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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.
In Germany, basic data on the biology, ecology and distribution of rare mosquito species are insufficiently recorded leading to knowledge gaps, for example regarding their vector potential. The introduction of new mosquito species and of the pathogens they transmit has increased the risk of diseases previously uncommon in Germany. These circumstances have led to increased efforts within the past 10 years to better understand the spatio-temporal occurrence and underlying habitat binding of mosquito species and to predict their future distribution, particularly with regard to the changing climatic conditions and changing landscape. A reliable morphological and genetic identification was lacking for several native mosquito species, which forms the basis for any robust monitoring within mosquito surveillance programs or insect conservation projects.
The aim of this thesis was to gain detailed knowledge on the current spatial and temporal occurrence, the habitat binding, and morphological and genetic features with regard to species identification for the non-native species Aedes albopictus (Skuse, 1895), the native species of the Aedes Annulipes Group, and the native and rare species Aedes refiki Medschid, 1928, Culex martinii Medschid, 1930 and Culiseta ochroptera (Peus, 1935).
The thesis compares the suitability of the local climate for the persistence of the species Aedes albopictus sporadically observed in Jena (Thuringia) from 2015 to 2018 with two populations in southern Germany. The focus was on the analysis of extreme winter temperatures and the duration below selected temperature thresholds. In addition to critical temperature conditions, aquatic habitat conditions were of importance. The results of this study suggest that the population could become established in the long term.
Through the monitoring conducted for this thesis, the very rare mosquito species Aedes refiki, Culex martinii in Thuringia and Culiseta ochroptera were rediscovered at several sites in northern and eastern Germany. It was possible to add new information on habitat binding, distribution and abundance for the considered mosquito species. The survival of these rare native mosquito species depends on the preservation of a few remaining habitats. In addition, it can be assumed that these species will become even rarer with future climate change in Germany and, therefore, should be considered endangered. In contrast, other mosquito species could benefit from an increase in average temperatures or precipitation in individual cases.
Due to the contribution to species identification, difficulties in the morphological and genetic identification of selected mosquito species native to Germany could be dispelled. Three forms each were assigned to the known morphological variants of Aedes refiki and Culiseta ochroptera and their peculiarities were described, as well as a new character for species identification was highlighted in the case of Culiseta ochroptera. Generated CO1 mtDNA sequences provide the first DNA-barcodes of Aedes refiki and Culex martinii for Germany.
In five native mosquito species of the Aedes Annulipes Group, twenty types of aberrant tarsal claws were illustrated and described in their morphology. Morphological peculiarities and an asymmetrical occurrence of the aberrant claw types were observed and possible causes for their development were discussed. Together with the development of a basic blueprint of mosquito tarsal claws, the results opened another field of research for the taxonomy, developmental biology and aquatic ecology of arthropods.
Presumably every organism on earth is involved in at least one mutualistic interaction with one or several other species. To interact with each other, the species need traits that provide benefits to the partner species. Surprisingly, the function of traits for the stabilization of mutualisms has rarely been investigated, despite of a general lack of knowledge how mutualisms are maintained. The aim of this work was to find functional traits, which stabilize the mutualism between a bat species and a carnivorous pitcher plant in Northern Borneo. Kerivoula hardwickii is the only bat species known to roost in pitcher-shaped trapping organs of Palaeotropical pitcher plants (Nepenthes). These bats fertilize the pitcher plant Nepenthes hemsleyana with their nutritious nitrogen-rich faeces while roosting inside the pitchers. The plants have outsourced capture and digestion of arthropod prey to the bats on which they strongly rely for nutrient acquisition. The bats in contrast are less dependent on their mutualism partner as they also roost in pitchers of two further Nepenthes species as well as in developing furled leaves of various plant species in the order Zingiberales. In earlier studies, we found that N. hemsleyana outcompetes alternative roosts by providing high-quality roosts for the bats. However, which traits exactly stabilize the mutualism between K. hardwickii and N. hemsleyana was still unclear. I found that both the bats and the pitcher plants show traits, which have the potential to stabilize their interaction. On the level of morphological traits, I found that the pitchers have a low fluid level and a particular shape that provide just enough roosting space for one individual of the solitary K. hardwickii, a mother with juvenile or a mating couple. The bats have enlarged thumb and foot pads that enable them to cling to the smooth surfaces of their roosts without using their claws. This avoids damage to the sensitive N. hemsleyana pitchers. On the level of communicational traits, again N. hemsleyana acquired morphological structures that act as effective ultrasound-reflectors, which guide the echo-orientating bats to the opening of the pitchers and help the bats to identify their mutualism partner. The bats’ calls on the other hand are characterized by extraordinary high starting frequencies and broad bandwidths, which enable K. hardwickii to easily locate pitchers of N. hemsleyana and other Nepenthes species in their dense habitats. Finally, on the level of behavioural traits the bats often but not always prefer their mutualism partner to other roosts when they can select roosts in their natural environment or in behavioural experiments. The reason for this behaviour seems to be a combination of 1) N. hemsleyana’s superior quality compared to alternative roosts and 2) different roosting traditions of the bats. In conclusion, the mutualism between bats and pitcher plants is asymmetric as N. hemsleyana is more dependent on K. hardwickii than vice versa. For the plants bat faeces present their most important nutrient source. In contrast, K. hardwickii can select between alternative roosting plants. This asymmetric dependency is reflected in the specifity and function of the traits that stabilize the mutualism in each of the two involved species. Especially on the morphological level, N. hemsleyana seems to have evolved several traits that perfectly fit to K. hardwickii. In contrast, the bats’ traits more generally facilitate their roosting in funnel-shaped plant structures and their occurrence in cluttered habitats. Thus, they are probably exaptations (i.e. traits that evolved for another reason) that are nevertheless functional and stabilize the mutualism with N. hemsleyana. This plant‘s superior roost quality is likely a consequence of the competition with alternative roosting plants and is a pre-requisite for the bats to prefer N. hemsleyana. Moreover, my study confirms earlier findings that asymmetric dependencies support the stabilization of mutualistic interactions. Finally, my work indicates that the specifity of functional traits can be used as a measure to determine mutual dependencies of mutualistic partners.
Having been regarded as wastelands until quite recently, wetlands are increasingly acknowledged as ecosystems of high biodiversity. Wetland restoration projects are often accompanied by the implementation of specific species management programs. Naturally, for effective management measures, profound knowledge of the target speciesʼ ecological requirements is obligatory, including habitat selection, feeding ecology as well as spatial behaviour such as movements within and between patches of suitable habitat. Yet, big knowledge gaps exist for many marshland birds which is particularly true for highly secretive species such as rails and crakes. Considered as the least known among the Palaearctic breeding birds, most information about the Baillon's Crake Zapornia pusilla is only anecdotic, resulting in strong uncertainties with regard to the species' distribution, population sizes, status, migratory behaviour as well as ecological requirements. This can be mainly attributed to the species' skulking behaviour and its seemingly highly erratic occurrence. Baillon's Crakes in the Western Palaearctic and Palaeotropics are referred to as the subspecies Z. p. intermedia. While European breeding birds are assumed to winter in sub-Saharan wetlands, African populations are considered rather to be itinerant with local movements induced by seasonal or anthropogenic habitat changes. However, for both migratory movements, major directions or routes are unknown. The discovery of a large number of Baillon's Crakes presumably wintering in the floodplains of the Parc National des Oiseaux du Djoudj (PNOD), situated in the Senegal River Delta, WAfrica, initiated this thesis. The main aim of the study was, firstly, to clarify the status and size of this population and assess its connectivity to European breeding population(s). Secondly, in order to improve the knowledge about the species' ecological requirements as a basis for the National Parks conservation management, habitat selection, spatial behaviour as well as dietary selectivity were investigated. The major part of the fieldwork was performed in PNOD in the course of the dry season during periods of 1.5 - 2.5 months from December - March 2009, 2010 and 2013. Baillon's Crakes were mainly caught with cage traps, ringed and common measurements were taken, including moult status. Skin tissue as well as one rectrice was sampled for DNA and stable isotope analyses. This was also done for Baillon's Crakes caught in European breeding grounds in Germany, Montenegro and Southern Spain. For dietary analyses, faecal samples were collected in PNOD in winter 2009/2010. Furthermore, some individuals were equipped with radio-transmitters to determine home range size and habitat selection. For the identification of the most relevant habitat parameters both on a population as well as on the individuals' level, we used a vegetation map based on satellite imagery covering the entire Djoudj area as well as maps generated on the basis of aerial photographs taken at two study sites.