Der p 1 is the primary activator of Der p 3, Der p 6 and Der p 9 the proteolytic allergens produced by the house dust mite Dermatophagoides pteronyssinus.

BACKGROUND: The enzymatic activity of the four proteases found in the house dust mite Dermatophagoides pteronyssinus is involved in the pathogenesis of allergy. Our aim was to elucidate the activation cascade of their corresponding precursor forms and particularly to highlight the interconnection between proteases during this cascade. METHODS: The cleavage of the four peptides corresponding to the mite zymogen activation sites was studied on the basis of the Förster Resonance Energy Transfer method. The proDer p 6 zymogen was then produced in Pichia pastoris to elucidate its activation mechanism by mite proteases, especially Der p 1. The role of the propeptide in the inhibition of the enzymatic activity of Der p 6 was also examined. Finally, the Der p 1 and Der p 6 proteases were localised via immunolocalisation in D. pteronyssinus. RESULTS: All peptides were specifically cleaved by Der p 1, such as proDer p 6. The propeptide of proDer p 6 inhibited the proteolytic activity of Der p 6, but once cleaved, it was degraded by the protease. The Der p 1 and Der p 6 proteases were both localised to the midgut of the mite. CONCLUSIONS: Der p 1 in either its recombinant form or in the natural context of house dust mite extracts specifically cleaves all zymogens, thus establishing its role as a major activator of both mite cysteine and serine proteases. GENERAL SIGNIFICANCE: This finding suggests that Der p 1 may be valuable target against mites.

Impact of living with kin/non-kin on the life history traits of Tetranychus urticae (Acari: Tetranychidae).

In many vertebrates and invertebrates, living in a group may influence the life history traits, physiology and behaviour of its individual members, whereas genetic relatedness affects social interactions among individuals in a group. The two-spotted spider mite Tetranychus urticae is characterised by a communal organization, in which silk production plays a key role. A silken web protects the colony against biotic and abiotic agents such as predators, competitors, humidity, wind, rain and acaricides. To evaluate the potential costs and benefits of being associated with genetically distant vs genetically close individuals in T. urticae, we assessed various fitness indicators (faecal pellet production, fecundity, death rate) in pure and mixed groups of two distinct populations of T. urticae: a red-form population from Tunisia and a green-form population from Belgium. If genetic origin had no influence, the values of fitness indicators in mixed groups composed of green and red individuals, would be intermediate between those of the pure green-form and red-form groups. Our results show that in a mixed group, faecal pellet production and death rate were statistically similar to the values obtained in the pure group of green-form individuals. Therefore, our study suggests that strain recognition ability may occur in T. urticae and that the genetic background of an individual may have a great impact on several of its life history traits.

The silk road of Tetranychus urticae: is it a single or a double lane?

Tetranychus urticae (Acari: Tetranychidae) is a phytophagous mite that forms huge colonies. All active members of a colony (immatures and matures, females and males) spin silken threads. These mites construct a common web that protects the colony from external aggression. The silk coverage is well-known to provide advantages to the colony but very little is known about the characteristics of the threads themselves. Here is the first quantification of the diameter of silken threads spun by two different stages (adult females and larvae) and its relationship with body size of the spinning individuals. Moreover, we observed how silk was deposited on the substrate through their two pedipalps. Threads were observed by means of transmission electron and fluorescence microscopy. Silken threads spun by larvae (0.055 ± 0.018 µm) were significantly thinner than threads spun by adult females (0.111 ± 0.038 µm). In the first step of the silk depositing behaviour, the mite attached the thread to the substrate by putting its pedipalps in contact with the surface (adhesion, double silken threads). When walking, silken threads became detached from the substrate and spitted up (silken threads were free). Finally, silken threads adhered to the surface. The presence of single and double threads makes thread diameter highly variable.

Testing for collective choices in the two-spotted spider mite.

Silk is a vector for collective behaviour in many spinning arthropods, including social spiders, social caterpillars, and some spider mites. In this study, the potential for silk-mediated collective choices is evaluated for the two-spotted spider mite Tetranychus urticae. This subsocial mite lives in large colonies on plants, sheltered under a collectively spun silk web. The silk has an attractive and arresting effect. We test whether the silk trails left by the spider mites can give rise to the collective choice of a path. The experiment consists in offering two identical paths to a group of migrating mites. Our results show that the presence of a silk trail influences the mites, but not sufficiently to systematically provoke a collective choice. In order to determine the trail following potential of T. urticae, we parameterise a theoretical trail following model to fit our experiments and those found in the literature. Our prediction is that even after a large number of mites have passed (200), a systematic collective choice of path should not be expected under the tested conditions. Our results, combined with what is known from the literature, allow us to propose a general scenario for the dispersal behaviour of T. urticae.

Probiotic Escherichia coli Nissle 1917 activates DC and prevents house dust mite allergy through a TLR4-dependent pathway.

Experimental animal and human studies have demonstrated that probiotic strains have beneficial effects on allergy. Here we report that the probiotic Escherichia coli Nissle 1917 strain (EcN) is able to activate DC, as shown by important cytokine synthesis together with up-regulation of membrane expression of CD40, CD80 and CD86. This EcN-induced DC activation was strictly dependent on the TLR4 signaling pathway and was also associated with stimulation of NF-kappaB and MAPK. We next investigated the prophylactic potential of i.n. co-administration of EcN with a recombinant form of Der p 1 (ProDer p 1) in a murine model of mite allergy. I.n. vaccinations with EcN plus ProDer p 1 prevented the subsequent allergic response following Der p 1 sensitization and airway challenge with aerosolized mite extracts through the induction of an allergen-specific IgG2a response, the prevention of specific IgE production and a strong reduction of IL-5 secretion by allergen-restimulated splenocytes. EcN alone or in combination with ProDer p 1 inhibited the development of airway eosinophilia and neutrophilia. This in vivo protective effect of EcN was, in part, mediated by TLR4 signaling. Our results suggest that EcN represents an efficient adjuvant to prevent allergic responses.

Effect of retentive markers on the dynamics of settlement: The case of arthropod silk.

During a settlement decision, the presence of conspecifics is crucial to species subject to Allee effects, for which the number of founders affects the subsequent growth of the colony. Marking the area (physically or chemically) conveys information about the number of conspecifics present in a new patch. Here, we study how an individual affinity for the marker affects the dynamics of a foundation process. A generic population model is presented, in which marking and affinity for the marker are at stake. Our results show that population size thresholds can appear, below which settlement is not possible. This model is then used to study the dynamics of migration and aggregation in a set of interconnected populations. We show that affinity for the marker can induce asymmetries in the population distribution. Anelosimus eximius is a social spider subject to Allee effects, for which silk potentially acts as a marker. We test our predictions with field experiments involving two populations of A. eximius in a Y-shaped setup. The agreement between our experimental and theoretical results strongly supports the validity of the model. This allows us to use the model to estimate a realistic set of parameters of biological significance to this social spider.

Triggering and persistence of trail-laying in foragers of the ant Lasius niger.

In the ant Lasius niger, the ability to ingest their own desired volume is the key criterion that rules the recruiting behaviour of scouts. This volume acts as a threshold triggering the trail-laying response of foragers. In this paper, we show that this desired volume is specific to each individual and is kept constant over successive trips to a food source. This individual specificity contrasts with the variability of all individual desired volumes within the colony. In this study, it is also shown that, among L. niger foragers, 14% never participate in the formation of the chemical pathway and never lay a trail over successive trips. Among the others foragers, interindividual differences in the persistence of trail-laying behaviour over successive trips are observed but do not rely on an individual specialisation, in which some ants would lay a trail more frequently and persistently than other scouts. We discuss how an individual in the foraging behaviour can play an essential role in the regulation of food retrieval dynamics.

Regulation of ants' foraging to resource productivity.

We investigate the behavioural rule used by ant societies to adjust their foraging response to the honeydew productivity of aphids. When a scout finds a single food source, the decision to lay a recruitment trail is an all-or-none response based on the opportunity for this scout to ingest a desired volume acting as a threshold. Here, we demonstrate, through experimental and theoretical approaches, the generic value of this recruitment rule that remains valid when ants have to forage on multiple small sugar feeders to reach their desired volume. Moreover, our experiments show that when ants decide to recruit nest-mates they lay trail marks of equal intensity, whatever the number of food sources visited. A model based on the 'desired volume' rule of recruitment as well as on experimentally validated parameter values was built to investigate how ant societies adjust their foraging response to the honeydew productivity profile of aphids. Simulations predict that, with such recruiting rules, the percentage of recruiting ants is directly related to the total production of honeydew. Moreover, an optimal number of foragers exists that maximizes the strength of recruitment, this number being linearly related to the total production of honeydew by the aphid colony. The 'desired volume' recruitment rule that should be generic for all ant species is enough to explain how ants optimize trail recruitment and select aphid colonies or other liquid food resources according to their productivity profile.

Reaching the ball or missing the flight? Collective dispersal in the two-spotted spider mite Tetranychus urticae.

The two-spotted spider mite is a worldwide phytophagous pest displaying a peculiar dispersal. At high density, when plants are exhausted, individuals gather at the plant apex to form a collective silk-ball. This structure can be dispersed by wind or phoresy. Individuals initiating the ball are enclosed in the centre and have a high risk to die. For the first time, the ultimate and proximate mechanisms leading to this group dispersal are examined. To explore if a particular mite genotype was involved in the ball formation, plants were infested with individuals of different genetic background. After the silk-ball formation, the mites in the ball and those remaining on the plant were collected and genotyped. The balls were harvested after 4h and 24h to determine the role of timing between the formation and dispersal on the mortality of mites. Mites do not segregate according to their degree of relatedness, stage, or sex. Mites parallel humans using public transportation: they climb up in the ball whatever their genetic background. Silk-balls composed of unrelated individuals may help avoiding inbreeding when colonizing a new plant. Our results also emphasize the importance of an adequate timing for efficient dispersal between the time spent between ball formation and dispersal.

Should I lay or should I wait? Egg-laying in the two-spotted spider mite Tetranychus urticae Koch.

Optimality theory predicts that females tend to maximize their offspring survival by choosing the egg-laying site. In this context, the use of conspecific cues allows a more reliable assessment of the habitat quality. To test this hypothesis, Tetranychus urticae Koch is an appropriate biological model as it is a phytophagous mite living in group, protected against external aggression by a common web. Experiments were conducted to determine the respective influence of substrate (living substrate: bean leaf vs. non-living substrate: glass plate), silk and presence of conspecific eggs on the egg-laying behavior of T. urticae females. On both living and non-living substrates, the presence of silk positively influenced the probability of a female to lay an egg, but had no influence on the number of eggs deposited. The egg-laying behavior was mainly determined by the nature of the substrate with mites laying fewer eggs on a non-living substrate than on a living one. The presence of a conspecific egg had no impact on either the probability of laying an egg or on the oviposition rate. This study showed a high variability among females in their fecundity and egg-laying performance. The physiology of females (individual fecundity), the egg-laying substrate and to a lesser extent the presence of silk impacted on the decision of spider mites to lay eggs.

Settlement decisions by the two-spotted spider mite Tetranychus urticae.

In silk-spinning arthropods, silk can be used for web building, protection, and communication. Silk is an informative material about the presence of conspecifics. It can therefore inform on habitat suitability and hence assist in habitat choice. In this context, we investigated the influence of silk on microhabitat choice by the two-spotted spider mite, Tetranychus urticae. Three factors that could potentially influence habitat choice were manipulated: the strain, number, and the stage of mites. Our study showed that these factors all influence the choice of microhabitat. The tendency of whether to settle on a silk-covered area was influenced by the origin of mites (strain effect). Adult females showed a higher tendency to settle on an area covered with the silk laid by numerous congeners (number effect). Moreover, larvae seemed to be more responsive to the presence of silk than adults (stage effect). This suggests that individuals use silk as a social cue in selecting their microhabitat and that the spatial organization and group behaviour seem to be shaped by the individuals' response to social cues, such as the amount of silk already present.

The locomotor activities on sites covered by silk produced by related and unrelated spider mites in Tetranychus urticae (Acari: Tetranychidae).

The two-spotted spider mite Tetranychus urticae is a silk producer known to live in groups with a common silk web that can cover entire plants and protect mites against predators, rain and wind. Silk also plays an important role during collective migration by aerial dispersal or by walking. In this context, we studied the locomotor activity i.e. time in movement, in resting, and in static exploration (probing behaviour) of an individual confronted to clean places and/or places covered by silk coming from its own population or from another. Two populations were tested, one of the red form (Tunisia) and another of the green form (Belgium). The experimental results showed that the presence or the absence of silk did not influence the relative proportion of each behavioural item either for the red or the green population. Individuals of the green form population spent more time moving and less time resting than individuals of the red form population and this, whatever the substrate (red/green silk, clean). Moreover, the silk from the red form population attracted individuals from both populations, whereas the silk produced by the green form population did not attract any individual either from the red or the green form. This surprising result might have been due to a difference in the quantity and/or quality of silk woven by the two forms. We discuss how the differences observed in the behaviour of these two populations may result from differences in their strategy to rapidly increase the population of the colony and reinforce the silk nest.

Discrimination through silk recognition: the case of the two-spotted spider mite Tetranychus urticae.

Choosing a suitable habitat is a main step in the settlement process, particularly for species having weak movement abilities. Reliable cues are thus needed for habitat selection. In silk-spinning arthropods, silk can be used as a social cue to select an appropriate location. Silk can also provide information on the presence of related or non-related individuals. In this article, we compare the settlement behaviour of two strains of the two-spotted spider mite, Tetranychus urticae, in response to the presence of silk woven by an individual of its own or another strain. We then examined how individuals behaved when confronted with both types of silk (own/another strain) simultaneously. Both strains were sensitive to related silk. Settlement decision for both strains did not differ according to the origin of the silk. Mites used the silk as a communication cue for habitat selection and strain discrimination. Our results provide experimental evidence for the use of multiple social cues in the settlement decision by weaving mites.

The formation of collective silk balls in the spider mite Tetranychus urticae Koch.

Tetranychus urticae is a phytophagous mite that forms colonies of several thousand individuals. These mites construct a common web to protect the colony. When plants become overcrowded and food resources become scarce, individuals gather at the plant apex to form a ball composed of mites and their silk threads. This ball is a structure facilitating group dispersal by wind or animal transport. Until now, no quantitative study had been done on this collective form of migration. This is the first attempt to understand the mechanisms that underlie the emergence and growth of the ball. We studied this collective behaviour under laboratory conditions on standardized infested plants. Our results show that the collective displacement and the formation of balls result from a recruitment process: by depositing silk threads on their way up to the plant apex, mites favour and amplify the recruitment toward the balls. A critical threshold (quorum response) in the cumulative flow of mites must be reached to observe the emergence of a ball. At the beginning of the balls formation, mites form an aggregate. After 24 hours, the aggregated mites are trapped inside the silk balls by the complex network of silk threads and finally die, except for recently arrived individuals. The balls are mainly composed of immature stages. Our study reconstructs the key events that lead to the formation of silk balls. They suggest that the interplay between mites' density, plant morphology and plant density lead to different modes of dispersions (individual or collective) and under what conditions populations might adopt a collective strategy rather than one that is individually oriented. Moreover, our results lead to discuss two aspects of the cooperation and altruism: the importance of Allee effects during colonization of new plants and the importance of the size of a founding group.

Food transport in ants: Do Lasius niger foragers maximize their individual load?

The decision for a Lasius niger forager to lay a chemical trail and launch recruitment to a food source is governed by an internal individual threshold. The value of this threshold triggering chemical communication is not set by the maximal capacity of the crop. Actually, trail-laying ants are still able to drink additional food encountered on their homeward journey. The partial filling of the crop by trail-laying ants may be a means for the ants to shorten foraging trips and to speed up the information updating within the nest. Moreover, by partially filling their crop, foragers keep a potential for sampling resources and for tasting other encountered food sources.

Spatial distribution and inbreeding in Tetranychus urticae.

In group living, species spatial distribution results from responses to environmental heterogeneity and/or mutual interactions between individuals. These mutual interactions can be regulated by genetic and/or epigenetic factors. In this study, we focus on genetic factors and investigate how the spatial distribution of some individuals colonizing a new environment is influenced by inbreeding. Our biological model is Tetranychus urticae, a phytophagous mite considered as a major pest of many cultivated plants. Groups of T. urticae were composed of individuals from successive inbreeding (sister-brother sib-mating). Our results show that the inter-individual distances increase with inbreeding. Indeed, inbreeding level seems to be an important factor affecting the intra-plant spatial distribution of mites. These results confirm that mites have the capability to discriminate their kin and, moreover, that they are able to accurately perceive differences between close relatives from sib-mating lines.

Starvation drives a threshold triggering communication.

The decision for an ant forager to launch recruitment is governed by an internal response threshold. Here, we demonstrate that this threshold (the desired volume) triggering trail-laying increases under starvation. As a consequence, highly starved foragers lay a recruitment trail and bring back to the nest higher quantities of food from large unlimited resources. In contrast, when the volume of the food source is under their crop capacity, the percentage of trail-communicating foragers is lower following a prolonged period of starvation. Such starvation-dependent changes in the "desired volume" threshold explain how ants optimize recruitment and select liquid food resources in order to prevent collective exploitation of low profitability.