Refine
Document Type
- Doctoral Thesis (2)
Language
- English (2)
Has Fulltext
- yes (2)
Is part of the Bibliography
- no (2)
Keywords
- Genetik (2) (remove)
Institute
Emerging infectious diseases are among the greatest threats to human, animal and plant health as well as to global biodiversity. They often arise following the human-mediated transport of a pathogen beyond its natural geographic range, where host species are typically not well adapted due to a lack of co-evolutionary host-pathogen dynamics. One such pathogen is the fungus Pseudogymnoascus destructans (Pd), which causes White-Nose disease in hibernating bats. While Pd was first observed in North America where it has led to mass-mortalities in some bat species, the pathogen originates from Eurasia where infection is not associated with mortality. Most of the Pd research has focused on the invasive North American range, which likely underestimated the genetic structure of the pathogen and the role it might play in the disease dynamics.
In my work, I therefore evaluated the genetic structure of Pd in its native range with the aim of uncovering cryptic diversity and further use population genetic data to address some key ecological aspects of the disease dynamics. With an extensive reference collection of more than 5,000 isolates from 27 countries I first demonstrated strong differentiation between two monophyletic clades across several genetic measures (multi-locus genotypes, full genome long-read sequencing and Illumina NovaSeq on isolate pools). These findings are consistent with the presence of two cryptic species which are both causative agents of bat White-Nose disease (‘Pd-1’, which corresponds to P. destructans sensu stricto, and ‘Pd-2’). Both species exist in the same geographic range and co-occur in the same hibernacula (i.e., in sympatry), though with specialised host preferences. I further described the fine-scale population structure in Eurasia which revealed that most genotypes are unique to single hibernacula (more than 95% of genotypes). The associated differences in microsatellite allele frequencies among hibernacula allowed the use of assignment methods to assign the North American isolates (exclusively Pd-1) to regions in Eurasia. Hence, a region in Ukraine (Podilia) is the most likely origin of the North American introduction.
To gain further insights into the spatial and temporal dynamics of White-Nose disease on a localised scale, several hibernacula were sampled with high intensity (artificial hibernaculum in Germany and natural karst caves in Bulgaria). Low rates of Pd gene flow were observed even among closely situated hibernacula. This indicates that Pd does not remain viable on bats over summer or it would be frequently exchanged among bats (and hence hibernacula) resulting in a homogenous distribution of genotypes. Instead, bats need to become re-infected each hibernation season to explain the yearly re-occurrence of White-Nose disease. Given the distribution and richness of Pd genotypes on hibrnacula walls and infected bats of the same hibernacula, bats become infected from the hibernacula walls when they return after summer. This means that environmental reservoirs exist within hibernacula (i.e., the walls) on which Pd spores persist during bat absence and which drive the yearly re-occurrence of White-Nose disease. In an experimental setup, I confirmed the long-term viability of Pd spores on abiotic substrate for at least two years and furthermore discovered temporal variations in Pd spores’ ability to germinate. In fact, these variations followed a seasonal pattern consistent with the timing of bats absence (reduced germination) and presence (increased germination) and could indicate adaptations of Pd to the bats’ life-cycle. The infection of bats from environmental reservoirs hence seems to be a central aspect of White-Nose disease dynamics and Pd biology.
Pds ability to remain viable for extended periods outside the host increases its risk of being anthropogenically transported and might have played a role in the emergence of White-Nose disease in North America. The existence of a second species (Pd-2) poses a great additional danger to North American bats considering that its introduction there could lead to deaths and associated population declines in so-far unaffected species given what is known about differing host species preferences in Eurasian bats. Even within the native range of Pd, the movement of Pd between differentiated fungal populations could facilitate genetic exchanges (e.g., through sexual reproduction) between genetically distant genotypes. Such genetic exchanges could lead to phenotypic jumps in pathogenicity or host-species preferences and should hence be prevented.
The native range of a pathogen holds great potential to better understand the genetic and ecological basis of a (wildlife) disease. My work informs about the dangers associated with the accidental transport of Pd (and other pathogens) and highlights the need for ‘prezootic’ biosecurity-oriented strategies to prevent disease outbreaks globally. Once a pathogen has arrived in a new geographic range, and particularly if it has environmentally durable spores (as demonstrated for Pd), it will be difficult/impossible to eradicate. Furthermore, a pathogen’s ability to remain viable outside the host and infect them from environmental reservoirs has been associated with an increased risk of species extinctions and needs to be considered when designing management strategies to mitigate disease impact.
Species persistence in the face of rapidly progressing environmental change requires adaptive responses that allow organisms to either cope with the novel conditions in their habitat or to follow their environmental niche in space. A poleward range shift due to global warming induced habitat loss in the south has been predicted for the lesser horseshoe bat, Rhinolophus hipposideros. Theoretical as well as numerous empirical studies link range expansion success to increased dispersal and reproduction rates due to spatial sorting and r-selection resulting from low population densities at the expansion front. R. hipposideros females however are highly philopatric and the species’ life history reflects a K- rather than an r-strategy, encompassing a long life span and limited individual annual reproductive output. I therefore investigated if adaptations in these traits determining range expansion success (dispersal and reproduction) can be observed in this bat species of high conservation concern. Genetic diversity presents a critical factor for adaptive responses to global change, both for range expansion and for coping with novel environmental conditions. I hence explored the genetic diversity levels of European R. hipposideros leading edge populations and their drivers for an assessment of these populations’ evolutionary potential and the development of conservation recommendations.
Comparing range expansion traits between an expanding R. hipposideros metapopulation in Germany and a non-expanding one in France revealed that range expansion was associated with an increase in juvenile survival and fecundity, and no decrease in adult survival. These results demonstrate than an increase in reproduction and growth rates is generally possible in R. hipposideros, indicating a potential adaptation (sensu lato) to range expansion. A positive correlation between adult and juvenile survival in the expanding metapopulation suggests higher resource acquisition in the expanding metapopulation, giving rise to the question if the observed demographic changes have a genetic basis or if they are rather induced by differences in environmental conditions between the two metapopulations. Long-term range expansion success requires adaptive evolutionary changes. The relative contribution of the former and that of undirected changes resulting e.g. from differences in resource availability therefore will have to be investigated in more detail in the future to allow predictions about range expansion dynamics in R. hipposideros.
The number of individuals within a radius of approximately 60 to 90 km around a population (as a measure of connectivity) was identified as the main positive driver of the studied populations’ genetic diversity. Overall genetic diversity levels in German R. hipposideros populations were found to be reduced compared to populations in France as a legacy of demographic bottlenecks resulting from severe population declines in the mid-20th century. This finding is alarming as future range expansion can be expected to entail a further decrease in genetic diversity. The resulting loss of genetic diversity can be expected to be particularly strong in R. hipposideros due to the detected dependence of genetic diversity on connectivity, because range expansion often results in small and patchy populations.
Protecting and ideally re-installing genetic diversity in R. hipposideros leading edge populations therefore presents a conservation goal of utmost importance. To achieve this endeavour, conservation efforts should target the protection of extensive networks of well-connected populations. Geographical concentration of individuals should be avoided and populations in key locations that connect clusters must be protected particularly well to prevent populations from becoming isolated. Continuous, regular monitoring of population trends is also important for a quick registration of disturbances or threats, and the subsequent rapid development of countermeasures to preclude further demographic declines.
The reduced levels of genetic diversity in the German metapopulation precluded a reliable quantification of dispersal rates due to the reduced power of discrimination between individuals. While ongoing re-colonization and the establishment of new maternity colonies provide evidence for increased dispersal in the expanding metapopulation, evaluating the expected range expansion velocity of R. hipposideros in relation to the estimated velocity of global warming induced habitat loss will require the confirmation of the existing preliminary dispersal data by employing more genetic markers.