Abstract Symbioses are significant drivers of insect evolutionary ecology. Despite recent findings that these associations can emerge from environmentally derived bacterial precursors, there is still little information on how these potential progenitors of insect symbionts circulate in trophic systems. Serratia symbiotica represents a valuable model for deciphering evolutionary scenarios of bacterial acquisition by insects, as its diversity includes gut-associated strains that retained the ability to live independently of their hosts, representing a potential reservoir for symbioses emergence. Here, we conducted a field study to examine the distribution and diversity of S. symbiotica found in aphid populations, and in different compartments of their surrounding environment. Twenty % of aphid colonies were infected with S. symbiotica, including a wide diversity of strains with varied tissue tropism corresponding to different lifestyle. We also showed that the prevalence of S. symbiotica is influenced by seasonal temperatures. We found that S. symbiotica was present in non-aphid species and in host plants, and that its prevalence in these samples was higher when associated aphid colonies were infected. Furthermore, phylogenetic analyses suggest the existence of horizontal transfers between the different trophic levels. These results provide a new picture of the pervasiveness of an insect symbiont in nature.
ABSTRACT Bacterial symbioses are significant drivers of insect evolutionary ecology. However, despite recent findings that these associations can emerge from environmentally derived bacterial precursors, there is still little information on how these potential progenitors of insect symbionts circulates in the trophic systems. The aphid symbiont Serratia symbiotica represents a valuable model for deciphering evolutionary scenarios of bacterial acquisition by insects, as its diversity includes intracellular host-dependent strains as well as gut-associated strains that have retained some ability to live independently of their hosts and circulate in plant phloem sap. These strains represent a potential reservoir for the emergence of new and more intimate symbioses. Here, we conducted a field study to examine the distribution and diversity of S. symbiotica found in aphid populations, as well as in different compartments of their surrounding environment. A total of 250 aphid colonies, 203 associated insects, and 161 plant samples associated with aphid colonies were screened for S. symbiotica . Twenty percent of aphids were infected with S. symbiotica , and the symbiont includes a wide diversity of strains with varied tissue tropism corresponding to different lifestyle. We also showed that the prevalence of S. symbiotica is influenced by seasonal temperatures. For the first time, we found that S. symbiotica was present in non aphid species and in host plants, and that the prevalence of the bacterium in these samples was higher when associated aphid colonies were infected. Furthermore, phylogenetic analyses suggest the existence of horizontal transfers between the different trophic levels examined. These results provide a completely new picture of the ubiquity of an insect symbiont in nature. They suggest that ecological interactions promote the dissemination of strains that are still free-living and poorly specialized, and for which plants are a proabable reservoir for the acquisition of new bacterial partners in insects.
Two types of dissemination propagules play a role in the epidemiology of Septoria tritici blotch (STB). Airborne ascospores are the major source of primary infections in autumn and winter, whereas the development of the epidemic in spring and summer is thought to be driven by splash-dispersed pycnidiospores. Recent observations in various countries, however, have shown that the airborne inoculum of Mycosphaerella graminicola can be produced all year round. In order to understand the role of airborne inoculum in STB epidemics in Belgium, a monitoring technique involving 7-day recording Burkard spore traps and real-time PCR was developed to quantify the total daily airborne inoculum at five locations in the Walloon region in Belgium over 2 years, from April 2009 to April 2011. Four traps were placed in wheat fields and a fifth trap was placed on the roof of a 25 m high building. The reliability of the quantification technique and the homogeneity of the distribution of the STB airborne inoculum at the field scale were controlled in preliminary assays. STB airborne inoculum was detected throughout both years at each of the five locations. Lower but sometimes important quantities were detected in the air above the building, indicating the possibility of long-distance transport. Differences in airborne inoculum quantities in fields were also observed at the network scale and might be linked to the STB level. From the seedling to stem extension stages, there were frequent peaks of detection at each site. The quantities trapped were correlated with the severity of the disease the previous year. Significant quantities of the airborne inoculum were also trapped between the cereal stem elongation and heading stages, suggesting the possible influence of this type of aerial inoculum on STB incidence on the upper leaves in spring and summer. The highest detections, however, generally occurred later, between heading and harvesting, especially in 2009 when disease pressure was high, supporting significant production of pseudothecia during that period. These results illustrate the presence of airborne inoculum all year round, suggesting a possible effect of M. graminicola airborne inoculum on STB epidemics throughout the growing season.
Fusarium head blight (FHB) is a common fungal disease in winter wheat in Belgium causing yield losses and sanitary problems due to the production of mycotoxins by species associated to the disease. Fusarium graminearum (teleomorph Gibberalle zeae) is one of the important species involved in the species complex causing FHB and is able to produce wind dispersed ascospores by sexual reproduction. In order to analyse the distribution of airborne inoculum of G. zeae along the year and specially between heading and flowering and to understand the role of this inoculum in the infection of the ears, a network of Burkard spore traps was set up in fields in Walloon Region in Belgium during the cultural season 2011-2012. Total DNA from each fragment of spore trap tape corresponding to 1 day sampling was extracted and the quantity of G. zeae was assessed using a real-time polymerase chain reaction (PCR) assay. First results showed the occurrence of G. zeae airborne inoculum between heading and flowering. The relationship between the distribution of airborne inoculum and the prevalence of G. zeae on infected ears collected in fields was studied in order to evaluate if spore traps coupled with real-time PCR can be used to improve the understanding of the epidemiology of FHB, the prediction of this disease and the control strategies.
Testing fluctuating rather than constant temperatures is likely to produce more realistic datasets, as they are ecologically more similar to what arthropods experience in nature. In this study, we evaluated the impact of three constant thermal regimes (7, 12, and 17 °C) and one fluctuating thermal regime (7-17 °C with a mean of 12 °C) on fitness indicators in the rosy apple aphid Dysaphis plantaginea, a major pest of apple orchards, and the parasitoid Aphidius matricariae, one of its natural enemies used in mass release biological control strategies. For some-but not all-traits, the fluctuating 7-17 °C regime was beneficial to insects compared to the constant 12 °C regime. Both aphid and parasitoid development times were shortened under the fluctuating regime, and there was a clear trend towards an increased longevity under the fluctuating regime. The fecundity, mass, and size were affected by the mean temperature, but only the mass of aphids was higher at 7-17 °C than at a constant 12 °C. Parasitism rates, but not emergence rates, were higher under the fluctuating regime than under the constant 12 °C regime. Results are discussed within the framework of insect thermal ecology and Jensen's inequality. We conclude that incorporating thermal fluctuations in ecological studies could allow for the more accurate consideration of how temperature affects host-parasitoid interactions and insect responses to temperature change over time.
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