickettsia infections at localities of greater snowfall; and higher Curculioniphilus, Sodalis, Serratia, Wolbachia and Rickettsia infections on acorns than on chestnuts. These patterns are discussed in relation to potential host-endosymbiont co-evolution via local adaptation across geographical populations.     

Endangered wild yak (Bos grunniens) in the Tibetan plateau and adjacent regions: Population size,distribution, conservation perspectives and its relation to the domestic subspecies

The yak (Bos grunniens) is a long-haired bovid, endemic to the Tibetan Plateau and the adjacent high-altitude regions. The domesticated subspecies of yak (B. grunniens grunniens) are abundant and closely associated with the livelihoods of herders, while the wild subspecies of yak (B. grunniens mutus) are endangered due primarily to anthropogenic effects. The endangered status of wild yaks calls for consideration, if we are to secure its long term survival, hence this study. Here we hope to provide baseline information necessary for further research and protection of the wild yak resources. We use published data to discuss their evolution, their characteristics as well as their distribution in the Tibetan Plateau and the adjacent high-altitude regions. We were able to come up with a world wild yak distribution map, which may be useful for establishing protected areas, as well as updating the species IUCN Red List Status. From the data available, we were also able to provide an estimate of the wild yak population in China (∼22,000 wild yaks living in China), corresponding to 90% of the total world population. We further discuss the major threats to yaks, and we give some suggestions for future and sustainable conservation.     

Testing the role of phenotypic plasticity for local adaptation: growth and development in time-constrained Rana temporaria populations

Phenotypic plasticity can be important for local adaptation, because it enables individuals to survive in a novel environment until genetic changes have been accumulated by genetic accommodation. By analysing the relationship between development rate and growth rate, it can be determined whether plasticity in life-history traits is caused by changed physiology or behaviour. We extended this to examine whether plasticity had been aiding local adaptation, by investigating whether the plastic response had been fixed in locally adapted populations. Tadpoles from island populations of Rana temporaria, locally adapted to different pool-drying regimes, were monitored in a common garden. Individual differences in development rate were caused by different foraging efficiency. However, developmental plasticity was physiologically mediated by trading off growth against development rate. Surprisingly, plasticity has not aided local adaptation to time-stressed environments, because local adaptation was not caused by genetic assimilation but on selection on the standing genetic variation in development time.