Nina Schwensow

MHC-associated mating strategies and the importance of overall genetic diversity in an obligate pair-living primate

Mate choice is one of the most important evolutionary mechanisms. Females can improve their fitness by selectively mating with certain males. We studied possible genetic benefits in the obligate pair-living fat-tailed dwarf lemur (Cheirogaleus medius) which maintains life-long pair bonds but has an extremely high rate of extra-pair paternity. Possible mechanisms of female mate choice were investigated by analyzing overall genetic variability (neutral microsatellite marker) as well as a marker of adaptive significance (major histocompatibility complex, MHC-DRB exon 2). As in human medical studies, MHC-alleles were grouped to MHC-supertypes based on similarities in their functional important antigen binding sites. The study indicated that females preferred males both as social and as genetic fathers for their offspring having a higher number of MHC-alleles and MHC-supertypes, a lower overlap with female’s MHC-supertypes as well as a higher genome wide heterozygosity than randomly assigned males. Mutual relatedness had no influence on mate choice. Females engaged in extra-pair mating shared a significant higher number of MHC-supertypes with their social partner than faithful females. As no genetic differences between extra-pair young (EPY) and intra-pair young (IPY) were found, females might engage in extra-pair mating to ‘correct’ for genetic incompatibility. Thus, we found evidence that mate choice is predicted in the first place by the ‘good-genes-as-heterozygosity hypothesis’ whereas the occurrence of extra-pair matings supports the ‘dissassortative mating hypothesis’. To the best of our knowledge this study represents the first investigation of the potential roles of MHC-genes and overall genetic diversity in mate choice and extra-pair partner selection in a natural, free-living population of non-human primates.

Functional associations of similar MHC alleles and shared parasite species in two sympatric lemurs

Several recent studies of animals in their natural surroundings found evidence for effects of certain major histocompatibility complex (MHC) immune gene alleles on the parasite load. However, in multi-infected individuals the particular selection pressure exerted by specific parasites has rarely been explored. In this study we took advantage of the parasitological and genetic data of two previously investigated Malagasy lemur species (Cheirogaleus medius and Microcebus murinus). We investigated whether the two sympatric and ecologically similar primates are infected by similar parasite species and explored if certain parasites are associated with particular MHC alleles. Our study revealed that most of the parasite egg morphotypes were found in both hosts. In each lemur species we identified one MHC allele which was positively associated with Ascaris-infection. Interestingly, these MHC alleles were very similar to each other but differed from all other investigated MHC alleles in an amino acid substitution in a putative functional important antigen binding site. Thus, our study gives first intriguing evidence for a direct connection between certain antigen binding sites of MHC molecules with a particular parasite in two wild primate populations. This may indicate that indeed certain parasites exert direct selective pressure on the MHC of wild living hosts.

Timing and severity of immunizing diseases in rabbits is controlled by seasonal matching of host and pathogen dynamics

Infectious diseases can exert a strong influence on the dynamics of host populations, but it remains unclear why such disease-mediated control only occurs under particular environmental conditions. We used 16 years of detailed field data on invasive European rabbits (Oryctolagus cuniculus) in Australia, linked to individual-based stochastic models and Bayesian approximations, to test whether (i) mortality associated with rabbit haemorrhagic disease (RHD) is driven primarily by seasonal matches/mismatches between demographic rates and epidemiological dynamics and (ii) delayed infection (arising from insusceptibility and maternal antibodies in juveniles) are important factors in determining disease severity and local population persistence of rabbits. We found that both the timing of reproduction and exposure to viruses drove recurrent seasonal epidemics of RHD. Protection conferred by insusceptibility and maternal antibodies controlled seasonal disease outbreaks by delaying infection; this could have also allowed escape from disease. The persistence of local populations was a stochastic outcome of recovery rates from both RHD and myxomatosis. If susceptibility to RHD is delayed, myxomatosis will have a pronounced effect on population extirpation when the two viruses coexist. This has important implications for wildlife management, because it is likely that such seasonal interplay and disease dynamics has a strong effect on long-term population viability for many species.

Rabbit haemorrhagic disease: virus persistence and adaptation in Australia.

In Australia, the rabbit haemorrhagic disease virus (RHDV) has been used since 1996 to reduce numbers of introduced European rabbits (Oryctolagus cuniculus) which have a devastating impact on the native Australian environment. RHDV causes regular, short disease outbreaks, but little is known about how the virus persists and survives between epidemics. We examined the initial spread of RHDV to show that even upon its initial spread, the virus circulated continuously on a regional scale rather than persisting at a local population level and that Australian rabbit populations are highly interconnected by virus‐carrying flying vectors. Sequencing data obtained from a single rabbit population showed that the viruses that caused an epidemic each year seldom bore close genetic resemblance to those present in previous years. Together, these data suggest that RHDV survives in the Australian environment through its ability to spread amongst rabbit subpopulations. This is consistent with modelling results that indicated that in a large interconnected rabbit meta‐population, RHDV should maintain high virulence, cause short, strong disease outbreaks but show low persistence in any given subpopulation. This new epidemiological framework is important for understanding virus–host co‐evolution and future disease management options of pest species to secure Australias remaining natural biodiversity.

Are associations of immune gene expression, body condition and parasite burden detectable in nature? A case study in an endemic rodent from the Brazilian Atlantic Forest

Host-parasite co-evolutionary processes are the most important drivers shaping the hosts immune system. During successful host immune responses to helminthic infections, usually a balanced cascade of different immune genes like MHC, T helper cell 1 and 2 (Th1 and Th2) cytokines is expressed. This information comes largely from human or laboratory studies. The situation under which the immune system has evolved, however, is more complicated and natural variation need to be included to provide a more complete picture of co-evolutionary processes. We employed quantitative real-time PCR (qPCR) to explore associations of immune gene expression, body mass index (BMI) and helminth burden in a wild population of a non-model rodent (Delomys sublineatus). Our study shows that a typical Th2 response with a combination of inflammatory and anti-inflammatory components is detectable also under natural conditions. Complex associations of the expression levels of TGF-β, IL-10, IL-4 and IL-2 with different parasites and with the number of different helminth infections, respectively, were detected. A positive association of the body mass index with the expression of IL-2 and IL-4 may indicate a link between host condition and the inflammatory part of an immune reaction. Our study shows for the first time that despite several potentially confounding parameters naturally present in a wildlife study, typical patterns of immune gene expression are detectable and influence helminth burden. Thus, in addition to structural variance of immune-relevant genes their expression might reflect host-parasite coevolutionary processes.

High adaptive variability and virus-driven selection on major histocompatibility complex (MHC) genes in invasive wild rabbits in Australia

The rabbit haemorrhagic disease virus (RHDV) was imported into Australia in 1995 as a biocontrol agent to manage one of the most successful and devastating invasive species, the European rabbit (Oryctolagus cuniculus cuniculus). During the first disease outbreaks, RHDV caused mortality rates of up to 97% and reduced Australian rabbit numbers to very low levels. However, recently increased genetic resistance to RHDV and strong population growth has been reported. Major histocompatibility complex (MHC) class I immune genes are important for immune responses against viruses, and a high MHC variability is thought to be crucial in adaptive processes under pathogen-driven selection. We asked whether strong population bottlenecks and presumed genetic drift would have led to low MHC variability in wild Australian rabbits, and if the retained MHC variability was enough to explain the increased resistance against RHD. Despite the past bottlenecks we found a relatively high number of MHC class I sequences distributed over 2–4 loci. We identified positive selection on putative antigen-binding sites of the MHC. We detected evidence for RHDV-driven selection as one MHC supertype was negatively associated with RHD survival, fitting expectations of frequency-dependent selection. Gene duplication and pathogen-driven selection are possible (and likely) mechanisms that maintained the adaptive potential of MHC genes in Australian rabbits. Our findings not only contribute to a better understanding of the evolution of invasive species, they are also important in the light of planned future rabbit biocontrol in Australia.

Resistance to RHD virus in wild Australian rabbits: comparison of susceptible and resistant individuals using a genome-wide approach

Deciphering the genes involved in disease resistance is essential if we are to understand host–pathogen coevolutionary processes. The rabbit haemorrhagic disease virus (RHDV) was imported into Australia in 1995 as a biocontrol agent to manage one of the most successful and devastating invasive species, the European rabbit (Oryctolagus cuniculus). During the first outbreaks of the disease, RHDV caused mortality rates of up to 97%. Recently, however, increased genetic resistance to RHDV has been reported. Here, we have aimed to identify genomic differences between rabbits that survived a natural infection with RHDV and those that died in the field using a genomewide next‐generation sequencing (NGS) approach. We detected 72 SNPs corresponding to 133 genes associated with survival of a RHD infection. Most of the identified genes have known functions in virus infections and replication, immune responses or apoptosis, or have previously been found to be regulated during RHD. Some of the genes identified in experimental studies, however, did not seem to play a role under natural selection regimes, highlighting the importance of field studies to complement the genomic background of wildlife diseases. Our study provides a set of candidate markers as a tool for the future scanning of wild rabbits for their resistance to RHDV. This is important both for wild rabbit populations in southern Europe where RHD is regarded as a serious problem decimating the prey of endangered predator species and for assessing the success of currently planned RHDV variant biocontrol releases in Australia.

Changes in Liver Gene Expression Indicate Genetic Pathways Associated with Rabbit Haemorrhagic Disease Infection in Wild Rabbits

Rabbit Haemorrhagic Disease Virus (RHDV) is of worldwide significance for both, domestic and wild rabbits (Oryctolagus cuniculus). While the loss of natural populations of wild rabbits in south-western Europe is of major conservation concern because rabbits are a key-stone species in natural ecosystems, the virus has been successfully used as a biological control of pest rabbits in Australia. However, rabbit numbers in Australia are currently rising again suggesting ongoing host adaptive changes. Accordingly, from both conservation and control perspectives it is important to know how rabbits are adapting and to work towards understanding the underlying genetic basis of this adaptation. Pathogenicity of a viral disease is not only influenced by the pathogen itself but likely also by the hosts immune gene expression which is poorly understood for wild animals. Here we used cDNA microarrays to obtain a general picture of the genetic pathways expressed in the liver during acute RHD infections in wild rabbits. We found that typical immune response pathways are activated during RHD but also identified differences in our results from those that might have been anticipated from laboratory studies. The down-regulation of cell surface protein genes in wild rabbits that avoided infection also suggests that the lower expression of certain surface proteins could confer protection from RHDV infection. This study expands our understanding of the molecular mechanisms at the host-pathogen interface during RHDV infection and pathogenesis and constitutes a step towards determination of genetic mechanisms that may eventually prove important in host-pathogen co-evolution under natural conditions.

Genetic perspectives on the historical introduction of the European rabbit (Oryctolagus cuniculus) to Australia

The introduced European rabbit (Oryctolagus cuniculus) is one of Australia’s most damaging invasive alien species, both in terms of ecological and economic impact. Biological control of rabbits using the myxoma and rabbit haemorrhagic disease viruses has been undertaken in Australia since the mid-1950s, and locally varying genetic resistance to these biocontrol viruses has been reported. The efficacy of biocontrol agents may be influenced, among several factors, by the genetic background of rabbit populations. Therefore, understanding the invasion process of rabbits in Australia, and their resultant population structure, remains crucial for enhancing future rabbit management strategies. Using reduced-representation sequencing techniques we genotyped 18 Australian rabbit populations at 7617 SNP loci and show that Australia’s invasive rabbits form three broad geographic clusters representing different ancestral lineages, along with a number of highly localised, strongly differentiated lineages. This molecular data supports a history of multiple independent rabbit introductions across the continent followed by regional dispersal, and the resulting patchwork genetic structure may contribute to variation across the country in rabbit resistance to the viral biocontrols. Our study highlights the importance of using genome-wide molecular information to better understand the historical establishment process of invasive species as this may ultimately influence genetic variabilty, disease resistance and the efficacy of biocontrol agents.