Samuel Venner

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.

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 2 mm) while large prey (length greater than 10 mm) 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.

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.

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 individuals 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.

A handbook for uncovering the complete energetic budget in insects: the van Handel's method (1985) revisited

Insects comprise relevant biological models for investigating nutrient acquisition and allocation processes in the context of life‐history ecology and evolution. However, empirical investigations are still partly limited by the lack of availability of simple methods for simultaneously estimating the four major energetic components (i.e. lipids, free sugars, glycogen and proteins) in the same individual. In the present work, we validate a fast, reproducible and cheap method for overcoming this problem that uses different solvents successively. First, proteins are solubilized in a phosphate‐lysis buffer and then quantified according to the classical Bradford assay procedure. In a second step, a chloroform–methanol mixture is added to the aqueous phase, which allows assay of the total lipid fraction, as well as the free sugars and glycogen in the same insect homogenate. In addition, a micro‐separation procedure is adapted to partition the total lipids into neutral (mainly stored lipids) and polar (mainly structural lipids) components. Although these assays are conducted sequentially in the same individual, the sensitivity of our method remains high: the estimated amount of each energetic compartment does not differ from that obtained with former, partial methods. Our method should thus largely improve our knowledge about nutrient acquisition and allocation among insects not only in laboratory‐reared individuals, but also in animals caught in the wild. Descriptions and recommendations are given at each step of the protocol to adapt the procedure to various insect species. Finally, to prevent misinterpretation of data generated in accordance with this protocol, the limits of our method are discussed in the light of life‐history studies.

Coexistence of Insect Species competing for a pulsed resource: toward a unified theory of biodiversity in fluctuating environments

Background One major challenge in understanding how biodiversity is organized is finding out whether communities of competing species are shaped exclusively by species-level differences in ecological traits (niche theory), exclusively by random processes (neutral theory of biodiversity), or by both processes simultaneously. Communities of species competing for a pulsed resource are a suitable system for testing these theories: due to marked fluctuations in resource availability, the theories yield very different predictions about the timing of resource use and the synchronization of the population dynamics between the competing species. Accordingly, we explored mechanisms that might promote the local coexistence of phytophagous insects (four sister species of the genus Curculio) competing for oak acorns, a pulsed resource. Methodology/Principal Findings We analyzed the time partitioning of the exploitation of oak acorns by the four weevil species in two independent communities, and we assessed the level of synchronization in their population dynamics. In accordance with the niche theory, overall these species exhibited marked time partitioning of resource use, both within a given year and between different years owing to different dormancy strategies between species, as well as distinct demographic patterns. Two of the four weevil species, however, consistently exploited the resource during the same period of the year, exhibited a similar dormancy pattern, and did not show any significant difference in their population dynamics. Conclusions/Significance The marked time partitioning of the resource use appears as a keystone of the coexistence of these competing insect species, except for two of them which are demographically nearly equivalent. Communities of consumers of pulsed resources thus seem to offer a promising avenue for developing a unifying theory of biodiversity in fluctuating environments which might predict the co-occurrence, within the same community, of species that are ecologically either very similar, or very different.

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.

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.

Mate-guarding strategies and male competitive ability in an orb-weaving spider : results from a field study

Competition between males to reproduce has led to various forms of resource-guarding behaviour. A recent theoretical investigation suggested that guarding strategies should be closely linked to the risk of take-over and the competitive ability of males, but experimental evidence is still scarce. We investigated precopulatory mate guarding of premoult females in a solitary orb-weaving spider Zygiella x - notata (Araneae: Araneidae). We tested the hypotheses that larger males should (1) be more successful at guarding, (2) acquire their mates by usurping them from other males (take-over), and (3) start guarding females as close as possible to their final moult. In a field study, we recorded the mating activity of marked males throughout a breeding season and conducted a male removal experiment to suppress male–male competition near randomly selected guarded females. We found strong male–male competition for guarding premoult females. Male take-over occurred frequently in the population, even though guarding was efficient at locally reducing male turnover near females. Large males were overall more successful in their guarding attempts because of their higher competitive ability, but guarding success did not increase linearly with body size. Large males mainly acquired females by evicting previous guardians. They spontaneously started guarding females closer to their moult than small males; however, this was not significant when no direct competition occurred between them. We discuss possible mechanisms to explain variation in guarding duration and success and the evolution of male body size in this spider.

Estimation of the Web’s Capture Thread Length in Orb-Weaving Spiders: Determining the Most Efficient Formula

Abstract Orb-weaving spiders modulate their web-building behavior in response to changes in their situation (internal or external state), and this capacity is expected to directly influence fitness. To understand the rules underlying the investment devoted to web construction, we need to quantify precisely the costs and benefits associated with web-building behavior. The sticky silk (capture spiral) is costly to produce and to set up and is a limiting factor in web building. Thus, it would be useful to have an efficient formula to calculate the total capture thread length (CTL) produced, which reflects the investment devoted to foraging activities. On the basis of web photographs we tested nine formulae on webs built by three spider species: Zygiella x-notata (Clerck), Araneus diadematus Clerck, and Larinioides sclopetarius (Clerck). We compared the calculated lengths with lengths directly measured from photographs. We showed that the most efficient formula (CTLvh) involved data recorded along both the horizontal and vertical axes. Because of its accuracy for all species tested, together with its ease of use in the field, this formula represents a useful tool for behavioral ecology studies on spiders.