Transmission of amphibian parasites: exploring the influences of host identity and exposure scenario on key transitions in the transmission pathway.

Batrachochytrium dendrobatidis (Bd) is an emerging fungal pathogen that threatens amphibian hosts globally. The observed mass mortality events in amphibian populations, multiple species declines and extinctions caused by this pathogen, have been attributed to its broad host range, widespread geographic distribution, and ability to infect all amphibian life-history stages. While the severity of the disease is worse in post-metamorphic animals, it is the pre-metamorphic (tadpole) stage that is seen as an important disease reservoir. The potential for tadpoles to act as a reservoir for Bd is dependent on their contribution to the environmental pool of zoospores, and therefore, their role in Bd transmission. However, few studies have explored transmission dynamics in multi-host tadpole communities. To explore Bd transmission in the tadpole communities, I break the transmission process into three discrete stages: (i) susceptibility (the probability of a naïve host becoming infected upon exposure to Bd zoospores in the environment), (ii) infectiousness (the rate at which infected hosts release Bd zoospores into the environment) and (iii) contact rate (the rate at which tadpoles encounter zoospores in the environment). I quantified the first two of these processes experimentally for a range of host species. I then used these data to parameterise a range of mathematical models to predict the consequences for Bd transmission in multi-host communities and in doing so, explored the sensitivity of my predictions to variations in contact rate (the third of the above processes). Standardised experiments exposing three tadpole host species individually to Bd showed that host susceptibility conformed to previous held species responses, with species lying along a continuum from largely resistant (rarely infected, or only infected to a very low level) to largely tolerant (highly likely to be infected, potentially to a high level), and there was little evidence of Bd-induced mortality for all species. I also co-exposed hosts to another amphibian pathogen of conservation concern, ranavirus, but this did not have a significant effect on Bd infections, although there were some additional mortalities of co-exposed hosts. Next, I explored zoospore shedding rates for the different host species, and found a clear power relationship between species-specific susceptibility and infectiousness. However, for a given pathogen load, hosts across all species did not differ in pathogen shedding rates. Hence these experiments revealed a strong host identity component to contribution to environmental zoospores, but this arises purely through variation in their own infection levels, which then directly determine their zoospore shedding rate. From these experimental chapters, I conclude that tadpoles can be classified into two broad host identities (tolerant and resistant), and I used this to develop and parameterise a series of mathematical models to assess Bd transmission dynamics in multi-host tadpole communities, considering variable contact and reinfection rates. The models revealed a clear effect of host community composition. Tolerant hosts contributed disproportionately to the environmental pool of zoospores, with the subsequent increase in the force of infection from the environment driving infection dynamics in a more resistant host species. These findings, however, suggest that the process by which reinfection occurs, which can lead to increases in on-host infection loads of tolerant host, can be highly influential in determining Bd-dynamics in the wider system. These predictions therefore highlight the need to understand the fate of zoospores shed from an infected host: whether they enter the aquatic environment and contribute towards the force of infection for other hosts, or whether they engage in immediate reinfection, contributing to increases in infection load of that host. My model predictions suggest that determining the relative occurrence of these two processes may be crucial for determining community-wide Bd transmission dynamics. Overall, my work shows that tadpole Bd-load dynamics and their resulting host identity associations, hold the potential to influence community-wide infection dynamics, an understanding of which could inform more targeted mitigation strategies than otherwise possible

Infectious genomic RNA of Rhopalosiphum padi virus transcribed in vitro from a full-length cDNA clone

AbstractAvailability of a cloned genome from which infectious RNA can be transcribed is essential for investigating RNA virus molecular mechanisms. To date, no such clones have been reported for the Dicistroviridae, an emerging family of invertebrate viruses. Previously we demonstrated baculovirus-driven expression of a cloned Rhopalosiphum padi virus (RhPV; Dicistroviridae) genome that was infectious to aphids, and we identified a cell line (GWSS-Z10) from the glassy-winged sharpshooter, that supports RhPV replication. Here we report that RNA transcribed from a full-length cDNA clone is infectious. Transfection of GWSS-Z10 cells with the RhPV transcript resulted in cytopathic effects, ultrastructural changes, and accumulation of progeny virions, consistent with virus infection. Virions from transcript-infected cells were infectious in aphids. This infectious transcript of a cloned RhPV genome provides a valuable tool, and a more tractable system without interference from baculovirus infection, for investigating replication and pathogenesis of dicistroviruses

Conclusive Evidence of Replication of a Plant Virus in Honeybees Is Lacking

The recent article by Li et al. (1) lacks adequate evidence to support the authors’ assertion that a plant virus propagates or replicates in honeybees. Instead, it is possible that tobacco ringspot virus (TRSV) virions associate with the honeybee and parasitic Varroa mites in the absence of TRSV replication

Social networks strongly predict the gut microbiota of wild mice

The mammalian gut teems with microbes, yet how hosts acquire these symbionts remains poorly understood. Research in primates suggests that microbes can be picked up via social contact, but the role of social interactions in non-group-living species remains underexplored. Here, we use a passive tracking system to collect high resolution spatiotemporal activity data from wild mice (Apodemus sylvaticus). Social network analysis revealed social association strength to be the strongest predictor of microbiota similarity among individuals, controlling for factors including spatial proximity and kinship, which had far smaller or nonsignificant effects. This social effect was limited to interactions involving males (male-male and male-female), implicating sex-dependent behaviours as driving processes. Social network position also predicted microbiota richness, with well-connected individuals having the most diverse microbiotas. Overall, these findings suggest social contact provides a key transmission pathway for gut symbionts even in relatively asocial mammals, that strongly shapes the adult gut microbiota. This work underlines the potential for individuals to pick up beneficial symbionts as well as pathogens from social interactions.Peer reviewe

Synchronous seasonality in the gut microbiota of wild mouse populations

The gut microbiome performs many important functions in mammalian hosts, with community composition shaping its functional role. However, the factors that drive individual microbiota variation in wild animals and to what extent these are predictable or idiosyncratic across populations remains poorly understood. Here, we use a multi-population dataset from a common rodent species (the wood mouse, Apodemus sylvaticus), to test whether a consistent “core” gut microbiota is identifiable in this species, and to what extent the predictors of microbiota variation are consistent across populations. Between 2014 and 2018 we used capture-mark-recapture and 16S rRNA profiling to intensively monitor two wild wood mouse populations and their gut microbiota, as well as characterising the microbiota from a laboratory-housed colony of the same species. Although the microbiota was broadly similar at high taxonomic levels, the two wild populations did not share a single bacterial amplicon sequence variant (ASV), despite being only 50km apart. Meanwhile, the laboratory-housed colony shared many ASVs with one of the wild populations from which it is thought to have been founded decades ago. Despite not sharing any ASVs, the two wild populations shared a phylogenetically more similar microbiota than either did with the colony, and the factors predicting compositional variation in each wild population were remarkably similar. We identified a strong and consistent pattern of seasonal microbiota restructuring that occurred at both sites, in all years, and within individual mice. While the microbiota was highly individualised, some seasonal convergence occurred in late winter/early spring. These findings reveal highly repeatable seasonal gut microbiota dynamics in multiple populations of this species, despite different taxa being involved. This provides a platform for future work to understand the drivers and functional implications of such predictable seasonal microbiome restructuring, including whether it might provide the host with adaptive seasonal phenotypic plasticity