Reproduction alternation in trees: testing the resource depletion hypothesis using experimental fruit removal in Quercus ilex

The keystones of resource budget models to explain mast seeding are that fruit production depletes tree stored resources, which become subsequently limiting to flower production the following year. These two hypotheses have, however, rarely been tested in forest trees. Using a fruit removal experiment, we tested whether preventing fruit development would increase nutrient and carbohydrates storage and modify allocation to reproduction and vegetative growth the following year. We removed all the fruits from nine adult Quercus ilex L. trees shortly after fruit set and compared, with nine control trees, the concentrations of nitrogen (N), phosphorus (P), zinc (Zn), potassium (K) and starch in leaves, twigs and trunk before, during and after the development of female flowers and fruits. The following year, we measured the production of vegetative and reproductive organs as well as their location on the new spring shoots. Fruit removal prevented the depletion of N and Zn in leaves during fruit growth. It also modified the seasonal dynamics in Zn, K and starch in twigs, but had no effect on reserves stored in the trunk. Fruit removal increased the production of female flowers and leaves the following year, and decreased the production of male flowers. Our results show that resource depletion operates differently for male and female flowering, because the timing of organ formation and the positioning of flowers in shoot architecture differ between male and female flowers. Our results suggest that N and Zn availability constrain flower production in Q. ilex, but also that other regulatory pathways might be involved. They strongly encourage further experiments manipulating fruit development over multiple years to describe the causal relationships between variations in resource storage and/or uptake, and male and female flower production in masting species.

Oak masting drivers vary between populations depending on their climatic environments

Large interannual variation in seed production, called masting, is very common in wind-pollinated tree populations and has profound implications for the dynamics of forest ecosystems and the epidemiology of certain human diseases.^1,2,3,4,5 Comparing the reproductive characteristics of populations established in climatically contrasting environments would provide powerful insight into masting mechanisms, but the required data are extremely scarce. We built a database from an unprecedented fine-scale 8-year survey of 150 sessile oak trees (Quercus petraea) from 15 populations distributed over a broad climatic gradient, including individual recordings of annual flowering effort, fruiting rate, and fruit production. Although oak masting was previously considered to depend mainly on fruiting rate variations,^6,7 we show that the female flowering effort is highly variable from year to year and explains most of the fruiting dynamics in two-thirds of the populations. What drives masting was found to differ among populations living under various climates. In soft-climate populations, the fruiting rate increases initially strongly with the flowering effort, and the intensity of masting results mainly from the flowering synchrony level between individuals. By contrast, the fruiting rate of harsh-climate populations depends mainly on spring weather, which ensures intense masting regardless of the flowering synchronization level. Our work highlights the need for jointly measuring flowering effort and fruit production to decipher the diversity of masting mechanisms among populations. Accounting for such diversity will be decisive in proposing accurate, and possibly contrasted, scenarios about future reproductive patterns of perennial plants with ongoing climate change and their numerous cascading effects.

Amplified Cyclicality in Mast Seeding Dynamics Positively Influences the Dynamics of a Seed Consumer Species

AbstractTemporal autocorrelation in environmental conditions influences population dynamics through its effects on vital rates. However, a comprehensive understanding of how and to what extent temporal autocorrelation shapes population dynamics is still lacking because most empirical studies have unrealistically assumed that environmental conditions are temporally independent. Mast seeding is a biological event characterized by highly fluctuating and synchronized seed production at the tree population scale as well as a marked negative temporal autocorrelation. In the current context of global change, mast seeding events are expected to become more frequent, leading to strengthened negative temporal autocorrelations and thereby amplified cyclicality in mast seeding dynamics. Theory predicts that population growth rates are maximized when the environmental cyclicality of consumer resources and their generation times are closely matched. To test this prediction, we took advantage of the long-term monitoring of a wild boar population, a widespread seed consumer species characterized by a short generation time (∼2 years). As expected, simulations indicated that its stochastic population growth rate increased as mast seeding dynamics became more negatively autocorrelated. Our findings demonstrate that accounting for temporal autocorrelations in environmental conditions relative to the generation time of the focal population is required, especially under conditions of global warming, where the cyclicality in resource dynamics is likely to change.

Mycobacterium tuberculosis genetic features associated with pulmonary tuberculosis severity

OBJECTIVES

Mycobacterium tuberculosis (Mtb) infections result in a wide spectrum of clinical presentations but without proven Mtb genetic determinants. Herein, we hypothesized that the genetic features of Mtb clinical isolates, such as specific polymorphisms or microdiversity, may be linked to tuberculosis (TB) severity.

METHODS

A total of 234 patients with pulmonary TB (including 193 drug-susceptible and 14 monoresistant cases diagnosed between 2017 and 2020 and 27 multidrug-resistant cases diagnosed between 2010 and 2020) were stratified according to TB disease severity, and Mtb genetic features were explored using whole genome sequencing, including heterologous single-nucleotide polymorphism (SNP), calling to explore microdiversity. Finally, we performed a structural equation modeling analysis to relate TB severity to Mtb genetic features.

RESULTS

The clinical isolates from patients with mild TB carried mutations in genes associated with host-pathogen interaction, whereas those from patients with moderate/severe TB carried mutations associated with regulatory mechanisms. Genome-wide association study identified an SNP in the promoter of the gene coding for the virulence regulator espR, statistically associated with moderate/severe disease. Structural equation modeling and model comparisons indicated that TB severity was associated with the detection of Mtb microdiversity within clinical isolates and to the espR SNP.

CONCLUSION

Taken together, these results provide a new insight to better understand TB pathophysiology and could provide a new prognosis tool for pulmonary TB severity.

The challenges, coping mechanisms, and recovery from the initial waves of the COVID-19 pandemic among academic radiographers

Introduction

The COVID-19 pandemic arrived in Europe in March 2020 and created major challenges across healthcare provision and for healthcare education programmes as well as having a major impact on society. Within the profession of Radiography changes in medical imaging, radiotherapy, and teaching practices have been reported along with the negative impact on radiographers and students. The aim of this study was to investigate key challenges relating to academic practice during the COIVD-19 pandemic; how radiography academics have coped and to identify recommendations for further support required to facilitate recovery of the academic faculty as the pandemic ebbs.

Methods

A survey was circulated using SurveyMonkey™ via personal, national and international networks, including the European Federation of Radiographer Societies (EFRS), to reach as many academic radiographers as possible. Open questions relating to the challenges of providing radiography education during the COVID-19 pandemic and the and coping strategies used were included. Thematic analysis was conducted using NVivo (QSR International, MA).

Results

533 academic radiographers responded to the whole survey from 43 different countries, with 340 responses relating to challenges and 327 for coping strategies. The main themes for the challenges were clinical practice, communication with colleagues, lack of face to face, managing change, students (support), and staff support. The coping strategy themes were communication with colleagues, physical exercise, self-care and wellbeing.

Conclusion

These data demonstrate a multitude of challenges for academic radiographers and the pressure they worked under during the first year of the COVID-19 pandemic was clear. However, the majority employed healthy coping strategies to help them deal with the pressure, uncertainty and trauma of the situation.

Implications for practice

COVID-19 had a significant impact on academic radiographers and while many reported good strategies for coping, the level of pressure is unsustainable. This study highlights the need to support academic radiographers to ensure a sustainable workforce.

The risk of burnout in academic radiographers during the COVID-19 pandemic

Introduction

The COVID-19 pandemic created major challenges across society, healthcare provision and also for those delivering healthcare education programmes. Clinical placements were disrupted and, in many incidences cancelled. Higher education institutions were required to move completely to online delivery methods with little notice. This created significant additional workload, stress and the need to learn new skills at a time of great uncertainty. This study explored the risks of burnout in academic radiographers during the first 12 months of the pandemic.

Methods

A survey was circulated using SurveyMonkey™ via personal, national and international networks, including the European Federation of Radiographer Societies (EFRS), to reach as many academic radiographers as possible. Disengagement and exhaustion were measured using the Oldenburg burnout inventory. Descriptive statistics and a one-way ANOVA were used to analyse the quantitative data using STATA V16 (Statacorp, TA).

Results

533 academic radiographers responded to the survey from 43 different countries. Mean disengagement was in the medium range and exhaustion was high for the total dataset. In a subset of countries with 10 or more responses, there was significant variation between countries, with the UK having highest mean exhaustion score and the UK, Ireland and France sharing the highest mean disengagement score. In the total dataset, 86% agreed workload had increased during the pandemic and 35% had considered leaving academia in the last year.

Conclusion

These data demonstrate the stark reality of the impact of the COVID-19 pandemic on academic radiographers’ workload, wellbeing, and intention to leave their roles.

Implications for practice

COVID-19 has had a significant impact on academic radiographers and this study highlights the urgent need for remedial measures to better support academic radiographers in order to ensure a sufficient, and sustainable workforce.

Counter-gradient variation of reproductive effort in a widely distributed temperate oak (Quercus petraea)

The genetic and phenotypic variability of life history traits determines the demographic attributes of tree populations and, thus, their responses to anthropogenic climate change. Growth- and survival-related traits have been widely studied in forest ecology, but little is known about the determinism of reproductive traits.Using an elevation gradient experiment in the Pyrenees we assessed the degree to which variations in reproductive effort along climatic gradients are environmentally or genetically driven, by comparing oak populations (Quercus petraea) growing under field and common garden conditions.In situ monitoring revealed a decline in reproductive effort with increasing elevation and decreasing temperature. In common garden conditions, significant genetic differentiation was observed between provenances for reproduction and growth: trees from cold environments (high elevations) grew more slowly, and produced larger acorns in larger numbers. Our observations show that genetic and phenotypic clines for reproductive traits have opposite signs (counter-gradient) along the environmental gradient as opposed to growth, for which genetic variation parallels phenotypic variation (co-gradient).The counter-gradient found here for reproductive effort reveals that genetic variation partly counteracts the phenotypic effect of temperature, moderating the change in reproductive effort according to temperature. We consider the possible contribution to this counter-gradient in reproductive effort as an evolutionary trade-off between reproduction and growth.

Rifampicin exposure reveals within-host Mycobacterium tuberculosis diversity in patients with delayed culture conversion

Mycobacterium tuberculosis (Mtb) genetic micro-diversity in clinical isolates may underline mycobacterial adaptation to tuberculosis (TB) infection and provide insights to anti-TB treatment response and emergence of resistance. Herein we followed within-host evolution of Mtb clinical isolates in two cohorts of TB patients, either with delayed Mtb culture conversion (> 2 months), or with fast culture conversion (< 2 months). We captured the genetic diversity of Mtb isolates obtained in each patient, by focusing on minor variants detected as unfixed single nucleotide polymorphisms (SNPs). To unmask antibiotic tolerant sub-populations, we exposed these isolates to rifampicin (RIF) prior to whole genome sequencing (WGS) analysis. Thanks to WGS, we detected at least 1 unfixed SNP within the Mtb isolates for 9/15 patients with delayed culture conversion, and non-synonymous (ns) SNPs for 8/15 patients. Furthermore, RIF exposure revealed 9 additional unfixed nsSNP from 6/15 isolates unlinked to drug resistance. By contrast, in the fast culture conversion cohort, RIF exposure only revealed 2 unfixed nsSNP from 2/20 patients. To better understand the dynamics of Mtb micro-diversity, we investigated the variant composition of a persistent Mtb clinical isolate before and after controlled stress experiments mimicking the course of TB disease. A minor variant, featuring a particular mycocerosates profile, became enriched during both RIF exposure and macrophage infection. The variant was associated with drug tolerance and intracellular persistence, consistent with the pharmacological modeling predicting increased risk of treatment failure. A thorough study of such variants not necessarily linked to canonical drug-resistance, but which are prone to promote anti-TB drug tolerance, may be crucial to prevent the subsequent emergence of resistance. Taken together, the present findings support the further exploration of Mtb micro-diversity as a promising tool to detect patients at risk of poorly responding to anti-TB treatment, ultimately allowing improved and personalized TB management.

Tuberculosis (TB) is caused by Mycobacterium tuberculosis (Mtb), bacteria that are able to persist inside the patient for many months or years, thus requiring long antibiotic treatments. Here we focused on TB patients with delayed response to treatment and we performed genetic characterization of Mtb isolates to search for sub-populations that may tolerate anti-TB drugs. We found that Mtb cultured from 9/15 patients contained different sub-populations, and in vitro drug exposure revealed Mtb sub-populations in 6/15 isolates, none related to known drug-resistance mechanisms. By contrast, drug exposure revealed Mtb sup-populations in 2/20 isolates in the control cohort of patients with fast culture conversion. Furthermore, we characterized a Mtb variant isolated from a sub-population growing in the presence of rifampicin (RIF), a major anti-TB drug. We found that this variant featured a modified lipidic envelope, and that it was able to develop in the presence of RIF and inside human macrophage cells. We performed pharmacological modelling and found that this kind of variant may be related to a poor response to treatment. In conclusion, searching for particular Mtb sub-populations may help to detect patients at risk of treatment failure and provide additional guidance for TB management.

Resource manipulation through experimental defoliation has legacy effects on allocation to reproductive and vegetative organs in Quercus ilex

BACKGROUND AND AIMS

In plants, high costs of reproduction during some years can induce trade-offs in resource allocation with other functions such as growth, survival and resistance against herbivores or extreme abiotic conditions, but also with subsequent reproduction. Such trade-offs might also occur following resource shortage at particular moments of the reproductive cycle. Because plants are modular organisms, strategies for resource allocation to reproduction can also vary among hierarchical levels. Using a defoliation experiment, our aim was to test how allocation to reproduction was impacted by resource limitation.

METHODS

We applied three levels of defoliation (control, moderate and intense) to branches of eight Quercus ilex trees shortly after fruit initiation and measured the effects of resource limitation induced by leaf removal on fruit development (survival, growth and germination potential) and on the production of vegetative and reproductive organs the year following defoliation.

KEY RESULTS

We found that defoliation had little impact on fruit development. Fruit survival was not affected by the intense defoliation treatment, but was reduced by moderate defoliation, and this result could not be explained by an upregulation of photosynthesis. Mature fruit mass was not affected by defoliation, nor was seed germination success. However, in the following spring defoliated branches produced fewer shoots and compensated for leaf loss by overproducing leaves at the expense of flowers. Therefore, resource shortage decreased resource allocation to reproduction the following season but did not affect sex ratio.

CONCLUSIONS

Our results support the idea of a regulation of resource allocation to reproduction beyond the shoot scale. Defoliation had larger legacy effects than immediate effects.

How does increasing mast seeding frequency affect population dynamics of seed consumers? Wild boar as a case study

Mast seeding in temperate oak populations shapes the dynamics of seed consumers and numerous communities. Mast seeding responds positively to warm spring temperatures and is therefore expected to increase under global warming. We investigated the potential effects of changes in oak mast seeding on wild boar population dynamics, a widespread and abundant consumer species. Using long-term monitoring data, we showed that abundant acorn production enhances the proportion of breeding females. With a body-mass-structured population model and a fixed hunting rate of 0.424, we showed that high acorn production over time would lead to an average wild boar population growth rate of 1.197 whereas non-acorn production would lead to a stable population. Finally, using climate projections and a mechanistic model linking weather data to oak reproduction, we predicted that mast seeding frequency might increase over the next century, which would lead to increase in both wild boar population size and the magnitude of its temporal variation. Our study provides rare evidence that some species could greatly benefit from global warming thanks to higher food availability and therefore highlights the importance of investigating the cascading effects of changing weather conditions on the dynamics of wild animal populations to reliably assess the effects of climate change.

Bacterial Transformation Buffers Environmental Fluctuations through the Reversible Integration of Mobile Genetic Elements

Horizontal gene transfer (HGT) promotes the spread of genes within bacterial communities. Among the HGT mechanisms, natural transformation stands out as being encoded by the bacterial core genome. Natural transformation is often viewed as a way to acquire new genes and to generate genetic mixing within bacterial populations. Another recently proposed function is the curing of bacterial genomes of their infectious parasitic mobile genetic elements (MGEs). Here, we propose that these seemingly opposing theoretical points of view can be unified. Although costly for bacterial cells, MGEs can carry functions that are at points in time beneficial to bacteria under stressful conditions (e.g., antibiotic resistance genes). Using computational modeling, we show that, in stochastic environments, an intermediate transformation rate maximizes bacterial fitness by allowing the reversible integration of MGEs carrying resistance genes, although these MGEs are costly for host cell replication. Based on this dual function (MGE acquisition and removal), transformation would be a key mechanism for stabilizing the bacterial genome in the long term, and this would explain its striking conservation.IMPORTANCE Natural transformation is the acquisition, controlled by bacteria, of extracellular DNA and is one of the most common mechanisms of horizontal gene transfer, promoting the spread of resistance genes. However, its evolutionary function remains elusive, and two main roles have been proposed: (i) the new gene acquisition and genetic mixing within bacterial populations and (ii) the removal of infectious parasitic mobile genetic elements (MGEs). While the first one promotes genetic diversification, the other one promotes the removal of foreign DNA and thus genome stability, making these two functions apparently antagonistic. Using a computational model, we show that intermediate transformation rates, commonly observed in bacteria, allow the acquisition then removal of MGEs. The transient acquisition of costly MGEs with resistance genes maximizes bacterial fitness in environments with stochastic stress exposure. Thus, transformation would ensure both a strong dynamic of the bacterial genome in the short term and its long-term stabilization.

Flower phenology as a disruptor of the fruiting dynamics in temperate oak species

Many perennial plants display masting, that is, fruiting with strong interannual variations, irregular and synchronized between trees within the population. Here, we tested the hypothesis that the early flower phenology in temperate oak species promotes stochasticity into their fruiting dynamics, which could play a major role in tree reproductive success. From a large field monitoring network, we compared the pollen phenology between temperate and Mediterranean oak species. Then, focusing on temperate oak species, we explored the influence of the weather around the time of budburst and flowering on seed production, and simulated with a mechanistic model the consequences that an evolutionary shifting of flower phenology would have on fruiting dynamics. Temperate oak species release pollen earlier in the season than do Mediterranean oak species. Such early flowering in temperate oak species results in pollen often being released during unfavorable weather conditions and frequently results in reproductive failure. If pollen release were delayed as a result of natural selection, fruiting dynamics would exhibit much reduced stochastic variation. We propose that early flower phenology might be adaptive by making mast-seeding years rare and unpredictable, which would greatly help in controlling the dynamics of seed consumers.

Subcultured Mycobacterium tuberculosis isolates on different growth media are fully representative of bacteria within clinical samples

Mycobacterium tuberculosis (Mtb) whole genome sequencing (WGS) plays an increasingly important role in tuberculosis diagnosis and research. WGS is typically performed on biobanked isolates obtained by subculture during diagnosis. Genetic variation upon culturing is known to occur in other bacterial species. However, little is understood regarding the impact of different subculture media on genome-wide diversity and variant selection in Mtb. Here we compared WGS derived from direct sequencing of sputa samples to WGS sequences from isolates subcultured on 3 different media. Based on analysis of single nucleotide polymorphisms (SNPs), there was no evidence of variant selection caused by the different culture media used, indicating that subcultured clinical strains can be reliably used to explore genetic determinants of Mtb pathogenesis and epidemiological features.

Increasing spring temperatures favor oak seed production in temperate areas

The changes in reproductive phenology (i.e. timing of flowering and fruiting) observed in recent decades demonstrate that tree reproduction has already been altered by climate change. However, understanding the impact of these changes in reproductive success and fitness remains a major challenge for ecologists. We describe here a previously unreported phenomenon: a significant increase in the reproductive effort (seed production) of temperate oaks with increasing spring temperature, observed over the last decade. In contrast, no relationship was found between seed production and precipitation. This sensitivity of seed production to temperature was confirmed by a "space-for-time" substitution based on elevation gradients. Our findings suggest that global warming may enhance oak reproductive effort in temperate ecosystems. Nevertheless, while fitness can be enhanced by higher levels of seed production, it also depends on the frequency and synchronization of mast seeding production, which may also be influenced by climate change.

Ecological networks to unravel the routes to horizontal transposon transfers

Transposable elements (TEs) represent the single largest component of numerous eukaryotic genomes, and their activity and dispersal constitute an important force fostering evolutionary innovation. The horizontal transfer of TEs (HTT) between eukaryotic species is a common and widespread phenomenon that has had a profound impact on TE dynamics and, consequently, on the evolutionary trajectory of many species' lineages. However, the mechanisms promoting HTT remain largely unknown. In this article, we argue that network theory combined with functional ecology provides a robust conceptual framework and tools to delineate how complex interactions between diverse organisms may act in synergy to promote HTTs.

From income to capital breeding: when diversified strategies sustain species coexistence

There is empirical evidence of many diversified ways for energy to be acquired and allocated to reproduction, notably with strategies ranging from strict income breeding (females fueling their gametes with energy gained concomitantly during reproduction) to strict capital breeding (females storing nutrients prior reproduction). Until now, the question of whether diversification of these strategies might impact the way communities are organized has not been considered. Here, we suggest that diversified resource allocation strategies among competing species may contribute to their coexistence. We examined this hypothesis by focusing on communities composed of four phytophagous insect species that coexist and compete for egg-laying sites. From wild-caught females, we determined precisely the breeding period of each species and we described their resource acquisition and allocation to reproduction dynamics. We quantified in each species the total amount of larval energy stored by newly-emerging females and then monitored the total energy budget of females caught in the field before and throughout their breeding period. We found that the four sibling weevil species are markedly segregated along the income-capital-breeding continuum, which is correlated with clear time partitioning in their laying activity. Our results suggest that diversified resource allocation strategies might contribute to time partitioning of plant resources exploitation and thus indirectly to their coexistence. This work should further encourage studies examining the extent to which competitive coexistence might be affected by diversification of income-capital breeding strategies together with the intensity of interspecific competition, and considering the divergent evolution of these strategies.

Endosymbiont diversity among sibling weevil species competing for the same resource

BACKGROUND

Whereas the impact of endosymbionts on the ecology of their hosts is well known in some insect species, the question of whether host communities are influenced by endosymbionts remains largely unanswered. Notably, the coexistence of host species competing with each other, which is expected to be stabilized by their ecological differences, could be facilitated by differences in their endosymbionts. Yet, the composition of endosymbiotic communities housed by natural communities of competing host species is still almost unknown. In this study, we started filling this gap by describing and comparing the bacterial endosymbiotic communities of four sibling weevil species (Curculio spp.) that compete with each other to lay eggs into oak acorns (Quercus spp.) and exhibit marked ecological differences.

RESULTS

All four species housed the primary endosymbiont Candidatus Curculioniphilus buchneri, yet each of these had a clearly distinct community of secondary endosymbionts, including Rickettsia, Spiroplasma, and two Wolbachia strains. Notably, three weevil species harbored their own predominant facultative endosymbiont and possessed the remaining symbionts at a residual infection level.

CONCLUSIONS

The four competing species clearly harbor distinct endosymbiotic communities. We discuss how such endosymbiotic communities could spread and keep distinct in the four insect species, and how these symbionts might affect the organization and species richness of host communities.

Make love not war: when should less competitive males choose low-quality but defendable females?

Male choosiness for mates is an underexplored mechanism of sexual selection. A few theoretical studies suggest that males may exhibit--but only under rare circumstances--a reversed male mate choice (RMMC; i.e., highly competitive males focus on the most fecund females, while the low-quality males exclusively pair with less fecund mates to avoid being outcompeted by stronger rivals). Here we propose a new model to explore RMMC by relaxing some of the restrictive assumptions of the previous models and by considering an extended range of factors known to alter the strength of sexual selection (males' investment in reproduction, difference of quality between females, operational sex ratio). Unexpectedly, we found that males exhibited a reversed mate choice under a wide range of circumstances. RMMC mostly occurs when the female encounter rate is high and males devote much of their time to breeding. This condition-dependent strategy occurs even if there is no risk of injury during the male-male contest or when the difference in quality between females is small. RMMC should thus be a widespread yet underestimated component of sexual selection and should largely contribute to the assortative pairing patterns observed in numerous taxa.

Dynamics of transposable elements: towards a community ecology of the genome

Like ecological communities, which vary in species composition, eukaryote genomes differ in the amount and diversity of transposable elements (TEs) that they harbor. Given that TEs have a considerable impact on the biology of their host species, we need to better understand whether their dynamics reflects some form of organization or is primarily driven by stochastic processes. Here, we borrow ecological concepts on species diversity to explore how interactions between TEs can contribute to structure TE communities within their genomic ecosystem. Whereas the niche theory predicts a stable diversity of TEs because of their divergent characteristics, the neutral theory of biodiversity predicts the assembly of TE communities from stochastic processes acting at the level of the individual TE. Contrary to ecological communities, however, TE communities are shaped by selection at the level of their ecosystem (i.e. the host individual). Developing ecological models specific to the genome will thus be a prerequisite for modeling the dynamics of TEs.

Dynamics of transposable elements: towards a community ecology of the genome

Like ecological communities, which vary in species composition, eukaryote genomes differ in the amount and diversity of transposable elements (TEs) that they harbor. Given that TEs have a considerable impact on the biology of their host species, we need to better understand whether their dynamics reflects some form of organization or is primarily driven by stochastic processes. Here, we borrow ecological concepts on species diversity to explore how interactions between TEs can contribute to structure TE communities within their genomic ecosystem. Whereas the niche theory predicts a stable diversity of TEs because of their divergent characteristics, the neutral theory of biodiversity predicts the assembly of TE communities from stochastic processes acting at the level of the individual TE. Contrary to ecological communities, however, TE communities are shaped by selection at the level of their ecosystem (i.e. the host individual). Developing ecological models specific to the genome will thus be a prerequisite for modeling the dynamics of TEs.

Unexpected male choosiness for mates in a spider

Sexual selection theory traditionally considers choosiness for mates to be negatively related to intra-sexual competition. Males were classically considered to be the competing, but not the choosy, sex. However, evidence of male choosiness is now accumulating. Male choosiness is expected to increase with an individual's competitive ability, and to decrease as intra-sexual competition increases. However, such predictions have never been tested in field conditions. Here, we explore male mate choice in a spider by studying size-assortative pairing in two natural sites that strongly differ in the level of male-male competition. Unexpectedly, our results demonstrate that mate choice shifts from opportunism to high selectivity as competition between males increases. Males experiencing weak competition did not exhibit size-related mating preferences. By contrast, when competition was intense we found strong size-assortative pairing due to male choice: while larger, more competitive males preferentially paired with larger, more fecund females, smaller males chose smaller females. Thus, we show that mating preferences of males vary with their competitive ability. The distinct preferences exhibited by males of different sizes seem to be an adaptive response to the lower reproductive opportunities arising from increased competition between males.

Dynamic optimization over infinite-time horizon: web-building strategy in an orb-weaving spider as a case study

Dynamic state-dependent models have been widely developed since 1990s for solving questions in evolutionary ecology. Up to now, these models were mainly run over finite-time horizon. However, for many biological questions an infinite-time horizon perspective could be more appropriate, especially when the end of the modeled period is state- rather than time-dependent. Despite this approach is widely used in the field of economics and operational research, thus far no work has been providing biologists with a general method to solve infinite-time horizon problems. Here we present such a method, through the exhaustive description of an algorithm that we implement to determine the strategy an organism should follow to reach a particular state as fast as possible while limiting mortality risk. To illustrate that method we explored web-building behavior in an orb-weaving spider. How are adult females predicted to build their successive webs to gain energy, grow, and lay their first clutch as fast as possible, without suffering from either predation or starvation? From this example, we first show how an optimal strategy over infinite-time horizon can be processed and selected. Second, we analyse variations of the optimal web-building strategy along with the spider's body weight and predation risk during web building. Our model yields two main predictions: (1) spiders reduce their web size as they are gaining weight due to body-mass-dependent cost of web-building behavior, and (2) this reduction in web size starts at lower weight under higher predation risk.

Spider webs designed for rare but life-saving catches

The impact of rare but positive events on the design of organisms has been largely ignored, probably due to the paucity of recordings of such events and to the difficulty of estimating their impact on lifetime reproductive success. In this respect, we investigated the size of spider webs in relation to rare but large prey catches. First, we collected field data on a short time-scale using the common orb-weaving spider Zygiella x-notata to determine the distribution of the size of prey caught and to quantify the relationship between web size and daily capture success. Second, we explored, with an energetic model, the consequences of an increase in web size on spider fitness. Our results showed that (i) the great majority of prey caught are quite small (body length less than 2mm) while large prey (length greater than 10mm) are rare, (ii) spiders cannot survive or produce eggs without catching these large but rare prey and (iii) increasing web size increases the daily number of prey caught and thus long-term survival and fecundity. Spider webs seem, therefore, designed for making the best of the rare but crucial event of catching large prey.

Body-mass-dependent cost of web-building behavior in an orb weaving spider, Zygiella x-notata

In numerous spider species, reproductive success of adult females has been shown to be positively correlated with their body mass. We suggest, however, that spiders may incur greater foraging costs as their body mass increases due to the numerous and complex locomotor bouts needed to build an orb-web. Such a body-mass-dependent cost should, in turn, affect the web-building decisions of spiders. In the laboratory, we tested the influence of body mass on energetic expenditure (measured as mass loss) during web-building behavior in Zygiella x-notata. Our results showed (1) that energetic costs associated with web-building were closely related to body mass and to web-building activity, and (2) that as their body mass increased, spiders reduced the amount of silk used per web, while their foraging effort simultaneously increased. This work gives new insights into web-building behavior and energy allocation strategies of weaving spiders.

Web-building behaviour in the orb-weaving spider Zygiella x-notata: influence of experience

Zygiella x-notata is an orb-weaving spider that often renews its trap daily. Web building has associated costs and benefits, and building successive webs may have consequences for lifetime reproductive success. In the laboratory, we tested the ability of Z. x-notata to modify its building behaviour in response to various stages in predation (prey detection, capture and ingestion) experienced with a previous web. We determined which stages provided information for the spiders. Spiders that detected, captured and ingested prey and then rebuilt their web used less silk and made a smaller capture area than in the previous web. There was no effect of prey detection alone on the next web. Capture without feeding gave the same results as capture followed by feeding. The spiders that ate prey without detection and capture (feeding by hand) had the same energetic gains as spiders that caught prey but delayed building a new web. The spiders thus showed plasticity in web-building behaviour and in the amount of silk used (energetic investment) in the short term (from one web to the next). Changes in body condition may therefore influence web construction. Moreover, information gained during prey capture appeared to influence the size and structure of the next web. This ability should enable spiders to adapt their web building to maximize their fitness. Copyright 2000 The Association for the Study of Animal Behaviour.