Host‐specific differentiation in the anther smut fungus Microbotryum violaceum as revealed by microsatellites

The anther smut fungus Microbotryum violaceum ( = Ustilago violacea) is a parasite of many species of the Caryophyllaceae. Its host specificity has been debated since early this century, when cross‐inoculation experiments indicated the existence of host‐specific lineages. Recently, on the basis of spore ultrastructure, all presumed host races were lumped within M. violaceum. To measure gene flow among natural populations of anther smuts from different host species, we used microsatellite variation at 5 loci among samples from 8 Silene, 2 Saponaria, 2 Dianthus and 1 Gypsophila species. Most of the 326 M. violaceum samples investigated originated from the Swiss Alps and close surroundings. Microsatellite variation revealed almost perfect isolation among anther smut fungi from different host species. In addition, differentiation was supported by the nonrandom distribution of null alleles among samples from different host species and host genera. Null alleles were most abundant in anther smut samples from non‐Silene hosts. The resolution of genetic differentiation among anther smuts from different host species was highest in those from Silene species. Genetic relationships among samples as indicated by Neighbour‐Joining analysis based on genetic distances are discussed with respect to host phylogeny and host ecology. One sample was identified as Ustilago gausseni because it had verrucose instead of reticulate spores and was collected from Silene italica. Neighbour‐Joining analysis revealed that this sample was similar to the M. violaceum samples from other Silene host species. Therefore, our data question spore morphology as a reliable character for anther smut systematics.     

Membrane rafts: a potential gateway for bacterial entry into host cells

Pathogenic bacteria have developed various mechanisms to evade host immune defense systems. Invasion of pathogenic bacteria requires interaction of the pathogen with host receptors, followed by activation of signal transduction pathways and rearrangement of the cytoskeleton to facilitate bacterial entry. Numerous bacteria exploit specialized plasma membrane microdomains, commonly called membrane rafts, which are rich in cholesterol, sphingolipids and a special set of signaling molecules which allow entry to host cells and establishment of a protected niche within the host. This review focuses on the current understanding of the raft hypothesis and the means by which pathogenic bacteria subvert membrane microdomains to promote infection.     

Geographic structure in the searching behaviour of a specialist parasitoid: combining molecular and behavioural approaches

An increasing number of studies have shown that the traits important to species interactions may differ geographically among populations or groups of populations within a single interacting species. We examined geographic structure in the searching behaviour of a recently discovered parasitoid in the genus Agathis (Hymenoptera: Braconidae) by examining the pattern of population structure obtained from sequence data of mitochondrial DNA cytochrome oxidase I and the pattern of population differentiation in female searching behaviour. Analyses of population structure showed no isolation by distance and suggested long distance dispersal among populations. This pattern is consistent with recent post‐glacial expansion of Agathis n. sp. Observations of searching behaviour demonstrated that populations of Agathis n. sp. differed in a subset of the behavioural traits examined and also one morphological trait. These population differences appear to be driven in part by local host plant characteristics, and based on the population structure of Agathis n. sp., have arisen relatively quickly in evolutionary time. This study suggests that the interaction between parasitoids and their host insects may exhibit substantial geographic variation, and studies that focus at the level of single populations or the species‐level may be missing much of the evolutionary dynamics of parasitoid–host interactions.     

The interaction between plant competition and disease

It is well documented that pathogens can affect the survival, reproduction, and growth of individual plants. Drawing together insights from diverse studies in ecology and agriculture, we evaluate the evidence for pathogens affecting competitive interactions between plants of both the same and different species. Our objective is to explore the potential ecological and evolutionary consequences of such interactions. First, we address how disease interacts with intraspecific competition and present a simple graphical model suggesting that diverse outcomes should be expected. We conclude that the presence of pathogens may have either large or minimal effects on population dynamics depending on many factors including the density-dependent compensatory ability of healthy plants and spatial patterns of infection. Second, we consider how disease can alter competitive abilities of genotypes, and thus may affect the genetic composition of populations. These genetic processes feed back on population dynamics given trade-offs between disease resistance and other fitness components. Third, we examine how the effect of disease on interspecific plant interactions may have potentially far-reaching effects on community composition. A host-specific pathogen, for example, may alter a competitive hierarchy that exists between host and non-host species. Generalist pathogens can also induce indirect competitive interactions between host species. We conclude by highlighting lacunae in our current understanding and suggest that future studies should (1) examine a broader taxonomic range of pathogens since work to date has largely focused on fungal pathogens; (2) increase the use of field competition studies; (3) follow interactions for multiple generations; (4) characterize density-dependent processes; and (5) quantify pathogen, as well as plant, population and community dynamics.