Host Specialisation, Immune Cross-Reaction and the Composition of Communities of Co-circulating Borrelia Strains

We use mathematical modelling to examine how microbial strain communities are structured by the host specialisation traits and antigenic relationships of their members. The model is quite general and broadly applicable, but we focus on Borrelia burgdorferi, the Lyme disease bacterium, transmitted by ticks to mice and birds. In this system, host specialisation driven by the evasion of innate immunity has been linked to multiple niche polymorphism, while antigenic differentiation driven by the evasion of adaptive immunity has been linked to negative frequency dependence. Our model is composed of two host species, one vector, and multiple co-circulating pathogen strains that vary in their host specificity and their antigenic distances from one another. We explore the conditions required to maintain pathogen diversity. We show that the combination of host specificity and antigenic differentiation creates an intricate niche structure. Unequivocal rules that relate the stability of a strain community directly to the trait composition of its members are elusive. However, broad patterns are evident. When antigenic differentiation is weak, stable communities are typically composed entirely of generalists that can exploit either host species equally well. As antigenic differentiation increases, more diverse stable communities emerge, typically around trait compositions of generalists, generalists and very similar specialists, and specialists roughly balanced between the two host species.

Comparative analysis of host-associated variation in Phytophthora cactorum

ABSTRACTPhytophthora cactorum is often described as a generalist pathogen, with isolates causing disease in a range of plant species. It is the causative agent of two diseases in the cultivated strawberry, crown rot (CR; causing whole plant collapse) and leather rot (LR; affecting the fruit). In the cultivated apple, P. cactorum causes girdling bark rots on the scion (collar rot) and rootstock (crown rot), as well as necrosis of the fine root system (root rot) and fruit rots. We investigated evidence for host specialisation within P. cactorum through comparative genomic analysis of 18 isolates. Whole genome phylogenetic analysis provided genomic support for discrete lineages within P. cactorum, with well supported non-recombing clades for strawberry CR and apple infecting isolates specialised to strawberry crowns and apple tissue. Isolates of strawberry CR are genetically similar globally, while there is more diversity in apple-infecting isolates. We sought to identify the genetic basis of host specialisation, demonstrating gain and loss of effector complements within the P. cactorum phylogeny, representing putative determinants of host boundaries. Transcriptomic analysis highlighted that those effectors found to be specific to a single host or expanded in the strawberry lineage are amongst those most highly expressed during infection of strawberry and give a wider insight into the key effectors active during strawberry infection. Many effectors that had homologs in other Phytophthoras that have been characterised as avirulence genes were present but not expressed in our tested isolate. Our results highlight several RxLR-containing effectors that warrant further investigation to determine whether they are indeed virulence factors and host-specificity determinants for strawberry and apple. Furthermore, additional work is required to determine whether these effectors are suitable targets to focus attention on for future resistance breeding efforts.

Cryptic host specialisation within Poecilochirus carabi mites explains population differences in the extent of co-adaptation with their burying beetle Nicrophorus vespilloides hosts

Symbiotic organisms adapt to one another but the extent of co-adaptation commonly varies among pairs of the same symbiotic species drawn from different populations. Here we identify some ecological causes of such differences between populations. We analysed the extent of local co-adaptation between burying beetles Nicrophorus vespilloides and their phoretic mites Poecilochirus carabi in Gamlingay and Waresley Woods, in Cambridgeshire, UK. Burying beetles transport mites to small vertebrate carrion upon which they both reproduce. We conducted reciprocal transplant experiments to test for evidence of local co-adaptation during reproduction. We found variation in the extent of local co-adaptation that was explained by cryptic host specialisation within P. carabi mites. P. carabi is a species complex, within which races of mites are specialised to associate with different species of burying beetle. We found that N. vespilloides from Gamlingay Wood carries a mixture of mite races, from each of the four Nicrophorus species that inhabits this wood. This mixture of races makes P.carabi harmful to Gamlingay N. vespilloides: together, they reduce beetle reproductive success. Experimentally purifying mites, so that Gamlingay N. vespilloides is associated only with the vespilloides mite race, improves beetle reproductive success. Waresley N. vespilloides, by contrast, carry a near pure race of vespilloides mites, which cause negligible damage to Waresley N. vespilloides reproductive success. This is probably because Waresley Wood harbours only two burying beetle species, which differ markedly in their reproductive biology. Cryptic host specialisation with P. carabi mites, combined with differences in the Nicrophorus guild between Gamlingay and Waresley Woods, therefore explain population differences in the extent of local adaptation between N. vespilloides and P. carabi.

Host specialisation and the disparate fate of Ceropales bipunctata (Hymenoptera: Pompilidae) in New Brunswick and Ontario, Canada

Ceropales bipunctata Say (Hymenoptera: Pompilidae) is a cleptoparasitic spider wasp that has declined significantly in parts of its range. New survey work has revealed that the species is common and widespread in dune habitat along the Gulf of Saint Lawrence in New Brunswick, Canada. Its host was determined as Anoplius cleora (Banks) (Hymenoptera: Pompilidae), with Arctosa littoralis (Hentz) (Araneae: Lycosidae) as prey, the first host record for this species. The relative abundance of C. bipunctata in coastal New Brunswick sharply contrasts with a decline in Ontario, Canada, where it was thought to be extirpated. Based on body size, habitat, and locality overlap we postulate that the primary host of C. bipunctata in Ontario and western Québec, Canada, is a different species, Anoplius aethiops (Cresson), with A. atrox (Dahlbom) as a likely secondary host in southwestern Ontario. Both species are closely related to A. cleora and have undergone a decline in eastern Canada. Based on these new findings, we reassess the conservation status of C. bipunctata in Canada.