Competitive index in mixed infections: a sensitive and accurate assay for the genetic analysis of Pseudomonas syringae–plant interactions

Mixed infections have been broadly applied to the study of bacterial pathogens in animals. However, the application of mixed infection-based methods in plant pathogens has been very limited. An important factor for this limitation is the different dynamics that mixed infections have been reported to show in the different types of models. Reports in systemic animal infections have shown that any bacterium has the same probability of multiplying within a mixed infection than in a single infection. However, in plant pathogens, bacterial growth in a mixed infection does not seem to reflect growth in a single infection, as growth interference takes place between the co-inoculated strains. Here we show that growth interference in mixed infection between different Pseudomonas syringae strains is not intrinsic to growth within a plant host, but dependent on the dose of inoculation. We also show that the minimal inoculation dose required to avoid interference depends on the aggressiveness of the pathogen as well as the type of virulence factor that differentiates the co-inoculated strains. This study establishes the basis for the use of mixed infection-based applications to the study of phytopathogenic bacteria. Analysis of the virulence of a type III effector mutant and an hrp regulatory mutant illustrate the increased accuracy and sensitivity of competitive index assays vs. regular growth assays. Several applications of this assay are addressed, and potential implications for this and other mixed infection-based methods are discussed.     

Changes in the composition of British butterfly assemblages over two decades

Changes in the abundance and distribution of individual species have been widely documented in Britain and other countries in recent decades, but little has been done to determine changes in community composition over broad geographic areas. Here, we studied species turnover in 51 butterfly assemblages in Britain since 1976, examining extinction and colonisation events together with variation in the abundances of the species. We showed that the species turnover that occurred over 20 years in Britain was associated with colonisation and extinction events and also with variability in the abundance of the species. These changes in community composition differed according to the habitat requirements of the species and their previous distributions, being more evident for habitat specialists and for southerly distributed species. Colonising species often became abundant components of the communities they joined, although this was more evident for generalist than for specialist species. The abundance of species following their arrival, increased with time since colonisation. Species turnover associated with southerly species expanding northwards is consistent with being a response to climate change. The results suggest that climate- and habitat-driven changes in the identity and abundance of species within communities are widespread, and probably ubiquitous. Similar changes are likely to be occurring in other groups of organisms that are similarly undertaking major range shifts associated with climate change.     

Long-term effects of ungulates on phytophagous insects

Abstract.  1. Most plants interact with a diverse suite of herbivores, allowing the opportunity for the existence of positive and negative interactions between highly dissimilar organisms. However, most studies on herbivorous interactions have been performed under the assumption that they occur mainly between similar species. Consequently, ecologists are still far from a full understanding of the ecological factors that determine insect population dynamics.
2. In this study, a 7-year field experiment was conducted that manipulated the presence of ungulates to evaluate their effects on the abundance, attack rate, and survival of four guilds of co-occurring herbivorous insects living on the same host plant: seed predators, stem borers, gall makers and sap suckers. These four guilds differed in habits and behaviour, the first three being sessile and endophytic and the last being free-living.
3. This study shows that the abundance of all four guilds was negatively affected by ungulates. However, the effect on attack rate differed among guilds, as mammals do not affect the seed predator attack rate. Ungulates also differentially affected insect survival, ingesting only seed predators and gall makers.
4. In summary, this study suggests that diverse mechanisms may affect different insect guilds in different ways. Therefore, competition between disparate herbivores appears to be complex and can be provoked by multiple mechanisms.     

Escape from natural enemies during climate-driven range expansion: a case study

Abstract.  1. A major, and largely unexplored, uncertainty in projecting the impact of climate change on biodiversity is the consequence of altered interspecific interactions, for example between parasitoids and their hosts. The present study investigated parasitism in the Brown Argus butterfly, Aricia agestis ; a species that has expanded northward in Britain during the last 30  years in association with climate warming.
2.  Aricia agestis larvae suffered lower mortality from parasitoids in newly colonised areas compared with long-established populations. This result was consistent over four consecutive generations (2 years) when comparing one population of each type, and also when several populations within the historical and recently colonised range of the species were compared within a single year. Thus, A. agestis appears to be partially escaping from parasitism as it expands northwards.
3. Reduced parasitism occurred despite the fact that several of the parasitoid species associated with A. agestis were already present in the newly colonised areas, supported predominantly by an alternative host species, the Common Blue butterfly, Polyommatus icarus .
4. As the species expand their distributions into areas of increased climatic suitability, invasion fronts may escape from natural enemies, enhancing rates of range expansion. The results suggest that the decoupling of interspecific interactions may allow some species to exploit a wider range of environments and to do so more rapidly than previously thought possible.