Uncovering trophic interactions in arthropod predators through DNA shotgun-sequencing of gut contents

Characterizing trophic networks is fundamental to many questions in ecology, but this typically requires painstaking efforts, especially to identify the diet of small generalist predators. Several attempts have been devoted to develop suitable molecular tools to determine predatory trophic interactions through gut content analysis, and the challenge has been to achieve simultaneously high taxonomic breadth and resolution. General and practical methods are still needed, preferably independent of PCR amplification of barcodes, to recover a broader range of interactions. Here we applied shotgun-sequencing of the DNA from arthropod predator gut contents, extracted from four common coccinellid and dermapteran predators co-occurring in an agroecosystem in Brazil. By matching unassembled reads against six DNA reference databases obtained from public databases and newly assembled mitogenomes, and filtering for high overlap length and identity, we identified prey and other foreign DNA in the predator guts. Good taxonomic breadth and resolution was achieved (93% of prey identified to species or genus), but with low recovery of matching reads. Two to nine trophic interactions were found for these predators, some of which were only inferred by the presence of parasitoids and components of the microbiome known to be associated with aphid prey. Intraguild predation was also found, including among closely related ladybird species. Uncertainty arises from the lack of comprehensive reference databases and reliance on low numbers of matching reads accentuating the risk of false positives. We discuss caveats and some future prospects that could improve the use of direct DNA shotgun-sequencing to characterize arthropod trophic networks

ELECTRIC FIELD ENHANCEMENT OF OPTICAL ABSORPTION IN GaAs-AlAs TYPE-II SUPERLATTICES

Nous observons l'augmentation de la probabilité de transition optique sous champ électrique longitudinal, dans des super-réseaux GaAs/AlAs de type "indirect". Cela prouve la séparation spatiale des électrons et des trous dans ces structures.The spatial separation of electrons and holes in GaAs/AlAs "indirect" type superlattices is evidenced by the enhancement of optical transition probability under a longitudinal electric field

Weeds mediate the level of intraguild predation in arthropod food webs

International audienceIntraguild predation, which is common for generalist predators, is a specific form of omnivory that may suppress the biological control of a pest. The dietary flexibility of a given organism depends on the choice of the C3 (banana crop) and the C4 (weeds) pathways they use and on the trophic level on which they feed. Understanding the conditions in which intraguild predation decreases biological control is a major issue in agroecosystems. We tested whether the contribution of different primary producer pathways in diets of generalist predators mediates the level of intraguild predation. We studied 10 agroecosystems in which banana plants (C3 metabolism) were diversely associated with weeds (C4 metabolism). Diversity in litter macrofauna was relatively low, with a mean between three and eight species per trap. Measurement of stable isotopes showed a significant decrease in the N-15 values of generalist predators when the C4 pathway contributed more than the C3 pathway to their diet. We rejected hypotheses that an increase in the abundance of prey and that a decrease in prey's N-15 values occur when the C4 pathway contributes more than the C3 pathway to their diet. The results are consistent with the diet modification hypothesis, that is, intraguild predation is lower when the C4 (weeds) pathway is preferred to the C3 pathway. Our results suggest that when the C4 pathway of weeds is more exploited by herbivores (or detritivores), generalist predators tend to consume these herbivores and thus neglect the intraguild prey. The diverse C4 plant community probably supports a diverse herbivore community that provides alternative prey. Our results provide evidence that increasing plant diversity in agroecosystems should decrease intraguild predation of generalist predators and should therefore improve pest regulation. In an applied perspective, plant diversity could be increased by establishing a more diverse cover-crop community

DX centers in AlAs and GaAs-AlAs selectively doped superlattices

DX centers have been investigated using deep level transient spectroscopy (DLTS) in Si doped AlAs and in selectively doped GaAs-AlAs superlattices (SLs) grown by molecular beam epitaxy. The activation energy for thermal emission is $E_{\rm a} = 0.42$ eV in both the SLs and AlAs layers. For the first time a study of the capture in a SL reveals a capture activation energy $E_{\rm cap} = 0.36$ cV, which locates the DX at $E_{\rm t}\approx 60$ meV below the conduction miniband. Taking into account the measured energies and trap concentrations, we show that the DX observed in the SLs lies in the AlAs layers.Des mesures de spectroscopie capacitives effectuées sur AlAs et sur des super-réseaux (SR) GaAs-AlAs dopés sélectivement et épitaxiés par jets moléculaires montrent que le centre DX a une énergie apparente $E_{\rm a} = 0.42$ eV aussi bien dans AlAs que dans les SR. Pour la première fois, une étude de la capture menée sur un SR nous permet de remonter à l'énergie d'activation de la section efficace de capture qui est dans notre cas de 0.36 eV, ce qui positionne le DX à $E_{\rm t}\approx 60$ meV de la minibande de conduction. La nature et la position de DX sont discutées en tenant compte de l'énergie et de la concentration du DX pour chaque structure, et nous montrons que le DX observé est lié au silicium dans les couches AlAs

Génération et détection cohérente d'onde THz par photomélange : vers la caractérisation d'échantillons à forte dispersion

Nous présentons quelques résultats préliminaires concernant la génération d'onde THz par photomélange. L'utilisation d'un second photomélangeur à la réception nous a permis de nous affranchir de la détection bolométrique généralement privilégiée dans cette bande spectrale et de préserver les informations sur l'amplitude et la phase de l'onde THz à détecter. De ce fait, les propriétés d'absorption et de réfraction d'un échantillon placé entre émetteur et récepteur peuvent être obtenues